The RF section of the television receiver, shown in the block diagram of Figure 144 as items Nos. 2, 3 and 4, consists of a radio frequency amplifier stage, a converter or first detector stage, and a local oscillator. This combination of circuit elements performs the same function as its counterpart in a conventional broadcast or short wave superheterodyne receiver.
A number of factors, not encountered in the reception of ordinary broadcast signals, require a more complicated design than is found in standard broadcast sets. Among these are:
1. The broad-band nature of the television channel, which requires the acceptance and amplification of a band of frequencies six megacycles in extent.
2. The frequency allocation of television channels (54mhz to 88mhz and 174mhz to 216mhz), which necessitates special types of coupling circuits to maintain uniform gain at the extremes of the bands.
3. Balanced types of input circuits matched in impedance to the characteristic of available parallel lead or coaxial transmission lines.
4. Rejection of undesired or "spurious" responses, due to radio transmissions outside of the desired television band.
a. The adjacent channel sound carrier.
b. Cross modulation due to other tele vision channels.
c. Direct transmission, through the RF system, of signals at the intermediate frequency.
d. Images due to other television channels and to FM stations.
e. Overloading of the input tube due to strong broadcast stations.
5. Radiation of local oscillator energy by the antenna. Such radiation must be sup pressed to prevent interference with neighboring television receivers.
Television RF input systems can be classified in several different ways, according to the mechanical means of channel selection, or the type of electrical circuit employed.
From the standpoint of mechanical arrangements, tuning systems may be grouped as, (a) continuous tuning systems (see Figures 167 and 169), and (b) step tuning systems: rotary switch selection, push button switch selection, and mechanical detent, or turret construction.
(See Figures 171, 173, 176, 178 and 180.) From the standpoint of listing according to electrical circuit, input systems may be broadly classified as balanced to ground (see Figures 172 and 181), or unbalanced (with single ended circuits), (see Figures 168, 170, 174, 175, 177, and 179). Another method of grouping, from the standpoint of circuit design, is possible:
1. Lumped equivalent of quarter-wave transmission line. (Figure 172.)
2. Circuits with a variable inductance as the tuning or "trimming" element. (Figures 168, 170, 172, 174, 175, and 177.)
3. Circuits with variable capacitance as the tuning or "trimming" element. (Figures 179 and 181.) All of the tuning systems illustrated in this section use the post war button base miniature construction. This has not been done primarily for the conservation of space, but rather is due to the fact that these small tubes are much more efficient in the V. H. F. region than their larger metal or "octal" counter parts. Another observation which can be made with respect to the choice of tubes, is the frequent use of three-element types, or triodes, especially in the RF stage. This choice of triodes is of interest since it serves to emphasize phenomena peculiar to operation in the V. H. F. region, coupled with the necessity for broad band (six megacycles wide) response.
At the lower frequencies, and with the modulation envelope extending not more than 10 khz either side of the carrier frequency, highly selective circuits of high "Q" are universally employed. The necessity for wide band response, in the television band, requires the use of low "Q" circuits, or circuits deliberately reduced in efficiency by means of parallel resistors, in order to "broaden" the response. These circuits are, by virtue of their loading, of low impedance (between 1000 and 10,000 ohms). A pentode is useful as an RF amplifier, in the lower frequency bands, by virtue of the fact that its plate resistance is extremely high and thus high-"Q", high-impedance plate load circuits can be used. In the television band the necessity for wide band response precludes the use of high impedance circuits and, for this reason, a pentode stage will not develop much more voltage gain than a triode. The triode, on the other hand, exhibits several advantages namely:
1. Appreciably lower tube noise level.
This is of extreme importance in the RF stage, where the noise energy, due to electrons in the tube and those circulating in the antenna circuit, may be comparable to the energy of the television signal itself.
2. The inter-electrode capacitances of triodes are lower than comparable pentodes.
3. Triodes are adaptable to grounded grid circuits and provide better stability than pentodes at V. H. F. We have indicated that a number of methods of . classification of input systems are possible. From the standpoint of interest to the radio service technician we have chosen to present our study on the basis of a grouping by the means of tuning or adjustment employed, i.e.: inductively tuned systems, and capacitively tuned systems.
INDUCTIVELY TUNED INPUT SYSTEMS: Two general types of continuously variable inductively tuned systems have appeared: The continuously sliding contactor type (Mallory-Ware "Inductuner"), and the sliding powdered iron core type (exemplified by the Belmont multiple core tuner). (See Figures 167, 168, 169 and 170.) Semi-fixed adjustable cores with a rotary switch for channel selection, are used in a number of arrangements. (Figures 171, 173 and 175.)
CONTINUOUSLY VARIABLE INDUCTIVELY TUNED INPUT SYSTEMS. Television channel tuning by variable inductance has been accomplished by two distinct methods, namely: inductance variation by sliding electrical contactor and by sliding iron cores.
Tuning by Continuously Variable Contactor type Inductor. Figures 167A and B illustrate a tuner which is unique in that the inductance of the circuit is made continuously variable. Such a method of tuning is particularly adapted to the V. H. F. range because it makes possible a high ratio of inductance to capacitance and consequently higher circuit impedance at the high frequency end of the range. In this tuner, a sliding contactor made of high silver content alloy, possessing spring properties, rides in "trolley fashion" on an inductor made of "fine" silver wire. End rings on the inductor permit circuit connection to both ends of the inductance. The sliding contactor "shorts" the unused portion of the inductance. The "Inductuner" is a three-gang inductor equivalent to a three-gang condenser, and in the input system shown in Figures 167C and 168, the three units are used as a coupled band pass selector stage and a local oscillator.
Fig. 167. A and B. A Continuously Variable Inductance Tuner for Television
Reception, employing the "Inductuner". Photo from Sample Courtesy
Allen B. Du Mont Laboratory, Inc.
Coupling elements C5, C7 and the shunt circuit L6-C6, are so proportioned that the acceptance band remains constant at six megacycles width over the entire tuning range of the system.
The tuning range extends continuously from 44 to 216 megacycles and includes not only the television channels but also FM stations, two amateur bands, aviation channels, V. H. F. radio telephone and commercial services. A unique dial arrangement shown in Figure 167C makes it possible to use the television receiver for the reception of the high band FM as well as the assigned television channels.
Adjustable end inductors L5, L7 and L9 of Figure 168 perform the same function circuit-wise as the high frequency trimmers of the variable gang condenser. The oscillator circuit shunt inductor (L8) is equivalent to the series "padder" of the conventional superheterodyne oscillator circuit. Since inductors L5 and L7, by structure, possess a higher "Q" than the "induc-tuner" itself, performance of the system is improved as the high frequency end is approached. A continuously variable, capacitively tuned system, on the other hand, would suffer in "Q" at the high frequency end of the tuning range.
The circuit of Figure 168 introduces to the television service technician a method of vacuum tube operation, which was introduced during the second World War, known as ''grounded grid''. Tube T1 is connected so that the signal voltage is introduced between cathode and grid, with the grid connected to ground. Such a connection is of advantage for two reasons: the grid acts as a shield between the input and the output of the tube, to suppress feed-back, or a tendency toward oscillation; injection of the input voltage in the cathode circuit allows a broad band "match" to low impedance parallel lead or coaxial transmission lines. Inductor L 1performs a dual function. It acts as a low pass filter to short circuit voltages which might be caused by nearby broadcast stations in the 550khz to 1600khz band, and it is broadly resonant to the television band since it is tuned by the cathode-to-ground capacitance of the tube and the associated circuit wiring. This tuning prevents the cathode -to-ground capacitance from decreasing the input impedance of the circuit.
The television service technician will recognize other features of the schematic diagram of Figure 168 as similar to the input systems of lower frequency entertainment receivers. The injection of local oscillator voltage by capacitance coupling between the oscillator and the control grid of the converter tube is a conventional method in the V. H. F. television region.
Fig. 168. Schematic of Inductively Tuned Input System Illustrated in Figure
167.
Continuously Variable Tuning by Powdered Iron Cores. Figure 169 shows a tuning system in which a special type of high frequency powdered iron core is used to change the value of the tuned circuit inductances. This permits selection of television channels on both the low and high frequency bands. In this case, a separate set of inductors and sliding cores are used for each band and electrical switching accomplishes transfer from one band to the other. Figure 170 shows the electrical circuit employed for this input system. Since tuning is continuous, both in this circuit and that of Figure 167, it is not necessary to provide separate fine tuning control. The tuning adjustment is set for optimum performance of the sound channel and this automatically assures a proper setting for the video or picture carrier.
Fig. 169. A Variable Inductance Tuner Employing Movable Powdered Iron Cores.
Photo from Television Receiver Courtesy Belmont Radio Corp.
The antenna primary coils , L1 and L3, are balanced to ground, with a center tap connected to the chassis-ground. This permits the use of balanced, parallel-lead transmission line with single ended tube circuits. Both the RF and converter stages employ the miniature type 6AK5 in pentode connection. Broad band response is obtained by loading the tuned circuits with parallel resistors of low value. In the high frequency band the use of parallel resistors is not required, due to higher circuit losses.
Artificial Line Type of input Tuner.
Figure 171 shows a type of input system which has been employed in many receiver designs.
The 616 twin triode is used and push-pull operation is employed in each stage. The artificial equivalent of a quarter-wave transmission line, made up of a series of inductors with their associated distributed capacitances, is employed in the RF, converter, and oscillator circuits to perform the functions usually associated with "lumped" tuned circuits.
These lines are balanced with respect to the chassis or ground. Tuning of the various television channels is accomplished by switching a "short circuit" progressively along the "line".
Fig. 170. Schematic of the Iron Core Tuning System Illustrated in Figure 169.
An understanding of the operation of this type of "front end" is essential to the television service technician both because of its ...
Fig. 171. Input Tuning System Employing "Lumped Equivalent" of Quarter-Wave
Trans mission Lines. Photo from Sample Courtesy R. C. A.
Fig. 172. Schematic Diagram of Tuning System Illustrated in Figure 171.
...unusual nature and its frequent use in television receiver design.
The input circuit of tube T1 (see Figure 172), fulfills three distinct functions:
1. Chokes L27 and L28 (center tapped to ground) provide a low frequency bypass for signal frequencies lower than the television band. This prevents cross-modulation effects in the input tube T1. Important sources of such low frequency interference may be:
a. 60 cycle pickup from power lines near the television antenna.
b. Local stations, in the broadcast band.
c. Local high frequency stations such as police, commercial transmitters, etc.
2. Balanced, series-tuned circuits, L29 C9, and L30-C16, can be adjusted to reject "images" due to local FM stations in the 88mhz to 108mhz band. Such images would fall on several of the television channels (depending on the IF frequency). Adjustment of inductors L29 and L30 permit reduction of such an image to such a point that it will not affect the picture.
Another use for these trap circuits is the elimination of the interference bet wee n two television stations in certain areas. In this case the image of a station in the low band group, i. e.: channels 2 through 6, may fall on a station in the high band group, channels 7 through 10. The trap circuits may be adjusted to minimize such interference.
3. Resistors R3 and R4 perform the dual functions of matching the 300 ohm transmission line to the tube input, and providing a grid return path for automatic gain control, which will be described later.
The RF amplifier tube T1 (6J6) is push pull connected and "cross neutralized" by means of capacitors C4 and C5. This neutralization increases the stability of the stage by preventing regeneration or oscillation and also reduces the transmission of oscillator energy back through amplifier tube T1 to the antenna.
The plate load of T1 consists of the equivalent of a quarter-wave transmission line, and is made up of inductors L1 through L26.
Rotary switch section SW1 "shorts out" sections of the line to tune it to any one of the twelve television channels.
Capacitors C2, C3 and inductive link L36 act as coupling elements between the "line" in the RF plate circuit, and the corresponding "line" comprised of inductors L31 through L57 in the converter grid circuit. The value of these elements is such as to keep the channel width constant over both television bands.
The series -tuned circuit, consisting of L61-C17, is series-resonant at the IF frequency and prevents the direct transmission of interfering signals (short wave broadcast) through the IF amplifier.
The oscillator circuit consists of a transmission line similar to those just de scribed, but with the additional feature that each channel inductance is separately adjust able (see inductors L67, L75 and L76 through L88). The oscillator tube (a push-pull connected 616) derives its plate-to-grid feedback, for sustained oscillation, from a crossed pair of capacitors (C25 and C27), whose values are greater than the grid-to-plate capacitance of the 616.
Injection of oscillator voltage into the converter grid circuit is accomplished by magnetic coupling bet we en the tuned transmission lines, augmented by the coupling link (L74).
Fig. 173. Another Example of a Quarter-Wave Line Type Input Tuning System.
Photo from sample
Fig. 174. Schematic Diagram of Tuning System Shown in Figure 173.
Another input tuning system which employs the quarter-wave resonant line principle is illustrated in Figure 173 and the schematic wiring diagram is shown in Figure 174. In this case, only two lines are employed. Inductors L2 through L25 are used as coupling means between the RF tube, T1, and the converter tube , T2. Inductors L26 through L49 constitute the resonant line for oscillator tuning.
Fig. 175. Input Tuning System Employing Rotary-Switch Selected Inductors.
Photo from Sample Courtesy Sarkes Tarzian.
Fig. 176. Schematic Diagram of Input Tuning System Illustrated in Figure 175.
The antenna input system is balanced to ground. Inductor L 1 serves as a low frequency shunt trap and, in conjunction with the circuit capacitance, is broadly resonant to the television bands.
Adjustment of the resonant-lines is accomplished by variation of the inductors associated with channels 6 and 13. Fine tuning is provided by the dual section vernier capacitor C10 and C11.
Input Systems with Switch-Selected variable Inductors. Figures 175 and 176 show an input system in which a rotary switch is employed to select the proper inductances for tuning to the desired television channel. Several features not previously discussed are evident in this circuit. The input circuit and method of connection of tube T1 constitutes a means of coupling between a balanced-to-ground transmission line, and a single ended tube circuit. In this case both the grid and the cathode act as input elements and are connected to opposite ends of the center-tapped inductor L3. On the high frequency band (channels 7 through 13) inductors L1 and L2, in series, are connected across L3 to resonate broadly at the higher frequencies.
The inter-stage coupling system consists of two tuned circuits with coupling adjusted to obtain proper band width for all channels. The plate and grid circuit inductances (L4 through L 15 and L16 through L27) are selected by the rotary switch. The tuning capacitances of these circuits are made up of the inherent capacitance of the tubes and the capacity of the wiring to ground. Both the circuit shunt loading (R6) and the coupling (C5, C7, C8, and C9) are switch-controlled to help provide uniform (6mhz) band width for all channels.
The oscillator circuit switching involves the selection of individually adjusted inductors (L30 through L40). Oscillator tuning capacitor (C13) is made variable for fine tuning.
Another circuit employing switch-selected inductors is shown in Figure 177. In this case the first tube is a triode-connected 6AU6 used as a grounded-grid amplifier. Since the input circuit is heavily loaded to provide matching for the 300 ohm transmission line, its resonance curve is very broad. Only 5 coils (L2 through L6) are required to cover both high and low TV bands.
Fig. 177. Schematic of Input Tuning System Employing Switch Selected Inductors.
(Broad Tuned, Cathode-Input System.) Courtesy General Electric Co.
Interstage coupling is provided by means of a series of wide band transformers (L7 through L33), which are individually switch selected for television channel. The windings are self resonant., being tune d by their own distributed capacitance, together with the tube capacitances. On the first two channels, image rejection trap circuits (L7-C5) and (L10-C6) are pre-tuned to reject FM station images which might occur on these channels.
The oscillator system (a variation of the Colpitts circuit) selects individually adjusted coils (L34 through L46) for each channel.
Oscillator trimmer capacitance C 12 is used for fine tuning.
Fig. 178. A Turret Type of Input Tuning Sys tem. Photo from Sample Receiver
Courtesy Philco Corp.
Fig. 179. Schematic of Input Tuning System Illustrated in Figure 178.
Turret Tuners. Figures 178 and 179 show a tuning system in which the input, converter and oscillator tuned circuits are mounted on a rotating turret. Only the circuit elements associated with a single selected television channel are connected in the circuit at any one time. The turret is octagonal in shape and provides tuning for eight channels.
The pair of coil assemblies (see Figure 178) for any channel are easily removed, and another pair for a different channel "snapped" in place. Since no more than seven channels are assigned to any given locality, this arrangement should accommodate the allocations of any specific area.
A set of spring backed contacts, associated with the various tube circuits, provide a means of connection to the terminals of the tuned circuits. As the turret is rotated by means of the channel selector knob, studs which terminate each set of coils are positively indexed into contacting position by a wiping action which assures low circuit resistance.
Separate input terminals are provided for the low and high frequency television bands, thus, individual antennas may be employed for high and low band reception. Since the coil design can be altered to accommodate the requirements of each channel, uniform band width is assured. (Figure 179 illustrates variations in wiring of the antenna and RF assemblies for individual channels.) An unusual feature of this circuit is the omission of the fine tuning control, made possible by automatic frequency control of the local oscillator. The first, or right half, of tube T3 (see Figure 179) functions as an ultra audion (or modified Colpitts) oscillator, while the second, or left half, acts as an automatic frequency control tube to automatically adjust the oscillator to correct frequency. The action of the circuit associated with the second section of T3 is similar to that of the reactance control tube described under AFC horizontal sync control. In this case, the DC control voltage is provided by the FM discriminator, or detector , at the end of the sound channel. The deviation of the oscillator frequency from the center of the correct sound IF frequency will result in the production of a positive or negative voltage on the grid (terminal 5 of tube T3). The plate-to-cathode capacitance of tube T3 (terminals 2 to 7), which is in shunt with the oscillator tuned circuit, can be made to vary in effective value by the change of the DC potential on the grid of the reactance tube, as determined by DC output of the sound channel discriminator.
An RF voltage is fed from the tuned circuit of the oscillator to the grid of the reactance tube through a series network consisting of capacitor Cl8 and resistor R9. Capacitor C 19, which parallels this series network, is employed to neutralize the grid-to-plate capacitance of the reactance tube. Due to the choice of values of capacitor C 18 and resistor R9, a phase shift of the oscillator voltage occurs, and causes the reactance tube to draw a plate current that is out of phase with the voltage appearing across the oscillator coil L5. The current drawn by the reactance tube is leading the voltage, and therefore the tube acts as a capacitor whose value at any time is determined by the magnitude of the plate current. In this manner, the grid bias of the reactance tube, which determines the plate current, can vary the effective capacitance of the ·tube, and thus the frequency of the local oscillator. Since the grid bias at any instant is determined by the frequency of the audio IF carrier, any drift of the oscillator which would produce a change of the IF carrier is automatically com pens ate d. This system, when properly adjusted, reduces the tuning operation to that of merely turning the channel selecting turret to the proper channel.
Fig. 180. An Input Tuning System Employing Push Button Selected Trimmer Tuned
Circuits.
Photo from Sample Courtesy The Hallicrafters Co.
CAPACITIVELY TUNED INPUT SYSTEMS:
Television channel selection by variable capacitance tuning has not appeared as frequently in television designs as have the inductively tuned types previously discussed.
In this type of tuning, it is possible to employ a switch-selected group of circuits which are adjusted by individual trimmers, or to employ a variable capacitor with fixed coils
Both methods have appeared commercially in television sets now on the market.
INPUT SYSTEMS WITH SWITCH-SELECTED "TRIMMER" TUNED CIRCUITS. Figures 180 and 181 show a capacitance-tuned input system in which individually adjusted trimming capacitors are connected into the circuit by a push button switch. A separate bank of 13 trimmers (one for each channel) is used for RF amplifier converter and oscillator tuning.
The input circuit, which matches 300 ohm parallel-lead transmission line, is similar to that described in connection with Figure 176.
The RF amplifier tube ( T1) has an input circuit balanced to ground, with half of the voltage injected from grid-to-ground and the other half from cathode-to-ground.
The coupling network between the plate of the RF tube and the grid of the converter consists of two tuned circuits with "high side" capacitance coupling. The value of the coup ling is varied individually for each of the low frequency channels (capacitors C24, C25, C26, C27, C28, and C29). When a channel of the high frequency group (channel 7 through 13) is selected, sufficient coupling is provided by inherent capacitance between the parts of the circuit (switch, wiring and trimmer banks). In this manner the bandwidth is kept sufficiently broad for all channels.
It will be noted from the schematic diagram of Figure 181, that the tuned circuits employ the trimmer capacitor in series with the tuning coil and the capacitance of the tube.
This allows the use of a reasonably large trimming capacitance and still makes it possible to maintain a high L to C ratio.
Fig. 181. Schematic of Input Tuning System Shown in Figure 180.
Fig. 182. An Input Tuning System Employing a Variable Gang Condenser with
Mechanical Detent for Channel Selection. Photo from Sample Courtesy General
Instrument & Appliance Corp.
INPUT SYSTEM EMPLOYING VARIABLE CONDENSER TUNING WITH MECHANICAL DETENT FOR CHANNEL SELECTION. Figure 182A and B illustrates the use of a special design of variable gang condenser for television tuning. The schematic diagram of the circuit employing this tuner is shown in Figure 183.
The gang condenser itself is of special construction and employs separate sections for the tuning of the low and high frequency bands.
The oscillator section (at the front of the unit) uses very thick soft copper plates to preclude the possibility of "microphonics". An ingenious mechanical arrangement provides a positive detent at each of the positions corresponding to a television channel. On the back s id e of the detent cam, there is a recessed groove in which a follower stud tracks continuously through 360 degrees of rotation.
However, between channels 6 and 7 there is a step in the groove which actuates a lever arrangement controlled by the follower stud.
This lever arrangement is attached to the shaft of a gang change-over switch , whose single pole, double throw sections are shown as S1, S2, S3, S4, S5, and S6 of Figure 183.
Referring to Figure 183, it will be noted that a separate set of coils, gang condenser sections, and trimmers, are employed for low band tuning as contrasted with high band tuning.
The mechanical arrangement just described, automatically switches from one group of components to the other group, as the channel selector knob is turned from the lower to the higher television bands.
Fig. 183. Schematic Diagram of Input Tuning System Shown in Figure 182.
The following circuit features should be noted:
1. All tubes are dual triode type 6J6, and are operated in push-pull connection.
2. The R-F stage is cross-neutralized for stability and reduction of oscillator radiation. The input system is similar to that employed in the quarter-wave line type of tuner illustrated in Figures 171 and 172. This system provides matching for a balanced parallel wire, 300 ohm, transmission line, and also provides a trap for reduction of F-M interference or an image due to another television channel signal. The use of separate coils and condensers for the two television spectrum locations has made possible optimum circuit design from the standpoint of L/C ratio and proper loading of circuits to provide the desired bandwidth.
The push-pull operation of the converter tube ( T2) allows rejection of spurious responses due to cancellation of even-order harmonics.