RE: [SI-LIST] : Unexpected results using series terminating resis tor

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From: Michael Nudelman (mnudelman@tellium.com)
Date: Mon Mar 05 2001 - 09:56:22 PST


John,

yes, it would be interesting to look at the sch; but 25 series term. at 50
Ohm should and will produce spikes; in order to get rid of them fully you
need to use 50 Ohm, but nobody does this. Any value lower than 50 Ohm will
get you aome degree of overshoot. (with 50 Ohm you first divide in half,
than double)

-----Original Message-----
From: Stubbs, Jason [mailto:Jason.Stubbs@marconi.com]
Sent: Monday, March 05, 2001 11:30
To: Si-List (E-mail)
Subject: [SI-LIST] : Unexpected results using series terminating
resistor

I have simulated a point to point unidirectional trace that I tried various
termination techniques on to reduce the signal reflections observed on the
unterminated trace. The trace is 7" long and 50ohm impedance and the signal
frequency is 125MHz. The ASIC receiver model is supposed to be 50ohm input,
but I don't know how I would go about proving that it is.

A sim of the unterminated trace shows what I would have expected to see if
the receiver had high input impedance (a positive reflection as large as the
incident wave and a voltage doubling at the receiver). This reflection
continues back to the receiver a second time producing a spike on the input.
I would have thought that by adding a source series resistor I could have
terminated this reflection at the source end.

I setup some whatif series resistors in ICX and continued the simulations,
but what I observed is an attenuation of the signal, the spike is still
there so I presume the signal is incorrectly terminated.

I tried a few other termination schemes; resistor to GND, AC term, resistor
to voltage, and the one that produced the best results was 50ohms to 1.25v
(using a Thevenin equivalent circuit because 1.25v was not on the board).

I would have expected the series term to have produced better results, and
because the board designer has already specified 25ohm series resistors for
these signals on the board he is beginning to doubt the simulation results.

Am I expecting too much from a series termination?

I didn't want to attach the files to this email, but if anyone is
interested, I have a set of PDF files that show the results I gathered from
the simulations.

Jason Stubbs
Marconi Communications
www.marconi.com
Tel: 724-742-6150
Fax: 724-742-6700

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