[SI-LIST] Re: FET probe caution
- From: "Alex Horvath" <alexh@xxxxxxxxxxxxxxxx>
- To: <doug@xxxxxxxxxx>, "si-list" <si-list@xxxxxxxxxxxxx>
- Date: Wed, 5 Sep 2001 15:05:51 -0700
Doug,
We have an application where we need to perform differential measurements on
a 1.25 Ghz differential digital signal traversing a backplane. Since it's a
50 ohm circuit (100 ohms differential), I reasoned that it was not necessary
to use a high impedance differential scope probe and somewhat inconvenient
since there is no easy way to permanently attach the probe to the circuit. I
decided to use a SMA or MCX with a divider circuit. The scope is Tek 7404 (4
Ghz BW). Can you make a recommendation as to the divider circuit and
resistor values, type of coax etc.
PS. We may require your services as a consultant when we get hardware in.
We're located in Los Gatos.
--------------------------------------------
Alex Horvath
Agile Storage
130 Knowles Dr. Suite B
Los Gatos, CA 95032
408-963-2419
--------------------------------------------
-----Original Message-----
From: si-list-bounce@xxxxxxxxxxxxx
[mailto:si-list-bounce@xxxxxxxxxxxxx]On Behalf Of Douglas C. Smith
Sent: Tuesday, September 04, 2001 8:14 PM
To: si-list
Subject: [SI-LIST] FET probe caution
Hi All,
A common accessory for FET probes can, in some cases, cause a
significant error in the measurement and cause a great deal of time lost
when debugging a circuit. My September Technical Tidbit at:
http://www.dsmith.org
describes the problem.
The accessory in question is a probe tip with a small ground "spring" or
"pin" on a hinge and uses a small ferrite core to improve probe
response. Its use can make the FET probe ten times more sensitive to
logic noise on the board and other EMI that you are not trying to
measure. Such a sensitivity can cause one to "see" a glitch that is not
there on a logic signal, a real time burner in the development lab or
troubleshooting environment. The article presents scope data to show the
magnitude of the problem which affects several brands of FET probes.
The Technical Tidbit area is at the bottom of the index page.
----------
In addition, I have posted pictures to the website from Montreal and the
IEEE EMC
Symposium that was held there a few weeks ago. The link to the pictures
is just above the Technical Tidbit area at the bottom of the index page.
Doug
--
-------------------------------------------------------
___ _ Doug Smith
\ / ) P.O. Box 1457
========= Los Gatos, CA 95031-1457
_ / \ / \ _ TEL/FAX: 408-356-4186/358-3799
/ /\ \ ] / /\ \ Mobile: 408-858-4528
| q-----( ) | o | Email: doug@xxxxxxxxxx
\ _ / ] \ _ / Website: http://www.dsmith.org
-------------------------------------------------------
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