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

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From: Stubbs, Jason (Jason.Stubbs@marconi.com)
Date: Mon Mar 05 2001 - 10:03:24 PST


Sorry, but I seem to have misled some of you with my explanation.

For the series resistor termination, the resistor was place at the driver.

For the other termination schemes I tried the resistor/capacitor was placed
at the receiver.

At the time of writing the original email I thought that this would have
been assumed, although in hindsight I suppose it would have been better if I
removed the ambiguity.

Regards

Jason

> -----Original Message-----
> From: Stubbs, Jason [mailto:Jason.Stubbs@marconi.com]
> Sent: Monday, March 05, 2001 11:30 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.
>
> 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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