Re: High purity 1kHz oscillator

On 25/10/2024 3:25 pm, Bill Sloman wrote:

On 25/10/2024 4:48 am, JM wrote:

On Thu, 24 Oct 2024 16:16:49 +0100, JM <sunaecoNoSpam@gmail.com> wrote:
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On Wed, 23 Oct 2024 02:05:52 +0100, JM <sunaecoNoSpam@gmail.com> wrote:
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On Tue, 22 Oct 2024 01:10:41 -0400, "Edward Rawde" <invalid@invalid.invalid> wrote:
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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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If you actually need a low distortion oscillator look for Viktor Mickevic's designs on diyaudio.   I've attached a schematic.
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PK


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It seems that I have a problem posting encoded binariesto this group with my provider.  Attached is a link instead.
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https://1drv.ms/u/c/1af24d72a509cd48/EakMPPRi-pdLgaAKtJ2rrwwBNMGZZsy84MV2QoH1dPcZJQ?e=o59V9a

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And that links to an incorrect schematic - the rectified output tap is connected to the wrong node.  Might be other errors..

 Even so, the third harmonic is 78dB below the fundamental. The simulation runs slowly on my computer, so it may take me a while to get the schematic to where the designer intended it to be.
 The choice of op amp is what I'd expect from an audio group - it does seem to be a low distortion part.
 I just seem to have wrecked the .raw file from the simulation, so it may be a while before I have more to say.

The file wasn't wrecked, just huge (4.063 GB) and slow to load, despite the fact that I've a solid state disk on my computer put in to hold LTSpice .raw files and load them tolerably quickly.
The circuit uses a half-wave rectifier, then runs the error signal into an integrator wrapped around U4. C4 -at 6.8u - is a biggish integrating capacitor. R7 - at 120k - has the main purpose of stabilising the feeback loop controlling the amplitude and also delivers around 6mV of 1kHz sinusoidal ripple into the gate of the FET. There about 12mV of 1kHz sine wave across the FET channel so this minimises any channel modulation. Ingenious.
It would have been even move impressive if he'd intended to do that from the start, but R5 and R6 make look like it was an afterthought.
My feeling is that an AD734 could do better. Jim Williams did better with his FET controlled version, but I've no idea how.
78dB below the fundamental is respectable, but not impressive.
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Bill Sloman, Sydney