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From: Phil Hobbs <pcdhSpamMeSenseless@electrooptical.net>
Newsgroups: sci.electronics.design
Subject: Re: High purity 1kHz oscillator
Date: Fri, 25 Oct 2024 00:44:23 -0000 (UTC)
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john larkin <jl@glen--canyon.com> wrote:
> On Thu, 24 Oct 2024 23:19:52 +0100, John R Walliker
> <jrwalliker@gmail.com> wrote:
> 
>> On 24/10/2024 21:56, Liz Tuddenham wrote:
>>> john larkin <jl@glen--canyon.com> 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. Your only
>>>> function now seems to be to generate childish insults aimed at most
>>>> everyone.
>>>> 
>>>> Here's some C-V notes. Series AC-coupling caps, or caps used in
>>>> filters, can introduce distortion too.
>>>> 
>>>> https://www.dropbox.com/scl/fo/akiyipqep6n67glfjhcea/ACfQ6By9LRZPKey4tgvpd
>>>> DE?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.
>>>> 
>>>> One could make a parametric amplifier using cap C-V effect.
>>> 
>>> Some ceramic caps make rather poor-quality contact microphones, so don't
>>> use them in audio pre-amps.
>>> 
>>> 
>> ... but COG/NPO ceramics are very well suited to this application.
>> 
>> John
> 
> They tend to be small values. We use one 10 nF part. Digikey offers a
> 0.88 uF C0G for $29. Each!
> 
> I had some N4700s cooked up special, to temperature compensate an LC
> oscillator.

Having a reel or two of reasonably consistent NTC caps would be a win.  

The datasheet limits are generally +-30% IME, so it takes a bunch of
cut-and-try to get good compensation.  With repeatable parts, ideally you’d
only have to do it once. 

How good is the consistency of your custom ones?

Cheers 

Phil Hobbs 


-- 
Dr Philip C D Hobbs  Principal Consultant  ElectroOptical Innovations LLC /
Hobbs ElectroOptics  Optics, Electro-optics, Photonics, Analog Electronics