Hi Doug:
In a former life, I worked for a famous scope manufacturer. For a while, I was
associated with probes.
A passive 10X probe comprises a voltage divider. The nose resistor is 9
megohms, and the scope input is 1 megohm, yielding a 10 megohm input impedance.
(Usually, these are precision resistors.) To be a frequency-flat divider, the
time constant of the probe nose RC must equal the scope input RC. The scope
input C is about 20 pF. That means probe nose C must be about 2 pF.
Scope input C is not well-controlled. That means the probe nose C or the scope
input C must be adjusted so that the individual probe RC and the individual
scope input RC are equal. The probe and scope input channel must be
“compensated” as a unique system. (Every scope has a square wave port for
“compensating” the probe to the input channel.)
Probes are made with either adjustable nose C or adjustable base C.
A properly “compensated” probe with a coaxial ground connection to the probe
tip will meet the accuracy and frequency response claimed by the probe
manufacturer. At high frequencies, the probe ground lead will reduce the
accuracy and frequency response.
As has been discussed here, high-frequency response of a probe is highly
dependent on the probe ground lead. The shorter, the better. Many probes come
with an assortment of ground leads for different measurements.
Rich
From: Doug Smith [mailto:d...@emcesd.com]
Sent: Monday, July 11, 2016 7:15 PM
To: EMC-PSTC@LISTSERV.IEEE.ORG; Richard Nute <ri...@ieee.org>
Subject: Re: [PSES] Oscilloscope probe calibration
Hi Rich,
Although useful, the literature from measurement companies and be misleading
and rarely descirbe the situation completely. Maybe I should hold a short free
webinar for anyone interested. I probably have more information on this than
any other source. If I do the webinar, the limit is 24 computers on the
connection, first come, first served. Contact me privately if interested, would
do it later this week in the morning Pacific Time, domestic US or UK only.
Doug
<http://emcesd.com/graphics/SYMBOL.JPG>
-
----------------------------------------------------------------
This message is from the IEEE Product Safety Engineering Society emc-pstc
discussion list. To post a message to the list, send your e-mail to
<emc-p...@ieee.org>
All emc-pstc postings are archived and searchable on the web at:
http://www.ieee-pses.org/emc-pstc.html
Attachments are not permitted but the IEEE PSES Online Communities site at
http://product-compliance.oc.ieee.org/ can be used for graphics (in well-used
formats), large files, etc.
Website: http://www.ieee-pses.org/
Instructions: http://www.ieee-pses.org/list.html (including how to unsubscribe)
List rules: http://www.ieee-pses.org/listrules.html
For help, send mail to the list administrators:
Scott Douglas <sdoug...@ieee.org>
Mike Cantwell <mcantw...@ieee.org>
For policy questions, send mail to:
Jim Bacher: <j.bac...@ieee.org>
David Heald: <dhe...@gmail.com>
