# RE: [SI-LIST] : What speed scope should I consider? / "transient impedance"

From: Jeremy Plunkett ([email protected])
Date: Fri Nov 12 1999 - 19:36:22 PST

A technique I've used is to measure the current using a differential probe
across a resistor in series with the DUT, and using another probe measure
the voltage on the pin relative to ground or vdd. You can then calculate
instantaneous Z = V/I using algebraic functions in the scope. The Tektronix
probes I used had a specified bandwidth of 1.5GHz; if you needed to you
might be able to set up a 3 or 4-channel TDR(i.e. 20GHz class scope w/
passive probes) in one chassis to get a measurement out to much higher
frequencies (2 channels are required to get the diff. meas).

Some difficulties of this method are:
1) Any measurement of transient/instantaneous impedance will be affected by
parasitic capacitance and inductance in your measurement setup (including
probe input capacitance).
2) Buffer behavior may be dependent on the load seen at the pin; if it's
practical you may want to take measurements with several different load
values, both resistive and capacitive.
3) The measurement described above includes the impedance of the IC package
power and ground connections in the results, therefore it will also be
affected by the number of buffers switching, supply bypassing on the board,
etc.

Hope you find this useful!

Jeremy Plunkett
Signal Integrity Engineer
Reliance Computer Corp.
[email protected]

-----Original Message-----
From: [email protected]
[mailto:[email protected]]On Behalf Of Muranyi, Arpad
Sent: Friday, November 12, 1999 4:47 PM
To: '[email protected]'
Subject: RE: [SI-LIST] : What speed scope should I consider?

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."

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Ronald B. Miller  _\\|//_  Signal Integrity Engineer
(408)487-8017    (' 0-0 ') fax(408)487-8017
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