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

About this list Date view Thread view Subject view Author view

From: D. C. Sessions (si-list@lumbercartel.com)
Date: Sat Apr 28 2001 - 07:23:56 PDT


On Friday 27 April 2001 11:25, Dagostino, Tom wrote:
> 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.

Pretty much. We're actually limiting the pullup/pulldown impedance mismatch,
so the main problem is the effect of reflections arriving during the transition when
the output is high-Z.

Me, I'm a hardcase. I'm inclined to just tell the systems designers to avoid
resonant line lengths. The others aren't confident that the users will even
know what we're talking about. (To be fair, quite a few applications don't have
fixed frequencies.)

> -----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> --------------+
> >
> > **** 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
> > ****
>
>
>
> **** 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
> ****
>

----------------------------------------
Content-Type: text/html; charset="iso-8859-1"; name="Attachment: 1"
Content-Transfer-Encoding: quoted-printable
Content-Description:
----------------------------------------

-- 
| 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 ****


About this list Date view Thread view Subject view Author view

This archive was generated by hypermail 2b29 : Thu Jun 21 2001 - 10:11:45 PDT