Signal Generator circuit

  
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The output level was set to 3. 8V peak to peak, not for any good reason, the first aim was to compare a few different op-amps before trying to further optimise the circuit. The op-amps were TL072, LM4562 and OPA2134. The distortion spectra are shown next, in all cases the 500k feedback level control was set to maximum resistance. Clearly this is
Signal Generator circuit - schematic

not the maximum value allowing oscillation, so some further improvement should be possible. For later tests the 5k6 shown with a dotted connection was added. These were not entirely what I had expected. The TL072 is rather ancient, but even so was not far behind the two `ultra-low distortion` types. Used as an inverting amplifier most op-amps perform reasonably well at 1kHz. All three op-amps produced more second harmonic than third. The most baffling result however is the LM4562, which appears to have the lowest 2nd and 3rd harmonics, but has additional components at very close to 100Hz intervals. The board is powered from 9V rechargeable batteries, so it is not a mains supply frequency problem. I had thought there may be some stability problems with this op-amp, its 55MHz gain-bandwidth specification is worrying, and suggests far more care is needed with layout. I built the circuit on a piece of veroboard, but included the sometimes recommended 100n capacitor between the op-amp supply pins. I can easily believe that layout is the cause of the problem, but why there would be frequency components at almost exactly 100Hz intervals remains a mystery, there was nothing obvious at 100Hz to cause an intermodulation effect. Anyway, not wanting to spend much time trying to solve that problem I decided to stick with the OPA2134. Its more modest 8MHz gain bandwidth is maybe more compatible with my seriously sub-optimum experimental...



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