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

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From: Dagostino, Tom (tom_dagostino@mentorg.com)
Date: Fri Apr 27 2001 - 11:25:39 PDT


I assume the problem is not having a fixed output impedance from the driver.
When the driver is high there is one impedance, when low a second impedance
and during the transition it varies from low to very high to low again.
This implies the reflection will be different depending on when the incident
waveform hits the driver.

Tom Dagostino
IBIS and Tau Modeling Manager
SDD
Mentor Graphics Corp.
503-685-1613
tom_dagostino@mentor.com

-----Original Message-----
From: Jim Freeman [mailto:freeman@broadcom.com]
Sent: Friday, April 27, 2001 9:58 AM
To: D. C. Sessions
Cc: si-list@silab.eng.sun.com
Subject: Re: [SI-LIST] : Reflected-wave drivers: quality metric

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)

Thanks
Jim Freeman
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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