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

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From: Larry Miller (ldmiller@rhapsodynetworks.com)
Date: Mon Mar 05 2001 - 09:09:02 PST


See below; I think you are mixing together source and receiver stuff.

-----Original Message-----
From: owner-si-list@silab.eng.sun.com
[mailto:owner-si-list@silab.eng.sun.com]On Behalf Of Stubbs, Jason
Sent: Monday, March 05, 2001 8:30 AM
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.

--->> most ASIC receivers look more or less like high impedance with some
small capacitance value to ground. Are you talking about the source
(transmitter) end?

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).

--->>> I think you mean voltage doubling at the transmitter

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.

--->>> only if the sum of the ASIC driver impedance and the series
terminator equal your transmission line impedance (50 ohms, you say). Most
ASIC drivers have a lower driving impedance in the 20 to 30 ohm range,
sometimes lower. So a 22 to 33 ohm series terminator at the SOURCE end will
absorb an reflection from a 50 ohm transmission line trace pretty well.

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).

--->>> these sound like receiver end schemes, not source (series) schemes.

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?

--->>> you can only get reflectionless transmission if BOTH the source and
receiver ends are terminated in the characteristic impedance of the
transmission line. But you should be able to get only one reflection back to
the source with a series terminator. It still sounds like you are confusing
source and receiver.

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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