Re: opamp sine wave oscillator

In <qlhmq354fae2q721u5tcqf81f9qc5f28ar@xxxxxxx>, John Larkin wrote:
On 6 Feb 2008 18:19:32 -0800 (PST), MooseFET <kensmith@xxxxxxxxx>wrote:

I think you may be down at the limit of the opamps and resistors etc.
For that matter this sounds like it also down at the limits of the
measurement system.

I'd imagine that most capacitors add distortion, too.

I know some do.

The electric field applies a force that squishes the dielectric, and
that will change the capacitor's geometry to some extent. I have heard
some change shape enough to audibly vibrate when AC is applied. I exect
the capacitance to be not perfectly constant through a cycle of AC.

I expect that use of capacitors with a higher working voltage (thicker
dielectric) will mitigate that one. I expect this to be worse with more
compressible dielectrics - such as paper soaked with oil.

I also suspect that dielectric constant varies a bit with electric field
due to electron orbits in the dielectric's molecules being squished out of
shape. I expect that effect to be mitigated by use of a thicker
dielectric (higher working voltage).

Some ceramic capacitors have dielectric constant so high that I would
not trust the dielectric constant to be all that constant. This is
probably worse with ones whose dielectric constant varies too much with
temperature anyway for the capacitor to be useful to make a good
oscilltor.

Not that electrolytics are used to make a good oscillator, but their
capacitance often varies with voltage. As I understand it, discharging
slightly erodes the dielectric and charging restores it. That would make
the capacitance run high at lower voltages. Most of the few times I
checked into that many years ago, I found this to be true.

- Don Klipstein (don@xxxxxxxxx)
.

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