FFT Rounding error?

There is a nice version of the Wein bridge oscillator that often gets used for this. There are two op-amps in the circuit and both of them are inverting. This removes the effects of common mode changes. The gain control is done either with a light controlled resistor or a couple of thermistors. It takes quite a while for the amplitude to settle but the noise and distortion values come out very good.

"When the only tool you have is a hammer, everything looks like a nail".

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I have a HP 8903B, which is a decent source. I'm not sure you could roll something together that is better.

I remember building the Wein bridge oscillator in school. We used a jfet for the gain control.

When the only tool you have is a claw hammer, everything looks like a bent nail. ;-)

(snip)

Yes, but I thought there was another one, but I suppose that works, too.

-- glen

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The JFET leads to a lot of even order harmonics. HP used a light bulb in their early designs. Some people have used things like car head lamps to get massive enough devices.

The Wein bridge design is not very accurate as far as frequency but it can be very low noise and distortion.

I did not expect it to, but held my peace. The tail is in part created by two things, the end around discontinuity, and the fact that your sampling frequency is insufficient to get a worthwhile spectrum of the switcher. Please note that the expected (mostly odd) harmonics are missing. Try about 10 (or 100) times the sample rate for about one 50th as long a time.

Then do the DFT twice and plot 50 us (5 cycles of the switcher) and see what you get. I promise that the 10 MHz sample rate will produce a much more believable waveform.