Path: utzoo!utgpu!news-server.csri.toronto.edu!rpi!zaphod.mps.ohio-state.edu!sdd.hp.com!hplabs!hpl-opus!hpcc05!hpldsla!tonya
From: tonya@hpldsla.sid.hp.com (Tony Arnerich)
Newsgroups: sci.electronics
Subject: Re: Re: IR Reception - Modulation and Quality
Message-ID: <1990015@hpldsla.sid.hp.com>
Date: 12 Apr 91 22:29:43 GMT
References: <159639@felix.UUCP>
Organization: HP Scientific Instruments Division - Palo Alto, CA
Lines: 28

Felix spoke of needing horrendously large capacitance to get rid of some
noise coming through his power supplies - 80000 uF, for instance.

Wow - there's a better way.

Most people think that a uF is a uF is a uF. Not so. There is a frequency
dependence that in fact is staggering for electrolytics. In fact, they begin
to be inductors, given a high enough freqency. Parallel inductance is a poor
low-pass filter.

The way they say to do it in "CMOS Cookbook" is to parallel capacitors of
varying size and type:

~2000 uF electrolytic
~  10 uF (tantalum?)
~   1 uF (ceramic?)
~ 100 pF (mica?)

Forgive me if I don't have the sizes and type right - get the book. The point
is that at high enough frequencies, the "smaller" capacitors in fact have a
higher capacitance than the "big" ones and carry the brunt of the filtering
load. 120 kHz could very easily be "high" to an electrolytic.

Also, there's no substitute for DISTRIBUTED capacitance throughout your circuit
if you want to kill noise propagation in a digital design. Analog? Beats me.
I work hard in digital circuits to *suppress* the analog. ;-)

tonya@sid.hp.com