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

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From: Scott McMorrow (scott@vasthorizons.com)
Date: Mon Mar 05 2001 - 09:00:46 PST


Jason,

Generally, many ibis models do not correctly account for termination
on an input line. If it is a single ended receiver, you can look at the
the I/V curve and look for a 50 ohm load line in the normal switching
region (outside of the clamp region.) If something approximating
a 50 ohm resistor curve is not there, then you can create it yourself
by adding the I/V curve of a 50 ohm resistor to the current receiver
I/V curve. This will correctly terminate the line and you will see the
correct results.

A series resistor will not make a difference, since it is in-line and
does not terminate to ground. As you correctly saw, it serves as
an attenuation element when placed at the end of a driven line.

regards,

scott

--
Scott McMorrow
Principal Engineer
SiQual, Signal Quality Engineering
18735 SW Boones Ferry Road
Tualatin, OR  97062-3090
(503) 885-1231
http://www.siqual.com

"Stubbs, Jason" wrote:

> 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 > > **** To unsubscribe from si-list or si-list-digest: send e-mail to > majordomo@silab.eng.sun.com. In the BODY of message put: UNSUBSCRIBE > si-list or UNSUBSCRIBE si-list-digest, for more help, put HELP. > si-list archives are accessible at http://www.qsl.net/wb6tpu > ****

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