On Mon, 02 Jan 2012 17:14:37 -0800, Hal Murray <[email protected]> wrote:
> >> Time constant is just R*C. If you have a 1000uF cap and a 1K resistor you >> have 1 second. In theory you could build 100s just by using a 100K resistor >> but I think real world components are not perfect enough. > >Does anybody know anything about the temperature coefficients of large caps? >I found data for ceramic caps, but when I added "electrolytic" all I got was >lifetime stuff rather than capacitance change with temperature. > >I'm not interested in the frequency shift of the filter as the temperature >but the voltage shift due to a fixed charge as the capacitance changes. That is not something you are likely to find specified because there is no need to test or guarantee it in the applications that electrolytic capacitors are generally used in. I would also expect problems because of dielectric absorption, leakage, and possibly noise. In the best case, you would probably need to qualify the capacitor yourself. I ended up doing that anyway with a polypropylene film capacitor used in a similar application because we needed to control leakage at high temperatures and that was not something that the manufacturer tested for or guaranteed. In that case, the integrator had a drift below 1 uV/sec at room temperature in a production environment using a 0.47uF polypropylene film capacitor and an equivalent time constant of about 5 seconds. The PC board used guard rings and the summing node was wired into the air although in a good environment that would not have been necessary. With care and the proper environment I think, 100 times better would not be out of the question but I suspect other factors like 1/f noise would become an issue. I think I would try a charge balancing scheme instead of integrating or averaging the output of the phase/frequency detector directly but maybe that is getting away from simple. _______________________________________________ time-nuts mailing list -- [email protected] To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
