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

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From: Jim Freeman (freeman@broadcom.com)
Date: Fri Apr 27 2001 - 09:57:41 PDT


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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