Re: [SI-LIST] : Transient impedance

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From: Chuck Hill (chuckh@altaeng.com)
Date: Tue Nov 16 1999 - 14:02:31 PST


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
>> > >majordomo@silab.eng.sun.com. In the BODY of message put: UNSUBSCRIBE
>> > >si-list, for more help, put HELP. si-list archives are accessible at
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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
>> > >
>> > >
>> > >**** To unsubscribe from si-list: send e-mail to
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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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