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

Howard Ji (howardji@cisco.com)
Thu, 09 Sep 1999 10:13:40 -0700

Doug is THE expert at Noise mesurements.

At 09:08 AM 9/9/99 -0600, Grasso, Charles (Chaz) wrote:
>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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