On 2006-05-26, Don Pearce wrote:
On 26 May 2006 10:22:30 GMT, John Phillips
wrote:

On 2006-05-25, Jim Lesurf wrote:
In article , John Phillips
wrote:

I noted, BTW, that the integrator used in the test setup employs another
capacitor which must be assumed to be linear for the test to work. ...

... my reaction was to feel the above didn't matter once we'd
established that the actual conditions of the test were of dubious
relevance. No point in worrying about details if the test situation was
orders of magnitude different to those which are of actual interest to us.

I agree that in the case of a coupling capacitor the ideal design drops
a negligible *signal* voltage across it. The article and its test are
of dubious relevance there.

However, consider the case of a single-pole RC filter. In the pass-band
substantially the full signal voltage appears across the capacitor.

But do remember that in a low pass filter, the opposite situation
applies. In the passband, the capacitor looks to all intents and
purposes like and open circuit, so it still doesn't matter what the
dielectric is doing. Now, at the top of the passband, where the
capacitor is starting to matter, some of this might happen, but at
that point the filtering action is going to attenuate any products. Of
course they will probably be outside the audio band as well.

Good point. However even if the current in the capacitor is low, the
RC product and therefore the -3 dB point may be modulating with the
voltage amplitude. I will have to think if this effect is negligible.

--
John Phillips