Audio Function Generator Provides Three Simultaneous Square, Triangle, Sine Waveforms

This simple, robust, and low-cost signal generator, based on the LM386 power amplifier IC, provides a trio of audio-band signals with three different simultaneous outputs at the same frequency: square/rectangle (SQW), triangle (TRG), and sine (SS).


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Each output can drive loads such as long cables, transformers, auto-transformers, audio couplers, or active loudspeakers. The amplitude of each output can be adjusted independently. The outputs can be connected to ground, the power supply, or between them for short time without damage.

The generator is useful for checking electrical or audio installations, audio or mains transformers, and similar components. The suggested maximum load for each output is 8 Ω, but lower-impedance loads also can be handled with some reduction in output. As a further convenience, the user can switch the output triplet between two frequencies.

The simplified block circuit shows IC1 operating as a square-wave generator with frequency determined mainly by R1 and C1 (Fig. 1). The signal in point A is approximately triangular with amplitude of about 0.35 V p-p and a supply rail of +9 V.

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The triangular signal goes to amplifier IC2 via potentiometer P1. It is also routed to the low-pass filter (LPF) or a band-pass filter (BPF). The quality of that filter determines the quality of the sinusoidal signal, which IC3 amplifies.

The 3-dB frequency of the LPF should be equal to or lower than the frequency of the triangular signal at point A. With a BPF, the central frequency or the resonant frequency should be approximately equal to that of the signal at the same point. In the simplest case, these can be second-order low-pass RC, RC band-pass, LC band-pass, or LC low-pass filters. IC3 amplifies the sinusoidal signal at point B, after potentiometer P2.

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