**From:** Peterson, James F (FL51) (*[email protected]*)

**Date:** Sat Feb 26 2000 - 07:42:16 PST

**Next message:**Ray Anderson: "Re: [SI-LIST] : Re: Opportunities"**Previous message:**[email protected]: "RE: [SI-LIST] : Opinions on SI CAE tools"**Maybe in reply to:**Shayle Hirschman: "[SI-LIST] : Fast edge termination choice"

Shayle-

Reduced, yes, but not for that reason : the voltage wouldn't be halved at

the load nor in the reflection (remember, in a series term', you'll have a

reflection coefficient of almost +1 at the load, so it will double).

Another interesting thing about series terminations is that the initial wave

injected on the t-line has the same rise time as the reflected wave which is

2x the initial wave, thus the edge-rate doubles in magnitude!

If the net is point to point and short, I always start out with what D.C.

suggested : no termination. That's the cheapest way and sometimes it works

(gotta simulate).

-jim

-----Original Message-----

From: Shayle Hirschman [mailto:[email protected]]

Sent: Friday, February 25, 2000 11:49 AM

To: [email protected]

Subject: RE: [SI-LIST] : Fast edge termination choice

That's interesting, Andy. Can I infer, then, that EMI or perhaps coupling

is reduced more by series source termination since the voltage and current

are halved near the source?

Shayle

At 05:19 PM 2/25/00 -0500, you wrote:

*>>Does series source termination defeat the purpose of selecting the fast
*

*>>slew rate option? Will it slow the edge transition time down, or will the
*

*>>signal still transition and travel as fast as if there was no termination,
*

*>>but simply be divided (thereby depending on the reflection at the
*

*>>destination)?
*

*>
*

*>If you imagine driving ideal transmission lines, then there is no
*

difference

*>on edge rates of the voltage at the far end of the line.
*

*>
*

*>But with a small capacitive load, series termination would slow the edge
*

*>rate down a little.
*

*>
*

*>If the line length is small, you have an RC network with some time
*

constant.

*>Adding series termination increases R. Adding parallel termination reduces
*

*>R somewhat.
*

*>
*

*>For a long line, it's more complicated. But if you looked just at the load
*

*>end and small time intervals compared to the line delay, your time constant
*

*>at that node is Zo*C without parallel termination, Zo*C/2 with it.
*

*>
*

*>> Could it be that it
*

*>>reduces EMI, not by slowing the edges down, but rather just simply because
*

*>>it terminates? And that, therefore, as far as EMI goes, parallel
*

*>>termination would have a similar EMI result?
*

*>
*

*>Either termination can help, but there is more to it than just that.
*

*>
*

*>Series termination reduces the size of both the voltage steps (delta-V) and
*

*>current steps (delta-I) of each pulse. The signal driven into the line is
*

*>approximately a half-sized replica of what it would have been even with
*

*>parallel termination. dv/dt and di/dt are about half. At the end of the
*

*>line, the voltage doubles, so the magnitude (and dv/dt) there are the same
*

*>as they would be for parallel termination, except for capacitive loading
*

*>effects noted above. di/dt at the load end is nearly zero for series
*

*>termination.
*

*>
*

*>Regards,
*

*>Andy
*

*>
*

*>
*

*>
*

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**Next message:**Ray Anderson: "Re: [SI-LIST] : Re: Opportunities"**Previous message:**[email protected]: "RE: [SI-LIST] : Opinions on SI CAE tools"**Maybe in reply to:**Shayle Hirschman: "[SI-LIST] : Fast edge termination choice"

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