# RE: [SI-LIST] : What speed scope should I consider?

From: Tom Dagostino ([email protected])
Date: Mon Nov 15 1999 - 14:59:53 PST

You should terminate the far end in the characteristic impedance of the
transmission line that takes the signal to the load. The value of the
driver's impedance does not affect the load's termination.

You should source terminate the driver's end at the impedance of the
transmission line. The value of this terminatin is the sum of the driver's
output impedance plus the resistor that sums to Zo of the line. Chances are
the output of the driver is going to be in either the high or the low state
when the reflection comes back so you will only have to deal with the static
output impedance of the driver. Unfortunately the output impedance of most
drivers is not the same in both the high and low states so you will have to
compromise. Trying to terminate at the value of the driver while it is in

Tom Dagostino

-----Original Message-----
From: [email protected]
[mailto:[email protected]]On Behalf Of Adrian Shiner
Sent: Monday, November 15, 1999 1:20 PM
To: [email protected]
Subject: Re: [SI-LIST] : What speed scope should I consider?

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

----- Original Message -----
From: Chuck Hill <[email protected]>
To: <[email protected]>; <[email protected]>
Sent: 15 November 1999 02:38
Subject: RE: [SI-LIST] : What speed scope should I consider?

>
> 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.
> >
>
>==========================================================================
> >
> >-----Original Message-----
> >From: Ron Miller [mailto:[email protected]]
> >Sent: Friday, November 12, 1999 3:54 PM
> >To: [email protected]
> >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
> >
> >
> >
> >
> >Does anyone know of any instrument that can measure transient impedance?
> >
> >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===========
> >
95131
> >
> >[email protected], [email protected]
> >
> >
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> >
>
>
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