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From: "Edward Rawde" <invalid@invalid.invalid>
Newsgroups: sci.electronics.design
Subject: Re: High purity 1kHz oscillator
Date: Tue, 22 Oct 2024 14:21:08 -0400
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References: <vf7c4h$1m5m$1@nnrp.usenet.blueworldhosting.com> <vf7slm$1e357$1@dont-email.me> <vf8bs8$27n8$1@nnrp.usenet.blueworldhosting.com> <91qfhj1tbi1ouh0cihrsjghk1q0t9ke345@4ax.com>
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"john larkin" <jl@glen--canyon.com> wrote in message news:91qfhj1tbi1ouh0cihrsjghk1q0t9ke345@4ax.com...
> 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'd probably ask the designers of the circuit Bill Sloman referenced, how they measured -110dB

https://www.analog.com/media/en/technical-documentation/application-notes/AN132f.pdf

I'd be happy with 70dB but I doubt that's achievable with the FET circuit.

>
>