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

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From: Nianci Wang (nwang@amcc.com)
Date: Mon Mar 05 2001 - 10:25:38 PST


Jason,

For a point to point unidirectional trace, I expect that either series source
termination or parallel termination at the Rx end should work. The question is
you should find out the characteristic of both driver and receiver I/O. The
information can be got from chip designer or you can figure out the output
impedance of driver and input impedance of receiver by turning the series or
parallel termination of your trace.

From your simulation, it seems that Rx input impedance is very large (much
larger than 50 ohm). Because you saw better results by terminating 50 ohm
resistor at 50 ohm trace. At driver size, it looks like the output impedance of
driver is around 25 ohm, because board designer has specified a 25 ohm source
series termination. But the chip output impedance may have larger process
variation and also have a big impacted by the package parasitic. So the 25 ohm
source series termination + the driver output impedance may not be absorbed all
the reflection wave from the Rx side but only attenuating it as you found in
your simulation.

Depend on the type of your trace (data or clock) , you may be able to
tolerance the spike you saw in your simulation. If that is a case than the
series termination is easier way to implement, otherwise the parallel
termination is a better approach, or you can use both series and parallel
termination.

Best Regards
Nianci

"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
>
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