Audio signal generator (ETI/TEST GEAR, 1977)



Fig. 1 --- Similar connection for both 8C108 and BC178; Unless otherwise indicated all resistance is in ohms and capacitance in micro-farads.

THIS ARTICLE gives all necessary data for constructing a reliable, stable audio signal generator which covers frequencies from 15Hz to 150kHz in four switch-selected ranges.

Both sine and square wave outputs are provided. Sine wave amplitude is one volt rms, square wave amplitude is one volt peak, adjustable by both fine and coarse attenuators in the emitter follower output stage.

The generator uses a total of seven silicon transistors, six of which are npn types and one a pnp type. The Wien bridge oscillator ( 01, 02, 03) is a slightly modified version of the well-known Mullard circuit which uses fixed capacitive elements and variable resistance elements in the bridge and includes a thermistor to ensure constant amplitude output. The modifications to the original Mullard Wien bridge circuit have been made to accommodate transistors readily available in Australia.

The full circuit is shown in Fig. 1.

Frequency is varied by the ganged potentiometers RV 1a and RV 1b; these form the resistive elements of the bridge. Constant amplitude output is ensured by the thermistor TH1 in the feedback loop to the emitter circuit of 01.


Our completed unit Front panel wiring details.

Fig. 5. Calibration scale (full size)

-------------- Simply constructed audio signal generator provides adequate performance for home and shop use.


Audio Signal Generator PARTS LIST R1 - Resistor 10k A watt 5% R2 - R3 R4 " 6.8k R5 " 1.2k R6 „ 100íZ R7 „ 6.8k R8 „ 30 R9 „ 6801 R10 - " 100e R11 - " 47k R12 - " 2.7k R13 - „ 1.5k R14 - " 1.2k R15 " 2.7k R16 - " 5.6k R17 - 100k R18 - - 820,Ç2 R19 - 30 R20 " 680S2 R21 „ 4.7k R22 " 68e2 R23 " 5.6k R24 " 680S- 2 " II R25 " 150£. 2 1 watt 10% RV1 ( a/b) - Dual gang potentiometer 25k linear RV2 - Potentiometer preset 10k linear RV3 RV4 - - Switch potentiometer 5k linear C1 - Capacitor - metallized 0.47 pf, 15 volt C2 - " 0.47 pf, " C3 0.047/if, C4 0.047pf, C50. 0047pf , C60. 0047pf , C1l - Capacitor - metallized 470 pf, 15 volt C8 470pf, (silver mica capacitors may be used for C7 and C8). CO - Capacitor - electrolytic 1000 pf, 16 volt

C10 - " _ „ 500 pf , C1 1 220 uf, C12 500 uf, C13 - - " 500 /if, (Note C10-C13 are single ended type capacitors)

C14 - " - " 1000 pf, D1 - diode - 0A91 (or equivalent)

02-5 diodes EM401 ( or equivalent)

213-1 - zener diode BZY96, 12v., 1 watt (or similar)

01 - Transistor BC108

02 BC178

03-07 - Transistors BC108 TH1 - Thermistor type STC type R53 Ti - Transformer 240 volt to 12 volt 150 ma ( Ferguson type PF 2851 or similar)

SVV1 ( a/b) - Switch rotary, two pole, four way SW2 ( a/b) - Switch rotary, two pole, two way SW3 - Switch rotary, single pole, three way SW4 - (rear of switch pot. R V4)

PC ET- 006 - Printed circuit board Case - ATC type plastic case

Various Output terminals, connecting block, mains cable and plug, control knobs.


Errata: Capacitors C1,C2,C3,C4,C5,C6,C7 and C8 shoLld be 100 volts not 15 volts as shown above.

Fig. 4. General layout of components, in our final version we found it was desirable to us screened signal reads between the circuit board and the panel mounted switches.

Fig. 2. Foil pattern - full size

The npn/pnp pair Q1 and 02 form a high gain amplifier which is coupled to the npn emitter follower 003 . For sine-wave output the signals are taken via SW2a and SW2b, and the fine attenuator control RV4, to the base of emitter follower 07. The output of 07 then goes via the switched attenuation circuit ( SW3, R21, R22, R23, R24) which can be used to adjust the sine-wave output to a maximum of 1 volt, 100 mV, or 10 mV rms.

For square-wave signals, the sine-wave output from 03 is taken to 04 and 05, which together form a squaring amplifier. The pre-set potentiometer RV2 is used to set the mark-space ratio to 1:1. The squared signal is then taken from the emitter follower 06, through pre-set attenuator RV3, and , then to the output transistor 07 The preset potentiometer RV3 is adjusted to produce a maximum square-wave output of 2 volts peak- to- peak. The coarse output attenuator will reduce this to either 200mV or 20mV peak-to-peak. Each of three output levels ( both sine-wave and square-wave) is then stepless variable from zero to maximum by means of RV4.

A circuit for a 12 volt power supply has been included in Fig. 1. However, if the generator is used infrequently or for short periods, it can be operated from a 12 volt battery. Current drain is less than 30mA. Our prototype unit was built on a printed circuit board the foil pattern of which is reproduced in Fig. 2. The layout of components is shown in Fig. 3; compare this with Fig. 4, which is a photograph of the completed unit. If desired, the unit can be assembled on Veroboard. The layout of components is not too critical, although we have found that it is necessary to use screened leads from the oscillator board to the output level switch, and from the output level switch to the output terminals.

It is essential that, metallized capacitors be used for C1, C2, C3, C4, C5 and C6. Silver mica capacitors may be used for C7 and C8 ( 470pF). DO NOT use ceramic capacitors.

The frequency range scale illustrated in Fig. 5 will prove accurate enough for most audio work. If greater accuracy is required, the unit must be checked against another oscillator of known accuracy.

The output waveform should, if possible, be checked with an oscilloscope to verify that a good sine-wave is being obtained and that the square-wave is uniform. The mark-space ratio of the square-wave output should be set to 1:1 by the pre-set potentiometer RV2 and the peak-to- peak output level by RV3. If no oscilloscope is available, RV2 and RV3 should be set to mid-range.

If an audio frequency voltmeter is available, the sine-wave output should be checked from 15Hz to about 100kHz to ensure that reasonably constant output is maintained.

The output signal should remain constant within ± -1dB up to 100kHz or so, but there may be changes in level whilst changing frequency. The envelope stability ( or the time take for the amplitude to stabilize after a change in frequency) is mainly a function of the quality of the dual ganged potentiometer.

Performance of the prototype unit is detailed above.


Frequency range 1: Frequency range 2: Frequency range 3: Frequency range 4: (range 4 will in fact extend Output variation: Distortion: Output impedance: Sine-wave output: Square-wave output: Square-wave mark/space ratio: Square-wave rise time: 15Hz - 150Hz-150Hz - 1,500Hz 1,500 Hz - 15,000Hz 15,000Hz - 150kHz beyond 150kHz)

Less than ± 1c18 from 15Hz - 150kHz.

<1% 600 ohms.

0-1 volt ( rms)

0-100mV ( rms)

0-10mV ( rms)

0-2 volts ( peak-to- peak)

0-200mV (peak-to- peak)

0-20mV ( peak-to- peak)

nominally 1:1 less than 1 µsec.



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