**From:** David Instone (*[email protected]*)

**Date:** Tue Apr 24 2001 - 05:05:49 PDT

**Next message:**Degerstrom, Michael J.: "RE: [SI-LIST] : Attenuation on a PCB trace"**Previous message:**Daniel, Erik S.: "RE: [SI-LIST] : D/A interfacing"**In reply to:**Ingraham, Andrew: "RE: [SI-LIST] : Source termination of transmission line"

Thanks for your time Andy,

So what I think you are saying is that if the line twixt driver and

connector is any longer than a small fraction of the rise time then it

had better match either the connector or the driver. Otherwise I get a

reflection both from the connector to line interface and the line to

driver interface, and of course the subsequent re-reflections between

the two. Which is where I was coming from in the first place except

that I missed the fact I could also use a line that matched the

connector instead of matching the driver.

Dave.

Ingraham, Andrew wrote:

*>
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*> Dave,
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*>
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*> Not enough time for a decent response, so this may be kind of rough....
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*>
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*> > > There is no minimum length. A t-line of any length is a t-line. When
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*> > it is
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*> > > very short, we might not bother to model it as a t-line (purely for the
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*> > sake
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*> > > of mathematical simplicity); but at any length, it pretty much has to
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*> > obey
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*> > > the Telegrapher's equations, Maxwell's equations, etc.
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*> >
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*> > I have a hard time visualising that in practice. When I TDR lines I
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*> > know that any discontinuities which last less than the TDR risetime do
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*> > not appear with their full value. So yes, to a zero risetime signal an
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*> > infinitely short line would appear as a line, but for signals with real
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*> > risetimes I think the line has to have a minimum length depending on the
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*> > actual risetime.
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*>
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*> OK, a t-line has to be some minimum length for you to *notice* it as a
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*> t-line. But a shorter t-line is a t-line nonetheless.
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*>
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*> As for reflections at impedance mismatches, or lack thereof when you match
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*> the impedances, the principle holds no matter how short the line is. It's
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*> just that you might not notice a MISmatch if the t-line is short compared to
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*> a risetime.
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*>
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*> When a t-line is very short compared to the risetime, then its
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*> characteristic impedance doesn't much matter either, and you can mismatch
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*> impedances to your heart's delight.
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*>
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*> ---
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*>
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*> > True but while some of the energy is reflected back towards the load the
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*> > greater part carries on towards the driver and if the driver is source
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*> > terminated it ends there and doesn't get re reflected so I get just one
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*> > ring and not a whole sequence. Now my thinking is - if the driver is
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*> > hard on the connector it's just 75 ohms trying to terminate 40 ohms,
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*> > doesn't work and in any case putting the driver hard on the connector is
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*> > generally not possible. If the driver is some way from the connector
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*> > then it needs a t-line to the connector and if that t-line matches the
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*> > driver impedance then anything that gets back to the driver stops
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*> > there. Saves the shunt resistor and it's imperfections
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*>
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*> Um, I think there's some flaws in this reasoning.
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*>
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*> If you start with a backward-moving wave in the 40 ohm medium, then what you
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*> want to do is match the *40* ohms, not the 75 ohms. The fact that you are
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*> connecting 40 to 75 ohms, means that tiere is a mismatch right there, with
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*> consequent reflections.
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*>
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*> Whether you have just the 75 ohm driver, or put it behind a 75 ohm t-line of
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*> any length, it looks like 75 ohms. Remember, a 75 ohm t-line of any length
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*> with a matched (75 ohm) load on the far end, looks just like 75 ohms. So
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*> your connector sees 75 ohms, looking back toward the source, NO MATTER HOW
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*> LONG THE LINE IS. (Well, ideally.)
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*>
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*> On the other hand, if you used a 40 ohm driver, and optionally connected to
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*> it with a 40 ohm trace, then there is (ideally) no mismatch between it and
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*> the connector. The only reflections you get are at the 40 / 50 ohm
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*> interface. (Ideally.)
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*>
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*> If you consider just the driver, connector, and the trace between them (and
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*> ignore what comes after the connector), then you have a time-dependent
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*> reflection problem. Think of it like a TDR, looking back toward the driver.
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*> At first, you have a reflection due to the mismatch between the 40 ohm
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*> connector and 75 ohm line. This part of the reflection lasts roughly twice
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*> the delay of the t-line. After that, and from then on until infinity, the
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*> reflection continues at the same magnitude (so that you don't notice any
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*> difference), but is now due to the mismatch between the 40 ohm connector ...
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*> considered in this case as your reference impedance for the TDR ... and the
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*> 75 ohm driver. If you somehow changed it to a 40 ohm driver, you would see
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*> only the t-line reflection, which would last only a finite time. If the 75
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*> ohm trace were much shorter than the risetime, you wouldn't even notice its
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*> reflection.
*

-- RegardsDave Instone. Compliance Engineer Storage Systems Development, MP24/22 Xyratex, Langstone Rd., Havant, Hampshire, P09 1SA, UK. Tel: +44 (0)23-92-496862 (direct line) Fax: +44 (0)23-92-496014 http://www.xyratex.com Tel: +44 (0)23-92-496000

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**Next message:**Degerstrom, Michael J.: "RE: [SI-LIST] : Attenuation on a PCB trace"**Previous message:**Daniel, Erik S.: "RE: [SI-LIST] : D/A interfacing"**In reply to:**Ingraham, Andrew: "RE: [SI-LIST] : Source termination of transmission line"

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