Integrating Capacitor

[Bissell, Harry]

If you contact the manufacturers of the polystyrene caps (such as F-Dyne or
Southern Electric USA), they say they are making some polycarbonate caps
that are intended for polystyrene replacement. Teflon is too large and too
expensive, and polyester have too much dielectric absorption (hysteresis).
Polystyrene and Polycarbonate are close, but Polystyrene wins in temperature
stability. You can still get polystyrene, so get some while you can and save
them for your most critical apps (S/H, VCO, etc).
I haven't tried any of the "improved polycarbonate" yet. But I will (when
the shit hits the fan)  :-) Harry

> -----Original Message-----
> From:	Steve Varner [SMTP:varner at k-online.com]
> Sent:	Friday, January 29, 1999 10:54 PM
> To:	synth-diy
> Subject:	Integrating Capacitor
> 
> This question may have been asked recently, so if you want to email me
> directly it's OK...
> 
> The current produced from the exponential converter in VCOs charges an
> integrating capacitor. This cap is usually labelled "poly" (for
> polystyrene capacitor). Now, these circuits were often layed out in the
> late 70s and early 80s (such as the Electronotes VCOs). Some designs
> were based on even older schematics from commercial synths. Back then I
> don't think there were so many alternatives to polystyrene. Ceramic and
> the older "mylar" caps would not be acceptable because of their specs I
> think. Modern caps (20 years later) have much better specs than those
> available in 1980 (temperature, tolerance, etc.). Some of the newer
> ceramic caps and the polyester film caps have great specs. Is there some
> other reason that polystyrene is important? They are listed as having
> "excellent electrical characteristics, high reliability and stability,
> low temperature coefficients, and small dissipation factor." I have a
> feeling it is the low dissipation at high frequency (<0.1% at 100kHz)
> that makes them good. Right? I bought some anyway, so I'll probably use
> them in my VCO design. I just want to know why, and why I can't use some
> other cap.
> 
> SV