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Path: utzoo!watmath!wateng!ksbszabo
From: ksbszabo@wateng.UUCP (Kevin Szabo)
Newsgroups: net.analog,net.ham-radio
Subject: Re: Telephone Glitch suppressor query
Message-ID: <1734@wateng.UUCP>
Date: Thu, 6-Dec-84 15:04:04 EST
Article-I.D.: wateng.1734
Posted: Thu Dec  6 15:04:04 1984
Date-Received: Fri, 7-Dec-84 01:10:27 EST
References: <1219@hplabs.UUCP>
Organization: VLSI Group, U of Waterloo
Lines: 30
Xref: watmath net.analog:105 net.ham-radio:2154

The problem with CMOS is that there are many P-N-P-N structures between
Vdd and Vss. As most of you know this is a SCR (Silicon Controlled
Rectifier) Structure, and is quite sensitive to High dV/dT (transients).
I would suggest putting a small capacitor across the line. The impedance
of a telephone line is 600 ohms, so the reactance of the capacitor
should be quite a bit higher so that it doesn't attenuate the high frequency
components. I would try a cap that gave 1K ohm at 5khz,

	Xc = 1 / ( 2 * pi * C ) ... === 31 * 10 ^ -9 Farad.

Which sounds a little high... maybe start with .001 micro farad and then
try moving up to to the .031 micro farad cap. Remember that the ring
voltage is 90v p-p supermiposed on the 45v dc that is normally on the
line. I forget the frequency, but it is below 1000 hz (20 Hz?). Anyway,
your cap shouldn't be polorized, and should be good for 200 V.

I trust your friend is on an old step-by-step Central Office... I think
capacitors on the line would probably confuse a modern CO.

With regards to Latch - Up, (plug time) we have some people doing interesting
research that will predict latch up paths in VLSI circuitry. It is a
difficult problem for CMOS designers since Latch - Up is a very large
problem yet one which still has to be check for manually (i.e. look
at the I.C. masks and identify the suspect P-N-P-N between Vss and Vdd).
A grad student here has modified a circuit extractor to Identify these
paths and analyze them for latch-up susceptibility. Very nice.

				Kevin
-- 
Kevin Szabo  watmath!wateng!ksbszabo (U of Waterloo VLSI Group, Waterloo Ont.)