RE: [SI-LIST] : Transient impedance

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From: Chris Rokusek (crokusek@viewlogic.com)
Date: Tue Nov 16 1999 - 16:01:02 PST


Charles,

IBIS is just a data format, what a particular simulator does with that data
may vary. The information to accurately model transient Z is often (if IBIS
cookbook recomendations are followed!) present in the data.

Chris

> -----Original Message-----
> From: owner-si-list@silab.eng.sun.com
> [mailto:owner-si-list@silab.eng.sun.com]On Behalf Of Chuck Hill
> Sent: Tuesday, November 16, 1999 2:03 PM
> To: si-list@silab.eng.sun.com; si-list@silab.eng.sun.com
> Subject: Re: [SI-LIST] : Transient impedance
>
>
> DC,
>
> That was the conclusion I reached as well. This was in conflict with the
> modeling approach used in IBIS. So I measured real devices to
> decide which
> view was correct. My conclusion is the assumption of low impedance during
> switching used by IBIS is a drawback, and not always true, in fact, mostly
> not true.
>
> Now, what are the implications? If the impedance is high during the
> switching interval, then the reflections off the source will have low
> attenuation (low return loss). Any waves incident to this driving source
> will be reflected. These could be reflected waves from loads on the bus,
> or near end crosstalk. The waves reflected off the source will appear at
> the loads as unwanted signals, (noise?) or intersymbol interference which
> causes timing jitter.
>
> Further, since IBIS assumes a low source impedance therefore a high return
> loss, its simulation results are better than reality in terms of this
> additional timing jitter mechanism.
>
> What do you think?
>
>
> Charles Hill, consultant
>
> P.S. IBIS is a very useful simulation environment for SI.
>
>
>
> At 03:46 PM 11/15/99 -0700, D. C. Sessions wrote:
> >Adrian Shiner wrote:
> >>
> >> If there are 2 active (cmos) devices in a push pull (totem pole) output
> >> stage then at some point both devices will be half way between on (low
> >> impedance) and off (very high impedance). The impedance must
> vary during
> the
> >> transition. Are you going to terminate both ends of the line which this
> >> device is driving so that the impedance variation is minimised? Does it
> >> matter? Perhaps we should return to current driven logic &
> stop worrying
> >> about the voltages???
> >
> >As a general rule, we design output stages to keep the crowbar
> current low.
> >Break-before-make, as it were, although it's no exactly sequenced so much
> >as just turning off faster than on. Necessarily, though, the output goes
> >through a high-impedance point.
> >
> >Compounding this is the fact that MOS devices aren't just modulated
> resistors.
> >If the gate-drain voltage is greater than about the threshold
> voltage, then
> >the output is resistive. Otherwise it's more or less a constant
> current, and
> >_that_ is certainly high impedance. As the driver turns on or
> off, you have
> >a situation where the gate-source voltage is decreasing while the
> drain-source
> >voltage is increasing -- the gate-source voltage drops REALLY
> fast, and the
> >transistor spends most of its transition time in high impedance.
> >
> >Therefore, the high-impedance window for a driver is quite a bit
> longer than
> >the nonoverlap interval. It's very nearly the entire edge time
> for a shunt
> >terminated line, since it starts shortly after the first device begins to
> >turn off and only ends when the second finishes turning on.
> >
> >> ----- Original Message -----
> >> From: Chuck Hill <chuckh@altaeng.com>
> >> To: <si-list@silab.eng.sun.com>; <si-list@silab.eng.sun.com>
> >> Sent: 15 November 1999 02:38
> >> Subject: RE: [SI-LIST] : What speed scope should I consider?
> >>
> >> > Arpad,
> >> >
> >> > I have a proprietary method of measuring the transient impedance. My
> >> > method is used to measure the output impedance of a buffer while
> switching
> >> > between high and low states. My measurements show that the output
> >> > impedance during buffer switching is often much higher than
> the on state
> >> > resistance.
> >> >
> >> >
> >> > Charles Hill, consultant
> >> >
> >> >
> >> >
> >> >
> >> > At 04:47 PM 11/12/99 -0800, Muranyi, Arpad wrote:
> >> > >I meant impedance vs. time, or non steady state but varying
> impedance,
> >> > >such as the impedance of a buffer turning on/off vs. time.
> >> > >
> >> > >Arpad
> >> >
> >>
> >=================================================================
> =========
> >> > >
> >> > >-----Original Message-----
> >> > >From: Ron Miller [mailto:rmiller@Brocade.COM]
> >> > >Sent: Friday, November 12, 1999 3:54 PM
> >> > >To: si-list@silab.eng.sun.com
> >> > >Subject: Re: [SI-LIST] : What speed scope should I consider?
> >> > >
> >> > >
> >> > >Try Tektronix 11801 or a similar sampling scope from HP with the
> >> > >TDR option.
> >> > >
> >> > >Ron Miller
> >> > >
> >> > >
> >> > >"Muranyi, Arpad" wrote:
> >> > >
> >> > >
> >> > >Does anyone know of any instrument that can measure
> transient impedance?
> >> > >
> >> > >Arpad Muranyi
> >> > >Intel Corporation
> >> >
> >========================================================================
> >> > >
> >> > >
> >> > >Quote from previous correspondence:
> >> > >
> >> > >
> >> > >"Network analyzers measure steady state impedance, not transient
> >> impedance."
> >> > >
> >> > >
> >> > >
> >> > >**** To unsubscribe from si-list: send e-mail to
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>> ****
>> > >
>> > >--
>> > >
>> > >Ronald B. Miller _\\|//_ Signal Integrity Engineer
>> > >
>> > >(408)487-8017 (' 0-0 ') fax(408)487-8017
>> > >
>> > > ==========0000-(_)0000===========
>> > >
>> > >Brocade Communications Systems, 1901 Guadalupe Parkway, San Jose, CA
>> 95131
>> > >
>> > >rmiller@brocade.com, rbmiller@sjm.infi.net
>> > >
>> > >
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>> >
>> >
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>>
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>
>--
>D. C. Sessions
>dc.sessions@vlsi.com
>
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>

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