Re: High purity 1kHz oscillator

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