RE: [SI-LIST] : Measuring noise with Differntial probe

Grasso, Charles (GrassC@LOUISVILLE.STORTEK.COM)
Thu, 9 Sep 1999 09:08:25 -0600

I would like to suggest you look at
Doug Smiths web page http://emcesd.com.

In this he descibes a low capacitance probe
with a 1GHz bandwidth and includes the
theory of operation.

Doug is an expert at Noise measurements.

Thank you
Charles Grasso
Advisory Engineer
StorageTek
1 StorageTek Drive
Louisville CO 80027 M/S 4247.
Tel:303-673-2908
Fax:303-661-7115
email:grassc@louisville.stortek.com
RMCEMC Web Site: http://www.ewh.ieee.org/r5/denver/rockymountainemc/

-----Original Message-----
From: Ingraham, Andrew [mailto:Andrew.Ingraham@compaq.com]
Sent: Thursday, September 09, 1999 7:34 AM
To: 'si-list@silab.eng.sun.com'
Subject: RE: [SI-LIST] : Measuring noise with Differntial probe

>When I measure PWR-GND noise of high-frequency(~100MHz) IC, I have two
>options.
>One is using the standard Active (FET) probes, the other is using a
>differential probe.
>
>The results of two measurement are slightly different.
>
>Which must I believe ?

Believe neither. Any hi-freq. measurement should be suspect. Every probe
will affect the signal and its display in some way.

Are you probing the same exact two points, and are both probe leads VERY
short? Probing a signal and ground pin that are only an inch apart from
each other, can introduce sizable errors.

The differential measurement *ought* to be better, *if* the diff. probe is
sufficiently accurate and wide bandwidth, as it avoids the ground loop.

Try doing a differential measurement with two FET probes, and see how it
differs from the other two. Try various combinations to get a feel for how
these changes affect what you measure.

Regards,
Andy Ingraham

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