[sdiy] ASM Capacitors/Bench Power Supplies

[Ian Fritz]

At 11:34 AM 7/31/2003, Magnus Danielson wrote:
> >
> > Does anyone know of a reason not to use NP0 ceramics for the integrating
> > cap? I briefly tried both NP0 and polystyrene in my last VCO and didn't 
> see
> > any difference in waveshape or tracking.  The NP0 has a smaller
> > tempco.  Also it seems to me that the high-grade mica units, although
> > expensive, should work better than polystyrene.
>
>In general, ceramics is bad due to leakage, memory effects, tempco etc.

Well, these are the words we always hear, of course, but what are the 
numbers???

Certainly the NP0 tempco (+/- 30 ppm/K) is significantly better than 
polystyrene (-100 ppm/K).

>If this is critical for NP0 in oscillators I can't say

Well, but that's what I'm asking!

>, but memory effects you
>want to avoid and leakeage is certainly a low-freq problem.

How do you think memory effects would show up?  I would think of memory 
involving some small change in capacitance from the cap being "soaked" at 
high voltage for a long period of time.  But would this be significant in a 
VCO where the voltage history is always a 0-5 V ramp (and the cap is rated 
at over 50 V)?  If we can figure out what the effect should be, maybe we 
can figure out an experiment to see it.

I agree about leakage coming in at low frequency, and I didn't look at the 
extreme low end when I compared ps vs. NP0.  From what I've seen, the 
dissipation factor may be about a factor of 5 better for 
polystyrene.  Usually, however, dissipation is modeled with a series 
resistance, which does not produce leakage. Rather, it would have the same 
effect as a Franco series resistance.  So it seems to me, it would be 
compensated out in the high-frequency tracking process.

So to characterize leakage, it would probably be good enough to look at the 
dc resistance.  Anybody have an electrometer handy?

   Ian