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From: antispam@fricas.org (Waldek Hebisch)
Newsgroups: sci.electronics.design
Subject: Re: High purity 1kHz oscillator
Date: Fri, 22 Nov 2024 00:56:35 -0000 (UTC)
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john larkin <jl@glen--canyon.com> 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.
>>>>
>>>> Otherwise it should be easy to get 60dB down on unwanted harmonics with a better filter.
>>>>
>>>> FWIW I likely won't be here for the next week.
>>>
>>> <snip>
>>>
>>> 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.
>>>
>>> Your revised circuit persists with this mistake, and the filter you've added around U1 doesn't do enough to compensate.
>>
>>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?

I was thinking about using passive filter with zero at the fundamental
frequency of the oscillator.  Fundamental would go down, distortion
would be changed, but not too much and in predictable way.  Then
one could measure distortion of signal that got trough the filter.
It would be weak signal but since it would have much higher distortion
it would be easier to measure.  I assume that passive filter is
sufficiently linear and that frequency is stable enough.

-- 
                              Waldek Hebisch