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From: Bill Sloman <bill.sloman@ieee.org>
Newsgroups: sci.electronics.design
Subject: Re: Random thoughts on sinewave oscillators
Date: Sat, 19 Oct 2024 23:32:21 +1100
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On 19/10/2024 2:36 am, john larkin wrote:
> On Fri, 18 Oct 2024 09:48:17 +0100, liz@poppyrecords.invalid.invalid
> (Liz Tuddenham) wrote:
> 
>> Edward Rawde <invalid@invalid.invalid> wrote:
>>
>>> Is the reason why this doesn't produce a better looking sinewave because
>>> the amplifier slew rate is faster going down than it is going up or some
>>> other reason?
>>>
>>> Ignore the wild decoupling, it took me long enough to get the concept to
>>> work at all.
>>>
>>> I'm aware that a single package containing two op amps could probably do a
>>> much better job.
>>
>> if noise is more important than waveform, I found amplitude control by
>> clipping gave the lowest noise.  The oscillators in this...
>>
>> <http://www.poppyrecords.co.uk/other/DistortionMeter/intermodmeter.htm>
>>
>> ...are amplitude stabilised by clipping.
> 
> If the gain of an oscillator loop is close to 1.00, say 0.98 to 1.02,
> one can add in a small tweak, a crude multiplier or even a clipper, to
> make up the difference.

Clipper's add harmonic content. Multipliers can do better. The AD734 
adds some harmonic content but it can be 70dB below the fundamental, and 
if the tweak you need is 50dD below the main signal path, your op amp 
will add as much.

> Of course, with many-bit DACs being cheap nowadays, it's easier to do
> a DDS sine wave generator, and get super-precise frequency and
> amplitude. A 70 cent uP can do that, and even use PWM to eliminate the
> DAC.

Sigma-delta DACs do rely on pulse width modulation.
A well-designed Wein bridge can keep the harmonic content of the output 
120dD below the fundamental, and DDS devices can't do that without heavy 
filtering of the output.

> All sorts of elegant analog circuits are blown away by cheap digital
> junk. Sigh.

Cheap digital junk doesn't blow away a well-designed Wein bridge. It may 
blow away anything that John Larkin can design, but that's a different 
competition.

-- 
Bill Sloman, Sydney