Re: [SI-LIST] : Reflected-wave drivers: quality metric

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From: D. C. Sessions (si-list@lumbercartel.com)
Date: Sat Apr 28 2001 - 07:19:11 PDT


On Friday 27 April 2001 09:57, Jim Freeman wrote:
> Hi DC,
> I don't see the CMOS on/off condition described below as a problem. It is an even
> greater problem to have crowbar current in the outputs due to both the pullup and
> pulldown being on simultaneously. This gives rise to simultaneous ground and VCC bounce
> as well as an oscillation caused by the capacitance from power to ground not responding
> instantaneously(in school I learned that capacitors don't like to change their voltage
> instantaneously)

Unfortunately, the simulations done by the WG indicate that having the driver in High-Z
when reflections come back does cause serious eye closure. So we have a choice
between specifying signal quality under varying driver impedance or else requiring the
receivers to be sensitive enough to not care. The receivers are already pushing pretty
far for the environment they're in, so we don't want to leave any performance on the
floor by excessively conservative driver specs.

Welcome to the world of standards creation. Laws, sausages, that kind of thing.

> P.S. VSWR may be a lost concept because most of the modern engineering programs have
> de-emphasized electromagnetics and emphasized Boolean algebra and its logical followon
> computer programming/hardware design. Most EE's think in terms of zeros and ones
> instead of volts and amps
>
>
> "D. C. Sessions" wrote:
>
> > JEDEC's JC-16 committee is in the process of revising the SSTL standards
> > to explicitly cover reflected-wave operation. One of the issues coming up
> > in the course of the work is that CMOS outputs are typically ill-behaved
> > during transitions (usually going to high impedance after one transistor
> > turns off and before the other turns on.)
> >
> > There have been several proposals to specify envelopes for the Z/T response
> > (majorly ugly), ignoring the matter (nope), and so forth. Right now the best bet
> > seems to be specifying a (two?) resonant stubs driven to have reflections arrive
> > back at the driver in the midst of transition.
> >
> > In other words, we're going to place bounds on the VSWR (you can stop laughing,
> > Ed. I had fun explaining to the kids that this wasn't exactly new and pointing them
> > to the ARRL handbook.)
> >
> > The question is: since these won't be pure sinusoids (anything but!) what would be
> > a good metric for the resultant resonant-mode waveforms that would reflect (as it
> > were) the cumulative S/N ratio at the receiver?
> >
> > D. C. Sessions,
> > Chair, JC-16
> >
> > --
> > | The race is not always to the swift, nor the battle to the strong. |
> > | Because the slow, feeble old codgers like me cheat. |
> > +--------------- D. C. Sessions <dcs@lumbercartel.com> --------------+
> >
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>

-- 
| The race is not always to the swift, nor the battle to the strong. |
| Because the slow, feeble old codgers like me cheat.                |
+--------------- D. C. Sessions <dcs@lumbercartel.com> --------------+

**** To unsubscribe from si-list or si-list-digest: send e-mail to majordomo@silab.eng.sun.com. In the BODY of message put: UNSUBSCRIBE si-list or UNSUBSCRIBE si-list-digest, for more help, put HELP. si-list archives are accessible at http://www.qsl.net/wb6tpu ****


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