Building a cheap sweep oscillator

Article By : John Dunn

The caveat is that the op-amps need 1K pull-up resistors from their outputs at U1 and U4 to the positive rail to ensure it will always start up.

I needed to make a really low-cost swept frequency tone generator using just some spare op-amp sections and no parts that were sole source. Fortunately, the sweep requirements weren't terribly critical. There was no particular linearity requirement and the frequencies were only low to mid audio range. I just needed to have a minimum frequency ratio from low to high of at least two to one.

I made the thing using TL082 op-amps and all went well. Unfortunately though, this happened so long ago that I lost my notes on what the exact design had turned out to be and thereby learned first-hand the true meaning of personal memory loss.

However, the gist of what I did is seen in the following SPICE simulation.

JD_oscillator (cr)
Figure 1: Cheap sweep oscillator

One small caveat not shown in this simulation is that the TL082 op-amps needed 1K pull-up resistors from their outputs at U1 (the sweep oscillator) and U4 (the swept oscillator) to the positive rail to ensure that this thing would always start up.

It did. The production run was approximately 500 units.

First published by EDN.

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