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From: Bill Sloman <bill.sloman@ieee.org>
Newsgroups: sci.electronics.design
Subject: Re: High purity 1kHz oscillator
Date: Fri, 25 Oct 2024 14:38:16 +1100
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On 25/10/2024 4:37 am, john larkin wrote:
> On Thu, 24 Oct 2024 20:39:03 +1100, Bill Sloman <bill.sloman@ieee.org>
> wrote:
> 
>> On 24/10/2024 4:44 am, john larkin wrote:
>>> On Tue, 22 Oct 2024 01:10:41 -0400, "Edward Rawde"
>>> <invalid@invalid.invalid> wrote:
>>>
>>>> But I suspect that component tolerances and mismatched FETs will ruin it.
>>>
>>> The very best bulk metal foil resistors have voltage-resistance
>>> coefficients of a few ppm/volt. Most resistors are much worse.
>>
>> Metal film resistors aren't all that much worse than bulk foil
>> resistors, and they are pretty cheap aznd widely available.
>>
>>> Just the resistors can kill distortion specs.
>>
>> Not that you can cite an example
>>
>>> At low frequencies, self-heating and tempco can add distortion too;
>>
>> Pull the other leg.
>>
>>> The old HP oscillators
>>> had that effect from the lamp filament.
>>
>> The lamp filaments ran rather hotter.
>>
>>> DGMS on capacitor nonlinearity.
>> The capacitors and resistors would have to remarkably bad to make much
>> difference to a well-designed Wein bridge.
>>
>> You've already got started on making an ass of yourself about resistor
>> non-linearities. Making bizarre claims about capacitors is more of the same.
> 
> Gosh, you keep getting nastier and crazier every year. 

I'm sure it makes you feel better to think that.

> Your only function now seems to be to generate childish insults aimed at most
> everyone.

By which you mean that I don't flatter you as fulsomely as you think you 
deserve.

> Here's some C-V notes. Series AC-coupling caps, or caps used in
> filters, can introduce distortion too.

You need ceramic caps - with pretty horrible voltage dependent 
capacitance - to get that. NP100 is roughly as good as mica or 
polypropylene.
> https://www.dropbox.com/scl/fo/akiyipqep6n67glfjhcea/ACfQ6By9LRZPKey4tgvpdDE?rlkey=is0vbs7m6xft5u9as5r8eyp30&dl=0
> 
> It's sort of outrageous that most ceramic cap data sheets specify
> capacitance and voltage and hide the fact that you don't get both.
> 
> Film caps can have C-V effects too, and those can matter at PPM
> distortion levels.

The 1% tolerance 15nF caps that Edward Rawde would use - if he ever 
built his Wein bridge - use a polypropylene dielectric film which is 
remarkably good - better than polycarbonate (which is marginally better 
than polyester).

> One could make a parametric amplifier using cap C-V effect.

But nobody has, certainly not with plastic film capacitors.

You really do seem to be sliding off into senility.

-- 
Bill Sloman, Sydney