Re: [SI-LIST] : Oscillation in lumped circuits and transmission l ines

J. Eric Bracken (bracken@ansoft.com)
Fri, 05 Feb 1999 08:54:38 -0500

OK, here are a couple of thoughts (turn on fixed-width fonts):

The model below corresponds most closely to a lossy transmission line
driven by a voltage source and open circuited at the output end.

R L
0--/\/\/\---() () ()------+---------0
Vin () () | Vout
----- C
-----
|
V

Being an open-circuited t-line, you fully expect a reflection
coefficient of 1 at the open end, (and -1 at the source end) and hence
big oscillations with a period determined by the sqrt(LC) time
constant. The effect of adding the series R is to damp these
oscillations--essentially changing from a lossless (crude) t-line
model to a lossy (crude) one. But the oscillations will still be
there--at least until you make the resistance larger than 2*sqrt(L/C),
at which point the circuit becomes more like a distributed RC
transmission line (familiar to people who deal with on-chip
interconnects) than the LC ones that PCB designers know and love.

--Eric

--
J. Eric Bracken, Ph.D.                          Tel: 1.412.261.3200 x135
Group Leader, Signal Integrity R&D              Fax: 1.412.471.9427
Ansoft Corp., Four Station Square, Suite 660    bracken@ansoft.com
Pittsburgh, PA USA 15219-1119                   http://www.ansoft.com   

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