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Any hi-fi installation requires one or more pieces of equipment to provide program material. Those commonly used may be de scribed as follows:
1. Record Players: There are two popular types. The most popular is the automatic record changer; for optimum results, the preferred type is the professional but plain turntable, manually operated. Both types are available in models that will play records of all sizes and speeds.
2. Tuners: These are a-m and fm radio receivers especially designed to receive, amplify, and rectify signals of wide band width to provide a hi-fi audio signal.
3. Tape: Playback equipment can provide several hours of re corded program material as does the record player. The primary advantage of tape is that one may easily record, store, and play back one's own recorded material of music, entertainment, or information for any purpose.
4. Television: The audio components of a television signal may be picked up by a special television audio tuner or from the television receiver and fed through a system to provide hi-fi audio to accompany the television picture program.
5. Microphones: These are electromechanical transducers de signed to convert sound waves into electrical impulses. Micro phones are used occasionally in hi-fi to record, entertain, or announce, or for other special purposes.
All record players have a motor, turntable, pickup arm ( tone arm), pickup cartridge, and needle. The professional high-quality reproduction systems generally have a manually operated turn table. Record changers with automatic devices to change records at the end of play of a record, providing continuous operation for long periods, are more popular. We will cover the component parts of both the manually operated and automatic types, starting with the needle.
To minimize wear and secure the maximum useful life from a recording, a properly shaped needle must be used. A properly formed reproducing needle will also minimize background noise.
Fig. 4-1 illustrates a group of playback needles seated in the grooves of a recording. The needle at (1) is of a theoretically ideal shape. At ( 2) and ( 3) the needles are too sharp and will gouge the bottom of the groove. The one at ( 4) is too blunt and will cause excessive wear on the walls of the groove, resulting in their eventual breakdown. At ( 5) is a needle of satisfactory shape. How ever, the cutting stylus that makes the groove that the playback equipment must track is shaped like a diamond. The difference in shape between the cutting stylus and playback needle causes track ability distortion and pinch distortion.
When a cutting stylus cuts a sine-wave groove, it moves back and forth in one plane, and the points of contact in the groove are in a line parallel to the radial from the center of the record being cut. However, when a round needle follows this groove, the only time the points of contact are in a line parallel to the radial from the center of the record is at the peaks of the sine-wave excursion ( Fig. 4-2A). At the mid-point, or steepest-slope point, of the sine wave groove, the points of contact of the cutting stylus are still parallel to the radial, but the points of contact of a round stylus are ...
(A) Points of contact at waveform peak in record groove. (B) Contact conditions away from waveform peak. (C) Pinch effect for round and elliptical needles.
... inclined nearly 45 degrees to the radial passing through the center of the record. The resulting tracking distortion is comparatively large.
This condition is mostly overcome by the use of an elliptically shaped needle. The needle is mounted in a position so that the long axis of the ellipse is nearly parallel to a radial passing through the center of the record. Thus, when the elliptically shaped needle tracks the groove, its points of contact nearly coincide with the original points of cutting-stylus contact at any given position ( Fig. 4-2B). The cutting stylus cuts at a 0-degree operating angle with the radius of the groove circle. ( See points 1 and 2 of Fig. 4-2B.) The round needle may follow, making contact on points ( 2 and 5 of Fig. 4-2B) of a line making a 40-degree angle with the line of contact of the cutting stylus, whereas the elliptical needle will follow with only a 5-degree difference angle ( points 3 and 4 in Fig. 4-2B) with a proportionate increase in fidelity or reduction of tracking distortion.
The elliptical needle has another advantage in that it also corrects for pinch-effect distortion caused by the changing width of the groove made by the moving cutting stylus. As the groove width narrows, the round needle rises because it is pinched upward. Since the elliptical needle rides almost the same points of contact as the constant-width cutting stylus, it maintains a fairly constant vertical position ( Fig. 4-2C). Regular records usually contain enough abrasive material to quickly wear a metal needle to the proper shape. Instantaneous or master recordings do not contain abrasive material, and because of this it is particularly important that a needle of the correct shape be used with these types of recordings.
Reproducing needles are made of a number of materials. Plain, hardened steel, and osmium needles are good for only a limited number of plays and therefore are seldom used in hi-fi playback.
Those made of sapphire are the most common. Sapphire needles have a longer life than steel needles. A properly used sapphire needle will give hundreds of plays. Diamonds are also used in reproducing needles and are considered to be the best. Diamond needles will give satisfactory performance for thousands of playings, and when properly shaped and polished they are superior to all other types. Their only disadvantage is that they are expensive and delicate and may be fractured by a slight impact. A chipped diamond or sapphire needle will quickly ruin any type of recording. The life of diamond needles in comparison with other materials varies according to the manufacturer and the user. A life expectancy for a diamond needle many times longer than that of competitive materials is not unusual. Wearing qualities of different needles are ...
OSMIUM 10 HOURS; SAPPHIRE 50 HOURS; DIAMOND 400 HOURS
... shown in Fig. 4-3. Prices of diamond needles are dropping to reasonable levels, making them best buys for top hi-fi playback.
The needle-tip size for monophonic (single-channel) records may be over 0.001 inch ( 1 mil), whereas needle tips for stereo records are either 0.5 or 0.7 mil.
The construction and the electrical characteristics of the various available pickups vary greatly. They can be classified into five distinct groups. These are: crystal, ceramic, magnetic, dynamic, and capacitance.
Units of varying quality can be obtained in each group. The tastes and desires of the user and the use to which the pickup is to be put may govern its choice. With systems designed to reproduce the voice for reference purposes only, an inexpensive pickup with a comparatively narrow frequency range is suitable. Where the highest quality of reproduction is required, a special highly damped pickup having a frequency characteristic flat to beyond 15,000 Hz is necessary.
Crystal and Ceramic Pickups--Crystal and ceramic pickups are the lowest-cost units mentioned in the foregoing. They are simple in design and construction, have fair frequency response characteristics, and hi-fi types have low distortion content.
Some crystalline substances possess the ability to produce an electrical charge under certain conditions. When they are stressed mechanically, a charge is produced on their surfaces. If a voltage is applied to the surfaces of a crystal with piezoelectric properties, a mechanical deformation of the crystal will take place.
The piezoelectric crystal acts as a generator and converts mechanical motion into an electrical charge. Crystal microphones and phonograph pickups can be thought of as piezoelectric generators.
A crystal is also similar to a motor. When a potential is applied to a crystal, it moves. It converts electrical energy into mechanical motion. Crystal headphones and record cutters are piezoelectric motors.
A piezoelectric crystal as used in microphones and pickups is a formation of crystalline Rochelle salt. The Rochelle-salt crystal possesses the piezoelectric property to a high degree. It is approximately 100 times as active as a regular quartz crystal.
Rochelle salt crystals are formed in large bars. These bars are cut into slabs or plates for use in the manufacture of crystal elements. The two commonly used crystal plates are usually referred to as expander and shear plates, as shown in Fig. 4-4. The crystal is either a shear or expander plate, depending on the way it is cut from the bar.
A crystal plate is said to have three axes. The latter are the electrical (AA), the mechanical (BB), and the optical (CC) axes. An expander plate is cut at a 45-degree angle to the optical and mechanical axes of a crystal bar. A shear plate is cut with its edges parallel to the mechanical and optical axes of the crystal bar.
When a potential is applied to the two large faces of each plate, mechanical motion is developed at an angle of 45 degrees from that of the mechanical and optical axes. ( When a force is applied, an electrical potential will be developed at the faces. ) Therefore, the expander plate will increase its length and, at the same time, de crease its width. If the polarity of the faces of the crystal is changed, the crystal will decrease its length and increase its width.
The same action takes place when a potential is applied to a shear plate, except that expansions and contractions occur along the diagonals of the plate instead of parallel to the edges, as in the case of the expander plate. When mechanical pressure is applied, a potential voltage is produced.
In order to form a crystal element for use in a crystal cartridge or other device, a number of crystal plates are cemented together. This makes possible more effective utilization of the properties of the crystal.
An element which consists of a number of expander plates cemented together is referred to as a bender element, while an element formed from a number of shear plates is known as a twister element. The names "bender" and "twister" refer to the action which takes place when an electrical potential is applied to the element.
The multiple crystal has a number of important advantages over a crystal employing a single plate. It greatly decreases the undesirable effects of saturation and hysteresis and reduces the effects of temperature on the impedance and sensitivity of the unit. Fig. 4-5 shows the construction of a bender and a twister crystal element.
The faces of each crystal plate are milled smooth, and foil or graphite electrodes are applied. Leads are connected to the electrodes, and after they have been properly oriented, the plates are bonded together with cement.
(A) Bender element. (B) Twister element.
The completed crystal element is coated with a special moisture proof material to protect it against deterioration under very dry or damp conditions. The crystal element is mounted in a nonconducting case and held at one end by a metal clamp. The other end of the crystal is free, permitting it to move torsionally. A bearing and chuck are mounted on the free end of the crystal. The bearing usually consists of rubber or a similar synthetic material. The chuck usually consists of a light metal such as aluminum. To restrain the crystal from vibrating at more than one mode, it is customary to restrain it slightly along its axes of motion. This is accomplished by cementing a strip of damping material along the length of the element. This strip also gives some damping effect in the other modes and helps to reduce the amplitude of the resonant peak of the crystal. Because the crystal is very stiff, its resonant frequency is normally not in the frequency range to be recorded.
As a circuit component, a crystal cartridge acts in the same way as does a capacitor and can be considered as such. Extremes of temperature drastically affect the operation of a crystal. The maximum sensitivity of a crystal is usually at about 75 degrees. As the temperature rises above or falls below 75 degrees, the sensitivity of the crystal falls off slowly. Cartridges using barium titanate have much improved temperature characteristics.
At temperatures in the neighborhood of 130 degrees, Rochelle salt crystals may permanently lose their piezoelectric properties.
As a rule, temperatures slightly below this, that is, from 110 degrees to 120 degrees will not injure a crystal.
Ceramic pickups have overcome this difficulty and are now commonly used in place of crystals. A ceramic pickup has the advantage that it is quite stiff, and, because of this, variations in mechanical load do not greatly affect the performance. Ceramic pickups have operational disadvantages in that they have some distortions and resonances which are almost always present. Also, most have rolloff of the high frequencies. Types such as the cartridge shown in Fig. 4-6 have overcome these faults to a major degree.
Magnetic pickups--The magnetic pickup is a current-operated device. The construction of magnetic pickups varies greatly. Essentially they consist of a coil and magnet and another magnet to which the needle is attached. The movable magnet to which the needle is affixed is damped. This is accomplished in a number of ways, de pending on the construction of the particular cartridge.
The coil is connected directly to the input of the preamplifier.
Current through it varies with the change in the density of the flux about the fixed magnet. This variation in flux is produced by variation in the force exerted on the movable magnet, for each change in position. Since the reproducing needle is connected to this magnet, it produces output in proportion to the movement.
Courtesy CBS Electronics, Div. of CBS, Inc.
The frequency range of a magnetic pickup is greater than that of the crystal type. The finest magnetic pickups have a frequency range of from 50 to over 15,000 Hz. The disto1tion content of a fine magnetic pickup may be as low as 0.1 percent at 400 Hz, and 1 percent to 4 percent at high frequencies. A typical hi-fi reluctance-type magnetic cartridge is shown in Fig. 4-7.
Dynamic Cartridges--The dynamic cartridge, as the name implies, is of the moving-coil type. The unit consists of a movable coil, to ...
(A) Pickup. Courtesy North American Philips Corp. (B) Construction.
(A) Pickup. (B) Construction.
... which the needle is mounted, and a permanent magnet. The coil is connected to a low-impedance preamplifier input. When the needle moves the coil, which is located in the magnetic field of the permanent magnet, a voltage is induced in the coil. The results obtained with a dynamic pickup are as good as or better than those obtained with magnetic pickups. A dynamic cartridge is illustrated in Fig. 4-8. This cartridge has a response that extends beyond 20,000 Hz, and it gives almost as much output as a good reluctance pickup.
Stereo Cartridges--In stereophonic application, one of the most critical components is the pickup cartridge. The basic principles of stereo cartridge operation were covered in Section 3. Now we shall consider those characteristics important in selecting a cartridge.
Fig. 4-9 shows some typical stereo cartridges.
The weight of a cartridge and pickup arm assembly is transmitted to the record as needle force. The maximum allowable needle force without excessive record wear is related to the size of the needle tip.
This is because wear is dependent on pressure, which is force per unit area; thus the smaller the needle tip, the less the needle force must be to prevent wear. If needle-tip size were made too small, the necessary reduction in needle force would result in loss of tracking; that is, the needle would skate over the record instead of staying in the groove. Thus the minimum needle-tip size is kept to 0.5 mil, and the range of needle force is roughly from ½ to 6 grams.
As is the case with all high-fidelity system elements, we are interested in the frequency response of a cartridge. Response is usually stated in terms of frequency range and the deviation, in dB, of the response over that range. We are naturally interested in the widest possible frequency range, but it is doubtful if response below 20 Hz makes much difference. However, the high limit of the range should extend to near 20,000 Hz or beyond to take full advantage of the better recordings.
The output voltage of a cartridge is important, not only because it is used to calculate how much amplification is needed, but also because the higher the output, the more chance there is of having a good signal-to-noise ratio. However, sometimes it is desirable to sacrifice some signal strength to ensure minimum distortion, best frequency response, and minimum record wear. Output voltage does not establish criteria unless the level at which the needle · is driven is also specified. Standard records are used to provide the drive for output voltage tests. Some cartridge manufacturers state output for a needle velocity of 5 centimeters per second ( cm/s), and others for a 10-cm/s needle velocity. Naturally, the output should be higher for the greater velocity. In Fig. 4-10, the needle velocity is shown in relation to needle force and frequency. For total outputs below 10 millivolts, a separate preamplifier may be necessary, depending on the gain of the system used.
Channel separation is the indication of how well the left signal is kept out of the right channel, and the right signal out of the left channel. Good separation is necessary for good stereo effect, since the difference between the two signals is what produces the spatial effect. If a cartridge were not carefully designed with separation in mind, one channel would affect the other, and the outputs of the two channels would tend to become the same. Tests have shown that a minimum of 15-dB separation should be maintained.
To operate properly, a cartridge must be connected to the proper amplifier input impedance. Generally speaking, ceramic and crystal pickups must work into a relatively high resistance ( 15 kilohms to several megohms) compared with magnetic cartridges ( 5 to 100 kilohms) . Each cartridge manufacturer specifies the proper load characteristics for his models.
Compliance is a measure of how easy it is to move the needle in the directions it must be driven during playing. It is ordinarily measured in millionths of a centimeter per dyne ( 10^-6 cm/ dyne) , i.e., the distance, in millionths of a centimeter, that the needle can be pushed by a force of 1 dyne in that direction. For stereo cartridges, it is important that they not only have a high compliance, but that this high compliance apply in all directions of motion. This is why many stereo cartridge manufacturers specify both lateral and vertical compliance. It will be shown that it is the low vertical compliance of most monophonic pickup cartridges that makes it necessary that stereo records never be played with monophonic pickups.
Another characteristic that is sometimes specified by cartridge manufacturers is channel balance. It is given as the deviation from
-------- Fig. 4-9. Typical stereo cartridges. Courtesy Pickering & Co., Inc. (A) Pickering Dustamatic. Courtesy Ortofon Div., Elpa Marketing Industries, Inc. (B) Ortofon SPE/T. Courtesy Shure Brothers Inc. (C) Shure V-15 Type II. Courtesy Empire Scientific Corp. (D) Empire 888P. Courtesy Stanton Magnetics, Inc. (E) Stanton longhair. (F) Dusting action of brush.
balance, that is, the difference in dB between the outputs of the two channels for the same drive. Actually, this characteristic is not very critical because just about all stereo systems include a channel balance adjustment to compensate for reasonable differences in output. However, a relatively large unbalance, such as more than 3 dB, is an indication of a defective cartridge. The optimum arrangement is a system that is naturally balanced throughout, and balance deviation of 1 dB or less should be considered good.
Application--The pickup cartridge should be mounted in a suit ably designed arm. The needle pressure should be left at the lowest value consistent with the design of the cartridge used. For optimum results, the arm should have an offset head to minimize tracking error, and side-to-side motion should be as free as possible.
Vertical sensitivity is an important characteristic of any cartridge.
This sensitivity is mostly related to the vertical movement sensitivity of the cartridge and stylus. Low vertical sensitivity in a properly designed monophonic unit will reduce hum and other turntable noises without affecting the quality of pickup of the lateral groove modulation from the record. However, as explained previously, vertical sensitivity is important for proper reproduction of stereo records. Hum may be picked up by induction in magnetic- and dynamic-type pickups. Hum sensitivity is another important consideration in pickup selection.
In general, reluctance-type magnetic pickups are most widely used. Dynamic pickups reproduce extremely well, but they are less sensitive. Ceramic units are best for medium-quality work requiring high output from the pickup unit; these units will drive a basic amplifier directly without need for a preamplifier. In comparison with ceramic units, the response of the better reluctance types is more nearly flat, and they have the least distortions and resonances.
While hi-fi crystal pickups are entirely suitable for most reproduction, the best crystal units usually have a number of small resonances and distortions. Capacitance units are suited to special applications requiring extremely wide frequency response. However, much of the operating quality of any cartridge depends on the pickup arm and the needle pressure.
The Pickup Arm (Tone Arm)
The pickup arm or tone arm is the carriage for the cartridge and needle. Its design and principle of operation are very important to high-fidelity reproduction. To permit the needle to respond without distortion to record groove deflections ( tracking ability), the tone arm should have free movement in all directions. The force of weight of the arm applied vertically to the needle should be adjust able to provide for the needs of various kinds of cartridges. Optimum pressure will generally be found between ½ gram and 6 grams, with an average setting of 1 recommended for most cases.
Several companies make a device available for measuring this weight near the point of application. The weight must be within the range required for a certain needle-cartridge combination. It must be heavy enough to keep the needle in the groove to follow the modulation, and light enough to allow free movement and low wear on the needle and the record. The pressure on the needle therefore should be adjustable and the adjustment made preferably with a calibrated scale.
The pickup arm should track as nearly as possible the original line of cut; otherwise, a tracking error will develop and cause distortion. See Fig. 4-11. The record is cut along line AB, and the usual pickup arm operates in an arc, as shown by line CD. Placement of the mounting for the pickup arm can vary the tracking to a large degree. When the playback is made with the arc of contact of needle to record too far off, the needle follows the modulation with a modified movement because the round needle tip is moving with a direction and amount per modulating element different from the direction and amount per modulating element of the original cutting stylus. This causes tracking distortion in the signal generated by the cartridge.
The arm is best designed to ride on precision aligned bearings for the greatest reduction of resistance to movement and for the lowest inertia. Some units use ball bearings.
Pickup arms that provide for different kinds of cartridges enable one to try and use different cartridges.
For stereo application, the pickup arm ( tone arm) and turntable and drive arrangement must meet special requirements which are more rigid than those sufficient for monophonic reproduction. Some pickup arms designed particularly for stereo are illustrated in Fig. 4-12. Let us consider some of the features to be considered when one is selecting a pickup arm for stereo.
Courtesy Thorens Div., Elpa Marketing industries, Inc. (A) Thorens TP14. Courtesy Rek-0-Kut Company, Inc. (B) Rek-O-Kut Micropoise. Courtesy Shure Brothers Inc. (C) Shure SME Series 2. Courtesy Empire Scientific Corp. (D) Empire 980.
Fig. 4-12. Typical pickup arms designed for stereo.
One of the characteristics most important in stereo pickup arms is the tracking error just described. When there is an angular difference between the direction of motion of the groove the needle is in and a line between the pickup-arm lateral pivot and the needle, tracking distortion and unbalance in the two outputs are produced.
In other words, instead of the needle pulling "in line" with the pickup, at some parts of the motion across the record it pulls in a different direction, producing proportional unbalance. The lateral component of force caused by tracking error in a stereo record groove produces more wear on one side of the record groove than on the other. Hence, one stereo component signal will be reduced in relation to the other. Also, stereo cartridges operate at much lower needle force than mono cartridges and cannot tolerate as much lateral tracking-error force without jumping out of the groove.
Tracking error is minimized in pickup arms by orienting the pivot point and needle point locations with respect to the record and making the angular offset of the pickup with respect to the longitudinal axis of the arm optimum. By proper adjustment of both of these variables, arm manufacturers have been able to reduce tracking error to a fraction of a degree in some models.
A good stereo pickup arm should have a needle-force adjustment finer than that adequate for monophonic pickup arms. Stereo models require more precision, because of the lower forces and smaller tolerances to which they must be adjusted. The normal arrangement is for adjustment first for perfect balance ( zero needle force), then adjustment of another control to the exact number of grams of force desired.
Sometimes pickup arm manufacturers state the tracking force.
This is the force that must be exerted laterally against the needle to move it in across the surface of the disc. In arms for stereo, the tracking force seldom exceeds 2 grams, and in many cases it is below 1 gram. The tracking force of an arm must be less than that of the cartridge used with it, or the cartridge rating would be exceeded as the needle moved inward on the record.
The construction of a turntable has a great deal to do with fidelity of the reproduction. Wabble, wow, and other turntable defects show up unfavorably when a record is played back.
Three standard turntable speeds are used. They are 78.26, 45, and 33½ revolutions per minute. Fixed speed is a very important factor in playback, since variations of as little as 1 percent are detectable.
Speed variations are caused by changes in the power source of the turntable and motor.
Wabble results when the turntable, spindle, and bearing assembly are not carefully aligned to be concentric. When wabble occurs, the surface of the record moves closer to and farther from the pickup head as the turntable moves. This results in changes in the strain on the needle, modifying the modulation as it is picked up. In extreme cases, the stylus may completely leave the surface of the record at one or more points.
The motor used to drive the turntable is usually powered by alternating current, and, as is universally the case, vibration at the power frequency is set up within the motor. If this vibration is trans mitted to the turntable and pickup, it will result in hum modulation of the pickup. This is known as rumble. In good turntable assemblies, the motor is insulated from the turntable sufficiently to eliminate rumble. Rubber mats lying on top of the table and under the record will further reduce this effect.
To avoid the defects described, careful design and construction of the motor and drive system are necessary. Motors can be designed to give fairly constant speed, but it is not possible to design and build a motor inherently of constant speed without some degree of undesirable fluctuation. The fluctuation will cause distortion. Fluctuation should be reduced by the turntable drive system. This may be achieved by use of resilient frictional drivers or application of governors or other frictional drags to provide regulation of the speed. The inertia of a heavy turntable will also contribute to regulating the speed and to reduction of the effect of motor fluctuation.
Frictional drives and drags must be carefully designed and applied so that the frictional force applied is constant, or the table speed will vary through each 360-degree revolution. This causes the effect known as "wow." Fig. 4-13 shows a number of driving methods commonly employed in record players and changers. The gear-train or direct motor drive (Fig. 4-13A) requires a very powerful motor free from vibration. If properly designed, this type of drive is very satisfactory. Because good units are very expensive, this drive system is rarely encountered.
The arrangement shown in Fig. 4-13B is the direct rim-drive method. This method provides a single speed. The turntable is driven by a rubber wheel attached to the motor shaft. The rubber wheel also serves to isolate the motor from the turntable, reducing the transmission of motor variations. Modem arrangements use one or more rubber idlers, further reducing transmission of motor fluctuations and vibrations.
One of the defects of this system arises from the deformation of the rubber wheels which results if the wheel is left in contact with the turntable rim when the equipment is not in use. The wheel is flattened at the point in contact with the turntable rim. In the better units, provisions are available for removing the pulley from con tact with the turntable when the equipment is not in use.
In Fig. 4-13C, a multiple-speed rubber-wheel drive system is shown. Two rubber wheels are provided, one giving a 78-rpm turn table speed, and the other a 33½-rpm turntable speed. The desired speed can usually be chosen by changing the position of a lever connected to the drive mechanism.
(A) Direct motor drive. (B) Rim drive. (C) Two-speed idler drive. (D) Belt drive.
The single- and dual-speed rubber-wheel drives are often constructed so that the rubber wheel mounted on the motor shaft drives an idler which, in turn, drives the turntable. This greatly simplifies the design of facilities for removing the wheel from contact with the turntable, and it permits better control of the pressures between the rubber wheel and the turntable rim. The possibility of slippage, which is a common fault of a direct rubber-wheel drive, is also greatly reduced. This system is also applied to three-speed players.
Courtesy Acoustic Research, Inc. (A) AR turntable.
Courtesy Marantz Co., Inc.(B) Marantz SLT-12 turntable.
Courtesy Matsushita Electric Corporation of America (C) Panasonic SP-10 turntable. Courtesy RASCO (D) RABCO turntable.
Fig. 4-13D shows a belt-drive arrangement often encountered in playback equipment. A rubberized composition belt is connected between pulleys on the motor shaft and turntable spindle. This sys tem is very good with respect to low transmission of vibration.
Another drive method uses a conical driving member that is continuously variable. This provides for adjustment to any of the three popular speeds of 78, 45, and 33¼ rpm and in addition allows for setting an exact speed around any one of the three standards to get a precise pitch from a particular record according to individual desire.
Record Changers Versus Turntables
Turntables have design and application advantages in that they are simple and have a limited function compared with an automatic record changer. It is generally conceded that the highest quality reproduction can be obtained from a good turntable. Record changers, on the other hand, can provide convenience with excellent quality. However, the additional mechanisms, devices, and controls necessary to change the record may cause problems in speed variations, rumble, and other defects. Each has its place-the turntable for flawless reproduction, and the record changer for hours of excellent-quality continuous program without attention.
Fig. 4-14 shows hi-fi turntables, and Fig. 4-15 shows typical record changers.
The cartridge, the pickup arm, and turntable requirements for stereo operation are so interrelated that many audiophiles prefer to buy the whole combination as a unit. Typical combinations are illustrated in Fig. 4-16. As explained in Section 3, variation of needle height, greater tracking force, and stronger rumble effects make record changers somewhat inferior to manual players for stereo.
Changers that are used for stereo are very similar to those employed for monophonic reproduction. However, they contain refinements to reduce rumble, tracking force, etc.
The turntable combination shown in Fig. 4-16B is a new design using solid-state electronics for speed control. Voltage from the ac line is reduced through a transformer, changed to de by a rectifier, and fed to a push-pull amplifier which powers the motor. Inasmuch as this amplifier can produce 20 watts and the motor needs only 5 watts, there is considerable margin for reliability. The amplifier is fed by a Wien bridge oscillator which determines the power frequency according to the speed of rotation selected by the user.
The pitch, or fine speed, control in this system is based on the introduction of changes in the oscillator circuit by means of a switch, controlled by an adjustment at the top center of the unit.
Courtesy Garrard, Div. British Industries Co. (A) Garrard SL55B. Courtesy JVC America, Inc. (B) Nivico Model 5204.
Fig. 4-15. Typical record changers.
The motor itself is a 16-pole synchronous type which locks to the frequency fed from the amplifier-oscillator circuit and thus maintains constant speed at each speed setting. With this motor-drive system, the turntable can be operated from power sources of 100 to 250 volts and at power-line frequencies of 50 to 60 Hz. With some modification at the factory, the unit can even be made to run from a de source such as batteries. The motor drives the platter through a rubber belt. The bearings are self-lubricating. The platter, a 12 inch nonferrous casting, weighs 7 pounds, 4 ounces; it is covered ...
Courtesy Garrard, Div. British Industries Co. (A) Garrard Zero 100. Courtesy Thorens Div., Elpa Marketing Industries, Inc. (B) Thorens TD-125.
... with a thick rubber mat; and its center piece may be inserted up side down to accommodate 45-rpm records. There are three controls: the speed selector (left), the off/ on switch (right), and ( center) the fine-speed adjustment and an illuminated strobe marker to permit accurate speed adjustment.
APPLICATION OF ELECTRON TUBES, SEMICONDUCTORS, AND INTEGRATED CIRCUITS
Over the first half of this century, electron tubes reached a high state of development in rf and audio applications. Semiconductors and integrated circuits are relatively new, but they have been developed to the point where they now can provide performance equal to that of tubes with the advantages of reduced weight, smaller size, improved reliability, and lowered cost of operation.
Semiconductor devices are small but versatile units that can per form an amazing variety of control functions in electronic equipment. Like electron tubes, they have the ability to control almost instantly the movement of charges of electricity. They are used as rectifiers, detectors, amplifiers, oscillators, electronic switches, mixers, and modulators.
Semiconductor devices have many important advantages over other types of electron devices. They are small and light in weight ( some are less than an ¾ inch in length and weigh only a fraction of an ounce). They have no filaments or heaters, and therefore re quire no heating power or warm-up time. They are solid in construction, extremely rugged, free from microphonics, and can be made impervious to many severe environmental conditions. The circuits required for their operation are usually relatively simple.
The simplest type of semiconductor is the diode. Crystal, silicon, and tunnel diodes are used in hi-fi equipment. Diodes have two elements. When another layer (element) is added to the semiconductor diode to form three layers ( with two junctions), the capability of amplification is added. The resulting device is called a bipolar transistor; it has an emitter, base, and collector ( similar in effect to the cathode, grid, and plate of a vacuum triode). Later transistor developments, the field-effect transistor (FET) and the integrated circuit, have contributed to improved performance in tuners, receivers, and amplifiers, and at the same time have made possible the mass manufacture of smaller equipment with efficiency equal to the most sensitive and powerful tube-type receivers. Some of the most recent improvements in the application of transistors have been provided for rf amplification, conversion, i-f amplification, detection, and control circuits in stereo equipment.
In the bipolar transistor, performance depends on the interaction of two types of charge carriers, holes and electrons. Field-effect transistors are unipolar devices ( i.e., operation is basically a function of only one type of charge carrier, holes in p-channel devices and electrons in n-channel devices). Early models of field-effect transistors used a reverse-biased semi conductor · junction for the control electrode. In the metal-oxide-semiconductor field-effect transistor (MOSFET), a metal control gate is separated from the semiconductor channel by an insulating oxide layer. One of the major features of the metal-oxide-semiconductor structure is that the very high input resistance of MOS transistors ( unlike that of junction-gate field-effect transistors) is not affected by the polarity of the bias on the control (gate) electrode.
In addition, the leakage currents associated with the insulated control electrode are relatively unaffected by changes in ambient temperature. Because of their unique properties, MOS field-effect transistors are particularly well suited for use in such applications as voltage amplifiers, rf preamplifiers, i-f amplifiers, and other circuits used in hi fi.
The distinguishing feature of an integrated circuit is that all components required to perform a particular electronic function are combined and interconnected on a common substrate. The constituent elements of integrated circuits lose their identities as discrete components, and the devices assume the appearance of "microminiaturized" function blocks. In comparison to their discrete-component counterparts, integrated circuits offer enhanced performance and new plateaus of reliability, at reduced costs. In addition, the avail ability of complete solid-state circuits in packages no larger than those of conventional discrete transistors makes possible further reductions in the size and weight of electronic equipment.
Tuners are available to receive fm or a-m signals separately or in one combined unit. Some tuners provide for the preamplifier equalizer functions because in some installations no program material source other than the tuner is required. In other cases, a separate preamplifier control is not as desirable as having all controls on the same panel with the tuning controls. The performance of such a tuner preamplification-compensation system should not be expected to be as good as a top-quality separate control pre amplifier, but the design of this arrangement usually provides as much quality as is usable for fm or a-m reception or other average pickup. For amplitude-modulation reception, the upper limit of frequency response is usually 10,000 Hz or less, because a-m broadcast stations are separated only by this amount, and reception of two nearly equal signals only 10 kHz apart will produce a strong 10-kHz beat note. This note is unpleasant to sensitive ears if not filtered out.
Frequency-modulation tuners can provide a greater range of reproduction and high fidelity over the entire audible range, if all elements of the systems involved are designed, arranged, and operated to achieve maximum performance.
A-M Tuners Amplitude--modulation tuners, covering the broadcast band from 540 to 1600 kHz, are usually of the superheterodyne type, having an rf preamplifier to reduce image interference, a first detector and oscillator to convert the signal to a lower frequency, where there are better rf amplification conditions, a second detector to remove the sound signal from the carrier signal, and an audio amplifier and output arrangement to bring the signal to a level sufficient to drive a basic audio amplifier or control unit of hi-fi type. In addition, there may be the usual control features, such as automatic volume control, noise limiting, and others.
Amplitude Modulation--Amplitude modulation is defined as the process of changing the amplitude of an rf carrier in accordance with the intelligence to be transmitted. When there is no modulation, the radio-frequency carrier portion of an amplitude-modulated wave is of constant frequency and constant amplitude, as shown in Fig. 4-17A. An audio modulating frequency is superimposed on this carrier in a manner that causes the amplitude of the carrier signal to vary as illustrated in Fig. 4-17B, leaving the carrier frequency un changed. The pattern shown in Fig. 4-17B is commonly referred to as a modulation envelope.
(A) Unmodulated rf wave. (B) Modulated rf wave.
Sidebands-An amplitude-modulated wave is composed of a number of frequencies: the radio frequency of the carrier wave, the modulating audio frequency or frequencies, and combinations of these frequencies. These combination frequencies are called the sideband frequencies and are the result of mixing the radio frequency and the modulating frequencies. Whenever any two frequencies are mixed together, two new frequencies are produced.
One of these is the sum of the two frequencies, and the other is the difference between the two original frequencies. Thus, for a modulating frequency of 5000 Hz and a carrier frequency of 1000 kHz, sideband frequencies of 995 kHz and 1005 kHz are produced. If the modulating frequency is increased to 10,000 Hz, sidebands will be produced at 990 kHz and 1010 kHz.
It is these sideband frequencies that carry the intelligence in an amplitude-modulated wave. When an rf carrier is modulated by many audio frequencies, such as occur in speech or music, the side frequencies consist of a band of frequencies above and below the carrier frequency. The width of each of these bands is determined by the highest modulating frequency. For this reason, hi-fi a-m signals must have an available bandwidth equal to twice the highest frequency to be reproduced.
A-M Superheterodyne Receivers
A superheterodyne receiver is one in which the desired signal is mixed with a locally generated signal to produce an intermediate frequency signal. This intermediate-frequency signal is then amplified and detected to produce the audio frequency. Fig. 4-18 is a simplified block diagram of a typical superheterodyne receiver.
The rf amplifier stage receives the weak signal intercepted by the antenna, amplifies it, and passes it on to the mixer. In the mixer stage, the received signal is heterodyned with the output of the local oscillator. The output of the mixer stage is an intermediate-frequency ( i-f) signal which has the same modulation characteristics as the received signal. The i-f signal then passes through a number of amplifiers, referred to as intermediate-frequency amplifiers, the output of which is applied to the second detector. This stage removes the i-f component from the signal, leaving the undistorted audio signal, which is then amplified and applied to the speaker.
Frequency Conversion--The converter stage consists of the mixer and local oscillator. The purpose of the frequency converter is to produce an intermediate-frequency signal having the same modulation characteristics as the received signal. This is accomplished by generating an unmodulated rf signal in the receiver and heterodyning it with the received signal. By this method, a third signal is generated, the frequency of which is equal to the difference between the locally generated and incoming signal frequencies.
Two circuits are required to generate the i-f signal, an oscillator and a mixer. Tubes of special design have been developed so that both functions can be accomplished by one tube. Many receivers, however, employ separate mixer and oscillator stages. A typical transistor-type converter using a separate mixer and oscillator is shown in Fig. 4-19. The rf input is coupled to the mixer tuned circuit, L2 and C1, by means of coupling coil LL This circuit ( L2-C1) is tuned to the frequency of the incoming signal and applies this signal to the mixer base. The oscillator operates at a frequency equal to the in coming signal frequency plus the intermediate frequency. Output from the oscillator is coupled to the mixer base through capacitor C2. The signal at the collector of the mixer is thus the result of both the incoming signal and the oscillator signal. Signals at the oscillator frequency, the receiver signal frequency, the difference frequency, and several others appear in the mixer output. The circuit of L3 and CS is tuned to the difference frequency, and this signal builds up to a high amplitude while other signals are largely eliminated.
Capacitors C1 and C3 are ganged so that when the mixer circuit is tuned to the frequency of an incoming signal, the oscillator is tuned so that its frequency remains equal to the incoming signal plus the intermediate frequency. The most common intermediate frequency is 455 kHz. If the receiver signal is at a frequency of 1000 kHz, the oscillator frequency must be 1455 kHz to produce an intermediate frequency of 455 kHz. If the mixer is tuned to a new signal at ( for example) 1100 kHz, the oscillator frequency must be changed to 1555 kHz.
Oscillator Signal Injection--In the converter described previously, a capacitor is used to inject the oscillator signal into the base circuit of the mixer. This arrangement is called capacitive injection. Two other methods of injecting the oscillator signal into the mixer circuit are shown in Fig. 4-20. In Fig. 4-20A, inductive injection is used.
The oscillator coil (L4) is inductively coupled to the mixer cathode circuit by means of coupling coil L3. In Fig. 4-20B, electronic injection is used. A dual-gate MOSFET is used in this circuit, pro viding a separate gate for the oscillator signal. The incoming signal applied to the control gate and the oscillator signal applied to a second control gate both act upon the electron stream through the transistor to produce the intermediate frequency in the output circuit. The injection method illustrated in Fig. 4-20B is superior to those of Figs. 4-19 and 4-20A in that it reduces interaction between the mixer and oscillator circuits.
RF AMPLIFIER MIXER OSCILLATOR I-F AMPLIFIER 2ND DETECTOR TO PREAMP
I-F Amplifiers--The i-f amplifiers provide most of the voltage amplification of the signal of a superheterodyne receiver. One or two and sometimes three i-f amplifier stages are used. A typical tube-type i-f amplifier circuit is shown in Fig. 4-21. The input and output circuits are inductively coupled by means of i-f transformers T1 and T2. The primaries and secondaries of the transformers are tuned. Since the incoming signal is always heterodyned to the same intermediate frequency, the four tuned circuits are operated at the same frequency at all times. This makes it possible to design and adjust the circuits to obtain maximum gain. The i-f transformers are mounted in small metal cans and are adjusted to the proper frequency by means of variable capacitors, as shown in the figure, or by means of movable powdered-iron cores. The capacitor-tuned types are often provided with fixed powdered-iron cores to increase gain and selectivity.
Because of the high gain of i-f amplifiers, coupling between input and output circuits must be kept to a minimum. This is accomplished by careful shielding and placement of parts and by providing suit able decoupling networks in plate, screen, and grid circuits. De coupling networks usually consist of a resistor and capacitor connected as shown in Fig. 4-21. The plate decoupling network consists of R2 and C5, while R3 and C6 provide screen decoupling.
Hi-Fi Bandpass and Image Rejection---The two most important factors influencing the choice of an intermediate frequency are bandpass and image rejection. For several reasons it is possible to obtain greater bandpass as the intermediate frequency is raised.
Therefore, when maximum bandpass is desired, the intermediate frequency is made as high as possible, consistent with other factors.
If the oscillator of a superheterodyne is tuned to 1455 kHz and the intermediate frequency is 455 kHz, signals at 1000 kHz ( oscillator minus intermediate frequency) and 1910 kHz ( oscillator plus inter mediate frequency) may be received by tuning the mixer to the desired signal. This is possible because both frequencies when heterodyned with the oscillator signal will produce the same difference frequency. In practice, the mixer is tracked so that it is always tuned to either the oscillator frequency plus the intermediate frequency or the oscillator frequency minus the intermediate frequency.
If the mixer frequency is equal to the oscillator frequency plus the intermediate frequency, then the oscillator frequency minus the intermediate frequency is referred to as the image frequency. If the mixer is tuned below the oscillator frequency, then the higher frequency is called the image frequency. Regardless of which frequency the mixer is tuned to, some signal energy will appear in the mixer output if a strong image-frequency signal is present. This difficulty occurs because the mixer circuit is not selective enough to reject the image signal. Suitable image rejection is obtained by choosing an intermediate frequency high enough to provide sufficient separation between the received-signal and image-signal frequencies. As the intermediate frequency is increased, the image frequency moves farther away from the frequency to which the mixer is tuned, and the image rejection increases. In the broadcast band and at some what higher frequencies, intermediate frequencies in the neighbor hood of 455 kHz are satisfactory; at higher frequencies, the inter mediate frequency must be increased to obtain suitable image rejection. Generally, it is necessary to make a compromise and choose a frequency somewhere between that which gives optimum image rejection and that which gives the greatest selectivity.
Transistor Tuned Amplifiers---In radio-frequency ( rf) and inter mediate-frequency ( i-f) amplifiers, the width of the band of frequencies to be amplified is usually only a small percentage of the center frequency. Transistors may be used in these applications effectively to select the desired band of frequencies and to suppress un wanted frequencies. The selectivity of the amplifier is obtained by means of tuned interstage coupling networks. A typical transistor i-f amplifier section is shown in Fig. 4-22.
Application of Integrated Circuits to 12-MHz I-F Amplifier for A-M Receiver---Fig. 4-23 illustrates the use of integrated circuits in an i-f amplifier for an a-m receiver. The amplifier is encased in a metal box, and adequate shielding and supply decoupling are provided. The i-f amplifier has three stages, each of which is designed to provide a gain of 25 dB. The source resistance to the input circuit was selected to provide a satisfactory compromise for gain, noise figure, and modulation-distortion performance. The input and output transformers, T1 and T4, have high unloaded Q's to preserve good noise performance and to maximize the output power.
The interstage transformers, T2 and T3, have low unloaded Q's to achieve the required gain. The second detector has a bandwidth of 10 kHz. Typical overall performance characteristics are as follows: Power drain = 83 milliwatts Power gain ( from input to second detector output) = 76 dB Age range ( first stage) = 60 dB Noise figure = 4.5 dB RF Amplifiers-An rf amplifier is not absolutely necessary in a superheterodyne receiver; in fact, many receivers do not include such a stage. However, the incorporation of an rf amplifier greatly improves the performance of a receiver. The purpose of an rf amplifier is to improve the image rejection and the sensitivity of the receiver. As explained in the discussion of if amplifiers, the mixer stage does not have sufficient selectivity to reject strong signals at the image frequency completely. The rf stage increases the image rejection by amplifying the desired signal. The image signal is not amplified, and thus image interference is reduced. Some receivers use as many as three rf stages to secure optimum image rejection in combination with an intermediate frequency low enough to permit high selectivity.
Considerable noise is generated in converter circuits. This noise is superimposed on the signal and appears in the output of the receiver. To be received, a signal must have an amplitude greater than the noise generated in the converter stage. An rf amplifier increases the amplitude of the incoming signal before it reaches the converter stage. Since the converter noise remains constant, the additional signal amplification makes it possible to receive signals which would otherwise be lower than the converter noise level.
Some noise is also generated by rf amplifiers, and, when such stages are employed, the absolute sensitivity of the receiver is determined by the noise in the first d stage. Radio-frequency amplifiers, how ever, generate much less noise than converters. The ability of an d stage to improve the sensitivity of a receiver is particularly important at frequencies above 10 MHz. Below 10 MHz, man-made noise is too great to make very high sensitivity useful.
(A) Using pentode tube. (B) Using MOSFET.
Fig. 4-24. Typical rf amplifier circuits.
A typical d amplifier circuit is shown in Fig. 4-24A. It consists of a pentode tube with a tuned grid circuit and an impedance load.
Pentodes are generally used because of their high gain and low interelectrode capacitance. Because of their high gain, d amplifiers must be carefully shielded and decoupled to prevent oscillation.
A typical neutralized d amplifier circuit using an n-channel MOS transistor is shown in Fig. 4-24B. The transistor shown is intended for operation at frequencies up to 60 MHz, and therefore it is highly suitable for use in a-m broadcast receiver circuits. Typically, its forward transconductance does not drop 3 dB until approximately 150 MHz. The stage shown in Fig. 4-24B has a typical power gain of 10 to 18 dB at 60 MHz. Cross modulation typically is less than one percent for interfering signal voltages up to 200 millivolts.
Since transistors do not have internal shielding, external feedback circuits are often used in tuned coupling networks to counteract the effects of the internal transistor feedback and thus provide more gain or more stable performance. If the external feedback circuit cancels the effects of both the resistive and the reactive internal feedback, the amplifier is considered to be uni-lateralized. If the external circuit cancels the effect of only the reactive internal feed back, the amplifier is considered to be neutralized.
Second Detector--The second detector removes the i-f component from the signal, leaving the audio that was impressed on the carrier at the transmitter. The simplest and most common type of detector is the diode detector ( Fig. 4-25). Grid-leak detectors overload too easily for use in superheterodyne receivers. The plate detector is sometimes used, but it is not as popular as the diode detector because it is more difficult to obtain ave voltage from the former.
The diode detector is used in many forms. It has the advantage of a cathode-follower output. Cathode-follower circuits have lower impedance, allowing use of longer lines to feed the amplifier unit.
Cathode-follower circuits also operate with low distortion and high stability.
Fig. 4-26A illustrates the use of an integrated circuit (IC) as an envelope detector for a-m. The internal circuit of the IC is shown in Fig. 4-26B. In this circuit, the emitter of the output transistor ( Q6) is operated at zero voltage by connection of an external resistor in the bias loop of constant-current transistor Q3.
The current in the differential-pair transistors ( Q2 and Q4) is increased to the point at which common-collector output transistor Q6 is biased almost to cutoff. For this current increase, constant current transistor Q3 is operated with terminal 4 open, and emitter resistor R is shunt loaded by the external resistor at terminal 3.
(A) Using vacuum diode. (B) Using solid-state diode.
Fig. 4-25. Diode detectors.
(A) Detector circuit. (B) Internal circuit of IC.
Fig. 4-26. Integrated-circuit a-m second detector.
Although the output transistor is nearly cut off, all the other active devices are operating in their linear regions. For small ac signals, therefore, the circuit provides linear operation except for Q6, which is turned on only by a positive signal. The maximum acceptable input signal depends on the linear range of the differential amplifier. An external filter capacitor is connected between terminal 8 and ground to remove the rf signal from the detected audio output.
The purpose of a detector is to eliminate alternate half-cycles of the waveform and detect the peaks of the remaining half-cycles to produce the output voltage ( Fig. 4-27). Between points A and B in Fig. 4-27, the capacitor at the detector output charges up to the peak value of the rf voltage. Then, as the applied rf voltage falls away from its peak value, the capacitor holds the cathode of the diode at a potential more positive than the voltage applied to the anode. The capacitor thus temporarily cuts off current through the diode. While the diode current is cut off, the capacitor discharges from point B to point C through the diode load resistor.
When the rf voltage on the anode rises high enough to exceed the potential at which the capacitor holds the cathode, current again passes through the diode, and the capacitor charges up to the peak value of the next positive half-cycle ( point D). In this way, the voltage across the capacitor follows the peak value of the applied rf voltage and reproduces the AF modulating signal. The jaggedness of the curve in Fig. 4-27, which represents an rf component in the voltage across the capacitor, is exaggerated in the drawing. In an actual circuit, the rf component of the voltage across the capacitor is small. When the voltage across the capacitor is amplified, the output of the amplifier reproduces the speech or music that originated at the transmitting station.
Another way to describe the action of a diode detector is to consider the circuit as a half-wave rectifier. When the signal on the anode swings positive, the diode conducts, and rectified current is delivered to the capacitor and load resistor. The voltage across the capacitor varies in accordance with the rectified amplitude of the carrier, and thus reproduces the AF signal. The capacitor should be large enough to smooth out rf or i-f variations, but should not be so large as to affect the audio variations. ( Although two diodes can be connected in a circuit similar to a full-wave rectifier to produce full-wave detection, in practice the advantages of this connection generally do not justify the extra circuit cost and complication.) In the circuits shown in Fig. 4-25, it is often desirable to forward bias the diode almost to the point of conduction to improve performance for weak signal levels. It is also desirable that the resistance of the ac load which follows the detector be considerably larger than the diode load resistor to avoid severe distortion of the audio waveform at high modulation levels.
Automatic Volume Control--The function of automatic volume control (ave), also called automatic gain control (age), is to maintain constant output from a receiver when the amplitude of the in coming signal changes. This is accomplished by rectifying part of the received signal, at the output of the i-f amplifier, and developing a voltage across a suitable resistor. The magnitude of the voltage is proportional to the amplitude of the incoming signal. This voltage may be applied as bias to the grids of remote-cutoff tubes in the rf and i-f stages of the receiver to vary the gain of the receiver inversely as to signal strength.
A typical ave circuit is shown in Fig. 4-28. Tube V1 operates as a conventional diode detector. Tube V2 is the ave rectifier. Signal voltage is fed from the detector plate to the plate of V2 through coupling capacitor C6. The rectified signal current produces a voltage across diode load resistor R5 such that the upper end of R5 is negative with respect to ground. This negative voltage is applied to the grid circuits of the rf and i-f amplifiers through a filter and individual decoupling networks. An increase in the amplitude of the incoming signal increases the ave bias and reduces the gain of the receiver to maintain constant output. If the signal amplitude de creases, the ave bias decreases, and the receiver gain is raised.
Resistors R6 and R7 form a voltage divider operating from the receiver B+ supply. The voltage divider places a positive potential on the cathode of the ave diode. This potential delays the development of ave voltage until the signal reaches a predetermined mini mum value. On weak signals, there is no ave bias, and the receiver operates at full gain.
Fig. 4-29. Combined ave, second detector, and audio amplifier.
Some receivers employ the circuit of Fig. 4-29. Here, the second detector, ave rectifier, and first audio amplifier are combined in one tube. The upper diode is the signal detector. The lower diode, which acts as the ave rectifier, is coupled to the detector plate through capacitor C4. A rectified voltage is developed across R2 and applied to the rf and i-f grid circuits through a filter consisting of R1 and C3. The cathode current of the audio amplifier section produces a bias voltage across R4 which delays ave action until the incoming signal is great enough to develop a voltage exceeding the bias.
The filters, R1 and C3 in Fig. 4-29, and R4 and C8 in Fig. 4-28, play an important part in the operation of these circuits. The filters remove audio-frequency variations from the ave voltage. Their time constants must be long enough to remove all audio fluctuations but not so long as to prevent the ave voltage from following rapid changes in input signal amplitude.
A simple method of producing reverse ave for transistor circuits is shown in Fig. 4-30. On each positive half-cycle of the signal voltage, when the diode anode is positive with respect to the cathode, the diode passes current. Because of the diode current through R1, there is a voltage drop across R1, which makes the upper end of the resistor positive with respect to ground. This voltage drop across R1 is applied, through the filter consisting of R2 and C, as reverse bias on the preceding stages. When the signal strength at the antenna increases, therefore, the signal applied to the ave diode increases, the voltage drop across R1 increases, the reverse bias applied to the rf and i-f stages increases, and the gain of the rf and i-f stages is decreased. As a result, the increase in signal strength at the antenna does not produce as much increase in the output of the last i-f amplifier stage as it would without ave.
When the signal strength at the antenna decreases from a previous steady value, the ave circuit acts in the opposite direction, applying less reverse bias and thus permitting the rf and i-f gain to increase.
The filter composed of C and R2 prevents the ave voltage from varying at an audio frequency. This filter is necessary because the voltage drop across R1 varies with the modulation of the carrier being received. If age voltage were taken directly from R1 without filtering, the audio variations in ave voltage would vary the receiver gain so as to reduce the modulation of the carrier. To avoid this effect, the ave voltage is taken from capacitor C. Because of the resistance ( R2) in series with C, the capacitor can charge and discharge at only a comparatively slow rate. The ave voltage therefore cannot vary at frequencies as high as the audio range, but can vary rapidly enough to compensate for most changes in signal strength.
There are two ways in which automatic gain control ( another name for ave, as previously stated) can be applied to a transistor.
In the reverse age method, age action is obtained by decreasing the collector or emitter current of the transistor, and thus its transconductance and gain. The use of forward age provides improved cross-modulation characteristics and better signal-handling capability than reverse age. For forward age operation, however, the transistor used must be specially designed so that its transconductance decreases with increasing emitter current. In such transistors, the current-cutoff characteristics are designed to be more remote than the typical sharp-cutoff characteristics of conventional transistors.
( All transistors can be used with reverse agc, but only specially designed types can be used with forward agc. ) Reverse age is simpler to use, and provides less bandpass shift and tilt with signal-strength variations. The input and output resistances of a transistor increase when reverse age is applied, but the input and output capacitances are not appreciably changed. The change in the loading of tuned circuits is minimal, however, because considerable mismatch already exists, and the additional mismatch caused by age has little effect.
In forward age, however, the input and output resistances of the transistor are reduced when the collector or emitter current is in creased, and thus the tuned circuits are damped. In addition, the input and output capacitances change drastically and alter the resonant frequency of the tuned circuits. In a practical circuit, the bandpass shift and tilt caused by forward age can be compensated to a large extent by the use of passive coupling circuits.
The variable-transconductance characteristic of the operational transconductance amplifier (OTA) integrated circuit is useful in an age amplifier. This circuit has all the generic characteristics of the operational voltage amplifier (OVA). The forward transfer characteristic is best described by transconductance rather than voltage gain. The output of the OTA is a current, the magnitude of which is equal to the product of transconductance and the input voltage.
The output circuit of this amplifier, therefore, may be characterized by an infinite-impedance current generator, rather than the zero impedance voltage generator used to represent the output circuit of an operational voltage amplifier. The low output conductance of the OTA permits the circuit to approach the ideal current generator.
When the OTA is terminated in a suitable resistive load impedance and provisions are included for feedback, its performance is essentially identical in all respects to that of an equivalent operational voltage amplifier. The electrical characteristics of the OT A circuits, however, are functions of the amplifier bias current. In the integrated-circuit OT A, therefore, access is provided to bias the amplifier by means of an externally applied current. As a result, the transconductance, amplifier dissipation, and circuit loading may be externally established and varied.
Internal details of a basic integrated circuit used in ave circuits are shown in Fig. 4-31. An understanding of this circuit is best obtained by analysis of voltages and currents with almost complete disregard for voltage gain and impedance levels.
Transistors Q1 through Q4 perform conventional functions, serving as a current mirror, a constant-current source, and a differential pair. An amplifier bias current is externally developed and applied to the current mirror, Q1 and Q2, to bias the differential pair, Q3 and Q4. The differential output signal currents of Q3 and Q4 are amplified by the beta of the differential pnp transistor pair, Q7 and Q8. Current mirror Q10 and Q11 then transforms the double-ended output of the pnp transistor network, Q5 through Q9, into a single ended output. The entire circuit functions in a class-A mode. The amplifier bias current ( abc) level establishes bias for all transistors in the amplifier.
Ideally, there is no need for a signal ground because the input signal is differential and the output signal is a current. The input and output terminals may operate at most ac and de potentials within the range of the supply voltages. When the OT A operates in the open-loop condition, the transconductance, and thus the amplifier gain, can be varied directly by adjustment of the abc level.
Therefore, an excellent age amplifier is obtained by rectifying and storing the amplifier output and applying this signal to the bias terminal. Fig. 4-32 shows a functional diagram of such a system.
Low-frequency feedback is provided around the gain-controlled stage to balance the amplifier. As the input signal increases, the amplifier bias current decreases and reduces the transconductance and therefore the system gain.
A plate-current signal-strength meter is shown in Fig. 4-33. A milliammeter is connected in the plate lead of several of the rf or i-f tubes which have ave voltage applied to their grids. As the signal increases, the ave voltage becomes more negative, and the plate current through the meter decreases. Resistor R is adjusted so that the milliammeter reads full-scale with no signal ( highest plate cur rent). This point is called "zero signal." Thus the meter indicator moves counterclockwise with increasing signal. In many commercial receivers, the meter is mounted in an inverted position, so that the pointer will move to the right with increasing signal strength.
A bridge-type signal-strength meter is shown in Fig. 4-34. Tube V1 is used to amplify the ave voltage. The current through R1, M, and R3 tends to cause the meter needle to move to the right, while the current through R2, M, and the tube tends to make the needle move to the left. At zero signal, these currents are made equal by adjusting the resistance of R1. The operation of this circuit is based on the fact that a change in grid bias will cause a variation in the de plate current of the tube. As the received signal amplitude increases, the ave voltage becomes more negative. The voltage is applied to the grid of V1, and the de plate current decreases. Thus, the meter needle moves to the right with increasing signal strength.
Automatic Frequency Control
Automatic frequency control ( afc) circuits are used in many superheterodyne receivers to compensate for frequency drift. This drift may be due to such factors as small changes in the oscillator or carrier frequencies. It is compensated for by automatically adjusting the oscillator frequency.
An afc system consists of two basic parts: a frequency detector and a variable reactance circuit. Fig. 4-35 shows a typical circuit.
The discriminator ( frequency detector) is of the Foster-Seeley type and is excited by the i-f signal from the final i-f amplifier stage. The discriminator output is a de voltage the polarity of which depends on whether the intermediate frequency has deviated above or below its correct value, and the magnitude of which is proportional to the amount of deviation. This dc voltage is applied to the control grid of the reactance tube, which is connected across the tank circuit of the local oscillator. A deviation in the intermediate frequency from its proper value causes a change in the de grid voltage of V2, which produces a change in the reactance V2 presents to the local oscillator. This change in reactance is such that the oscillator is automatically adjusted to bring the intermediate frequency back to its correct value.
requency-modulation tuners operate in the very high frequency (vhf) region and consequently have operating requirements different from those of a-m tuners. The rf circuits and components must be of optimum design to provide sufficient sensitivity to pick up fm stations under all conditions, and the audio aspects of all circuitry must be of sufficient quality of design and construction to receive and pass audio frequencies to 15 or 20 kHz.
High-fidelity a-m reception is usually limited by the presence of high-level man-made noise and atmospheric disturbances which a-m receivers cannot reject without loss of fidelity. Also, because of the propagation characteristics of frequencies used for standard a-m broadcasting, out-of-area broadcast station signals can interfere with local reception ( especially at night). At the frequencies used for fm reception, distant stations usually do not interfere, and the inherent noise-rejection characteristics of the fm receiver minimize the noise problem.
In frequency modulation, the frequency of the rf carrier is varied in accordance with the AF or other signal to be transmitted. Amplitude and frequency modulation are compared in Fig. 4-36. Fig. 4-36A shows an unmodulated carrier, Fig. 4-36B shows an amplitude-modulated carrier, and Fig. 4-36C shows a frequency-modulated carrier.
In the a-m carrier, the frequency remains constant and the amplitude is varied during modulation; in the fm carrier, the amplitude remains constant and the frequency is varied during modulation.
Receivers for frequency modulation are of the superheterodyne type and are somewhat similar to ordinary amplitude-modulation superheterodynes. Block diagrams of the two most widely used types of fm receiver and an a-m superheterodyne are shown in Fig. 4-37. All three receivers employ rf amplifiers, mixer stages, oscillators, and i-f amplifiers. The most important differences between a-m and fm receivers is in the detector circuit. A number of fm detectors have been developed. The ratio detector used in the receiver in ...
(A) Unmodulated carrier. (B) Amplitude-modulated carrier. (C) Frequency-modulated carrier.
Fig. 4-36. Comparison of amplitude and frequency modulation.
... Fig. 4-37B removes the audio signal from the carrier and at the same time rejects amplitude impulses which may accompany it. The receiver in Fig. 4-37C employs a discriminator detector to remove the audio signal from the carrier. This detector is sensitive to amplitude impulses, and in order to eliminate them before detection, limiter stages must be provided. The limiter removes all amplitude fluctuations from the carrier before it is applied to the detector.
RF Amplifier--In fm receivers as in a-m receivers, radio-frequency amplifiers are used to secure improved signal-to-noise ratio, higher gain and selectivity, and improved image rejection. Improvement in signal-to-noise ratio is more important to secure in an fm rf amplifier than in an a-m rf amplifier because considerably more noise is generated in converter stages, and, as described, the addition of amplification before the converter increases the signal amplitude without increasing the noise.
When tubes are used in vhf rf amplifiers, they must have high mutual conductance, low interelectrode capacitance, and high input resistance. In the design of low-level solid-state tuned rf amplifiers, careful consideration must be given to the transistor and circuit parameters which control circuit stability, as well as those which maintain adequate power gain. The power gain of an rf transistor must be sufficient to provide a signal that will overcome the noise level of succeeding stages. In addition, if the signals to be amplified are relatively weak, it is important that the transistor and its associated circuit provide a low noise figure at the operating frequency.
In stereo receivers, the noise figure of the rf stage determines the sensitivity of the receiver and is, therefore, one of the most important characteristics of the device used in the rf stage.
Field-effect transistors combine the inherent advantages of solid state devices ( small size, low power consumption, and mechanical ruggedness) with a very high input impedance and a square-law transfer characteristic that is especially desirable for low cross modulation in rf amplifiers. The output of the MOSFET in rf amplifier circuits is low in harmonics, and the MOSFET has a practical dynamic range capability about five times as great as for bipolar transistors. Also, MOSFET's provide less loading of the input signal, less change of input capacitance under overdrive conditions, and other characteristics that make the MOSFET particularly well suited to this application.
(A) Using vacuum tube. (B) Using bipolar transistor.
Fig. 4-38. Typical vhf rf amplifiers.
Components and circuits used in fm rf amplifiers must generate as little noise as possible. Because of the high frequencies at which these circuits operate, short leads, careful shielding, and high quality insulation must be used.
The rf amplifier, mixer, and local oscillator taken together are called the front end of the receiver. Typical circuits used in fm front ends are shown in Figs. 4-38 through 4-41. In each circuit, the antenna transmission line is coupled to the input coil by means of a separate winding. This is required to match the high input impedance of the stage to the comparatively low impedance of the antenna transmission line. Most receivers are designed to match a 300-ohm line. To pass the complete fm signal, the rf amplifier must respond to a wide band of frequencies. This is accomplished by using low-Q coils to broaden the response curve.
Figs. 4-38A and 43B show the application of a highly sensitive electron tube and a vhf bipolar transistor, respectively, to vhf rf stages. The bipolar transistor can provide a sensitivity and signal to-noise ratio sufficient to permit top-quality hi-fi stereo reception, but it presents difficulty in matching and cross-modulation effects.
Mixer-Oscillator--Frequency-modulation receivers generally use separate mixer and oscillator tubes or transistors to achieve greater efficiency, although in some cases these functions are combined in one device specifically designed for this application. The circuits employed are similar to those found in a-m receivers, with modifications to make them more suitable for use at high frequencies. The difficulties encountered in using a combined mixer-oscillator stage stem from interaction between the mixer and oscillator, which be comes troublesome at high frequencies and results in oscillator pulling and instability. These difficulties are largely avoided by using separate components and loose oscillator-mixer coupling.
It is much more difficult to minimize oscillator drift at the frequencies used for fm broadcasting than it is at a-m broadcasting frequencies. Heating, humidity, and B+ supply-voltage variations (regulation) all contribute to oscillator drift. The effects of changing humidity are minimized by coating circuit components with moisture-proofing materials and by permitting a certain amount of temperature rise in the area surrounding critical components. Heating causes drift because it expands parts of critical components, which results in increased capacitance. It is minimized by using insulation materials with low temperature coefficients and by shunting tuned circuits with negative-temperature-coefficient capacitors to counteract the increase in capacitance taking place in other components. The effects of poor regulation in B+ voltage supplies are minimized by careful decoupling of the various circuits in the receiver.
The fm tuner circuit in Fig. 4-39 uses an MOS field-effect transistor in the rf amplifier stage and bipolar transistors in the mixer and local oscillator stages, to achieve an over-all front-end-section gain of 35 dB. This is 15 to 20 dB more than silicon high-frequency transistors usually provide. The tuner operates from a de supply of -15 volts.
The rf amplifier in the tuner is designed to minimize the spurious responses normally found in fm receivers as a result of mixing of the harmonics of unwanted incoming signals with harmonics of the local-oscillator signal to produce difference frequencies within the i-f passband. This objective necessitates some compromise between optimum receiver sensitivity and spurious-response rejection in the selection of the source and load impedances for the rf amplifier.
Achievement of minimum spurious responses requires that the gate input to the rf amplifier be obtained from a tap as far down on the antenna coil ( L1) as gain and noise considerations permit.
This arrangement assures the smallest practical input voltage swing to the gate and, therefore, makes possible optimum use of the available dynamic range of the MOSFET. In addition, the low spurious-response objective requires that the entire rf interstage coil ( L2) be used as the load for the MOS transistor. This coil, selected on the basis of the optimum compromise between gain and band width requirements, provides a load impedance to the rf amplifier of 3800 ohms, which presents a slight mismatch to the 4200-ohm output impedance of the MOS transistor. Although the compromises in the input and output circuits of the rf amplifier result in a slight loading of interstage coil L2 and cause some degradation in the selectivity of the front end, these undesirable effects can be tolerated because the antenna coil is not loaded by the gate of the MOS transistor. The effectiveness of these compromises is demonstrated by the excellent spurious-response rejection ( more than 100 dB) which the circuit can provide.
The MOSFET used in this rf amplifier has a maximum available gain of 24 dB. The compromises in circuit design between optimum receiver sensitivity and spurious-response rejection, however, result in a total mismatch and insertion loss of 11.3 dB. The actual net gain of the rf amplifier, therefore, is 12.7 dB. This stage amplifies the frequency-modulated rf signal coupled from a 300-ohm fm antenna by the antenna coil and applies this amplified signal to the base of the mixer transistor.
The bipolar transistor used in the mixer stage is operated in a common-emitter circuit configuration that provides a conversion power gain of 21.8 dB. Both the frequency-modulated rf input signal and the continuous-wave local-oscillator signal are applied to the base terminal of this transistor. The two signals are heterodyned in the mixer stage to produce the 10.7-MHz difference frequency used as the intermediate frequency in fm receivers.
The bipolar oscillator transistor is operated in a common-collector circuit that generates an extremely clean oscillator waveform.
Fig. 4-40 shows applications of dual-gate protected MOSFET's in an arrangement that provides optimum use of the available dynamic range of the MOSFET in both the rf amplifier and mixer stages. The dual-gate MOSFET is very good for use as a mixer because the signals to be mixed are applied to separate gate terminals.
This reduces oscillator radiation in the antenna circuits.
Fig. 4-41 shows application of integrated circuits to an fm stereo front end. For the optimum performance that can be achieved with such a circuit, the differential mode is used. The gain is about 40 dB overall, and the noise figure is 7.5 dB, which is higher than that achieved by other circuits shown in Figs. 4-38 through 4-40. How ever, where space is at a premium, this circuit will provide excellent performance, at lower cost.
To improve tuning operation of the modem hi-fi receiver, tuning diodes ( varactors, or voltage-variable capacitors) have been introduced to replace variable capacitors and all their related mechanical apparatus. Tuning diodes are pn junction diodes in which the junction capacitance is varied by changing the applied reverse bias voltage. They are used in fm receivers as the variable element that changes the resonant frequency of series and parallel resonant circuits.
Tuning diodes offer the following advantages over mechanical capacitors :
1. Mechanical linkage and switching contacts are eliminated.
2. Channel or station changes can be made in less complicated arrangements by push-button, continuous-tuning, or signal search systems or by sweep methods.
3. Precision automatic fine tuning is simplified.
4. Faster response time is provided.
5. Remote tuning is simplified.
6. Tuning components are many times smaller than the mechanical components they replace.
7. Circuitry can easily be adapted to modular or microcircuit types of packaging.
8. Miniaturization is simplified.
Fig. 4-42 shows a tuned circuit of this type as used in the rf circuit of an Altec Lansing stereo receiver. In the complete front end of this receiver, four tuning diodes (balanced varicap tuning) are all controlled by one external variable control voltage. This control voltage is varied by a potentiometer, but it could be con trolled by voltages provided from automatic, remote, or other tuning subsystems.