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(source: Electronics World, Aug. 1964)
By ROBERT H. DOUGLAS
Construction of device that will produce a scope display of frequency response of audio amplifiers, bandpass filters, and audio compensating networks.
above photo: Front-panel view of the author's compact a.f. sweep generator.
HAVE you ever wanted to monitor the frequency response of an audio system while you were working on it? If so, here is an instrument that will indicate the frequency response of audio amplifiers, bandpass filters, compensating networks, or any other audio circuit.
Audio sweep generators produce a varying tone which covers the range from the low end of the audio band to above 20 kc. This changing frequency can be applied to a circuit lender study and the output of that circuit connected to an oscilloscope. If the sweep of the scope is synchronized with the sweeping of the generator, the scope display will be a plot of gain versus frequency for the circuit under study.
Most audio sweep generators sweep the audio spectrum at a constant rate. This type of sweeping distorts the plot of frequency response and does not give the user a standard frequency response curve. If you look at the frequency response curves given for hi-fi equipment, you will notice that the frequency scale is not linear, that is, one inch is not equal to, say, 1 kc. at all points on the scale. Instead a logarithmic scale is used, where the scale is graduated so as to give greater space to the low frequencies than the high ones. The audio sweep generator to be described can plot frequency response on a logarithmic as well as a linear scale. (See scope-trace photos in Figs. 3, 4, and 5.)
In this transistorized audio sweep generator, the audio frequency output is obtained by heterodyning two r.f. signals and utilizing the difference frequency. If one of the r.f. signals is held fixed and the other varied slightly in frequency, the audio difference will also vary. Referring to Fig. 1, the circuit block diagram, we see Q1 connected as a fixed-frequency oscillator operating at about 1500 kc. Q3 operates as a variable-frequency oscillator that sweeps in frequency from 1500 to 1520 kc. These two oscillators feed buffer amplifiers Q2 and Q4, whose outputs are mixed and the difference frequency generated in the detector. This difference signal will then sweep from zero to 20 kc.
The output of the detector is coupled to an audio amplifier, Q5 and Q6, where the r.f. signals are filtered out and the difference signal amplified. The signal used to sweep the variable-frequency oscillator is generated by Q9, a saw--tooth generator. Q7 and Q8 serve to modify the slope of the sawtooth when the generator is in the normal (logarithmic) mode of sweeping.
Looking at the circuit in more detail (Fig. 2), we see Q1, the fixed-frequency oscillator, connected in the grounded-base configuration. The combination of C1, C2, and T1 resonate at approximately 1500 kc. The output of the oscillator is developed across R4 and coupled through C5 to Q2. The output of Q2 is coupled through C7 to the detector.
The variable-frequency oscillator and amplifier, Q3, Q4, T2, and associated circuitry, is identical to the fixed-frequency oscillator except for the addition of Dl. The operation of the oscillator causes a reverse-bias to be applied across this diode. When a diode is reverse-biased, a region forms around the junction, called the depletion region, where no current carriers are available. This depletion region becomes the dielectric of a small capacitor with the p and n regions forming the plates. If the voltage across the diode increases, the depletion region becomes wider; and the capacity across the diode's terminals decreases. Since D1 is connected across T2, its capacity helps to determine the oscillator's frequency. Varying the voltage across D1 will vary the oscillator's frequency.
The outputs from the two buffer amplifiers are mixed via C7 and C14. At this point we have an r.f. signal whose amplitude varies at a frequency equal to the difference between the two oscillator frequencies. D2, D3, C15, and R15 make up a detector which produces an output equal to the instantaneous amplitude of the r.f. input. This difference frequency is coupled through C16 to the output control and then to Q5.
C17, connected between the collector and base of Q5, serves to reduce the gain of the amplifier above 20 kc. This prevents any r.f. signals from reaching the output. Q6, an emitter follower, gives the generator a low output impedance. It will deliver a 1-volt signal into a 600-ohm load.
The sweep generator, Q7, Q8, and Q9, is a rather unusual circuit. The heart of this circuit is Q9, a unijunction transistor. Unijunction transistors are similar in operation to gas-discharge tubes. The emitter-base (B1) circuit is essentially open until the emitter is raised to a critical voltage. At this point the emitter impedance drops to a low value until the emitter current is reduced below a critical point. Then the emitter returns to the open state.
In this circuit, C22 charges through Q8 until the unijunction fires and rapidly discharges the capacitor. While the unijunction is discharging C22, a negative-going spike is developed across R31 and coupled through C24 to synchronize the oscilloscope with the generator sweep. After C22 is discharged, it begins to slowly charge again. This action forms a sawtooth waveform across the capacitor. This saw--tooth appears across R23 and a portion of it is coupled through C20 to D1, causing the variable-frequency oscillator to sweep in frequency.
In the linear mode of sweeping, the sawtooth across C22 has constant slope due to the fact that the current through Q8 is constant. In the normal (logarithmic) mode of sweeping, the collector current of Q8 is varied to produce a saw--tooth with the curved slope.
To follow the circuit action when the generator is in the normal mode of sweeping (S1 is closed), let us consider the circuit immediately after Q9 has fired and the voltage across C22 is at its lowest point. Due to the coupling action of C21 the voltage at the base of Q7 is also very low, hence the collector current of Q7 is very small. This means that the input to Q8 will be very small and its collector current will also be low. Note that the rate of charge of C22 is determined by the collector current of Q8. The capacitor will, therefore, charge very slowly.
However, as the capacitor charges, the input to Q7 increases. This increases the output current of Q7, the input to Q8 increases, and the output current of Q8 increases. The rate of charging of C22 is increased which further increases the input to Q7. This action is regenerative and would rapidly charge C22 to the supply voltage if Q9 did not fire, discharging C22 and allowing the whole cycle to be repeated. Therefore, with Si closed we have a sawtooth across C22 whose slope increases as its amplitude increases.
When this waveform is applied to D1, the generator produces a logarithmic sweep. With S1 open, the currents through Q7 and Q8 are constant and the slope of the sawtooth is constant.
This makes the generator sweep at a constant rate.
Construction and Alignment There are no critical parts in this circuit. All the semiconductors, with one exception, are standard, readily available units. Equivalent types for the units listed in the parts list should work satisfactorily. D1, however, may be difficult to obtain. The 1N950 is a special type made by Hughes for use as a voltage-sensitive capacitor. The author found that low-power zener diodes will work in this circuit. The requirements are that the diode be of the low-power type (250 to 400 mw.) and have a zener voltage of around 30 volts. The two usable types are 1N725 and 1N972. The builder may find that this modification will reduce the amount of sweep but the circuit should still sweep to 20 kc.
It is recommended that the builder adhere to the values of the components as specified, especially in the feedback amplifier. While none of the parts is critical, circuit performance will be affected by major changes in component values.
If the constructor does not need the logarithmic mode of sweeping, this feature can be easily eliminated. Replace the entire feedback amplifier, Q7 and Q8, R24 through R30, S1, and C21 by a 2200-ohm resistor connected between C22 and the junction of C23 and R32. This modification will give the same sweep as when S1 is open. Even with no help from the builder's junk-box, the entire sweep generator can be built for under $50.00. The author assembled the sweep generator in a 4 " x 5 x 2" aluminum chassis which was then mounted inside a chassis box. While this particular size chassis gives a compact unit, it makes a very difficult wiring job for the average builder.
Unless you are particularly adept at miniature wiring, assemble the unit on a larger chassis.
Layout of the circuit is not critical as long as the two oscillators are kept isolated from each other. To accomplish this in the author's model, the variable-frequency oscillator and the amplifier were mounted with the transistors and oscillator coil on top of the chassis and the wiring on the bottom. The fixed-frequency oscillator was mounted "upside down" with the wiring on the top and the transistor coil on the bottom. The prospective builder should make a sketch of the parts layout before the chassis is drilled. This avoids the problem of squeezing too many parts into a small space or, on the other hand, having components hanging from 1.5" leads on both ends. Neither situation is desirable in any electronic circuit.
Top view of the 4" x 5" x 2" aluminum chassis employed.
Once the sweep generator has been built and checked, the unit must be aligned. Set the output control fully up and the upper sweep limit control down. Connect an oscilloscope to the output terminals. Set the scope to observe a signal of about 1 volt. With the plates of C1 halfway meshed, take an alignment tool or insulated screwdriver and turn the slug of either T1 or T2 until a signal is observed on the scope. Adjust the slug until the signal is of maximum amplitude. (This is not a very critical adjustment; all you are doing is getting the difference signal within the bandpass of the audio amplifier.) If nothing is observed by turning one slug, put that slug in the middle of its travel and turn the other.
When the oscillators are approximately adjusted, connect a lead from the sync terminals on the generator to the external synchronization input of your oscilloscope and set the scope to external sync. Momentarily connect the scope input to the generator sync terminals. One should see a train of narrow pulses about 3 volts in amplitude. Adjust the sweep controls of the scope to display one pulse. Without touching the sweep control of the scope, reconnect the scope to the output terminals. By turning the upper sweep limit control and adjusting the zero beat control, one should see a display similar to Fig. 3.
With the generator applied to a hi-fi power amplifier. the trace shown in Fig. 4 was observed with the generator set for linear mode. When switched to the logarithmic mode, the same amplifier produced the curve shown in Fig. 5. The response at 20 kc. is about 1.5 db below the mid-frequency output.
In some applications it may be desirable to know the actual frequency range that the generator is sweeping.
This is easily accomplished with a calibrated audio oscillator and the circuit shown in Fig. 6. To use the circuit, adjust the audio oscillator for an output equal to that of the sweep generator.
With the scope synchronized to the sweeping of the sweep generator, a pulse Or marker will be observed at the point where the sweep generators frequency is equal to that of the audio oscillator.
If, for example, the audio oscillator is set for 5 kc., a marker will be observed at the point on the trace where the sweep generator is passing through 5 kc.
Besides uses in the hi-fi field, several unusual applications for this trait have been suggested. The generator can be used to analyze the rejection of filters and the narrow bandpass of systems used in tone telemetry. In these applications the generator does not necessarily have to sweep from zero to some frequency. By adjusting the controls the unit can be made to sweep from, say, 8 to 12 kc. or from any lower to any higher frequency within the bandpass of the audio amplifier. The generator can also be used to produce a fixed frequency by turning down the upper sweep limit control.
The hi-fi enthusiast soon realizes that one of the weakest links in his system is between the system's speakers and his ears. The acoustic design of the room in which the system is installed greatly affects the over-all frequency response of the system. With the audio sweep generator connected to the system and a good quality microphone connected to an oscilloscope, you can display the response of your entire audio system from preamp input to your ear. It is surprising what a different listening position, speaker placement, or even the location of furniture in the room can do for system response.
When you have built the generator, You will find it a valuable aid in checking all audio-frequency systems. Everyone from the hi-fi enthusiast to the broadcast engineer can undoubtedly find more than one use for this transistorized audio sweep generator.
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