Re: [SI-LIST] : Source termination of transmission line

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From: David Instone ([email protected])
Date: Tue Apr 24 2001 - 05:05:49 PDT

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.


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


Dave 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 Tel: +44 (0)23-92-496000

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