Re: High purity 1kHz oscillator

On 23/10/2024 5:02 am, john larkin wrote:

On Tue, 22 Oct 2024 10:12:23 -0400, "Edward Rawde"
<invalid@invalid.invalid> wrote:
 

"Bill Sloman" <bill.sloman@ieee.org> wrote in message news:vf7slm$1e357$1@dont-email.me...

On 22/10/2024 4:10 pm, Edward Rawde wrote:

But I suspect that component tolerances and mismatched FETs will ruin it.
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Otherwise it should be easy to get 60dB down on unwanted harmonics with a better filter.
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FWIW I likely won't be here for the next week.

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<snip>
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My message was that the current sucked out of U2 through D1 and D2 was a narrow spike, peaking at 0.3mA and repeating at 1kHz,
which distorted the voltage at the output of U2.
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Your revised circuit persists with this mistake, and the filter you've added around U1 doesn't do enough to compensate.

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That's what I thought you'd say, because there are now two spikes, but it does seem to reduce distortion.
So I'd leave it in any experimental prototype and take the decision to remove it if real testing shows it's not sufficiently
beneficial.
The filter can be redesigned when a real circuit is tested. I didn't have time to do a more elaborate active filter.

 If you do build an ultra-low-distortion oscillator, how would you
measure the distortion?

Put a spectrum analyser on the output and measure the harmonic content.
The number cruncher's version of that is to digitise the sine wave with the best A/D converter you can get, fit a sine wave to the data and average the deviations from that closely fitted sine wave.
Random noise will go down as you average over more cycles. The distortion won't.
--
Bill Sloman, Sydney