Hi Ken and the group, I would highly recommend renting a spectrum analyzer for this job. You would need to get the scope signal on the order of 20 millivolts or more (possibly much more) to keep the digitizing noise low enough to be usable. For a 10 uA common mode current that would mean about 60 dB of gain. To cascade three preamps may itself create a noise problem. Keep in mind scopes do not have anywhere near the dynamic range of spectrum analyzers (a few Volts to maybe 10mV for a scope (note somewhat larger than most sensitive setting for noise reasons), maybe 50 dB compared to 80 or so for a spectrum analyzer.
Add to that the difficulty in interpreting the FFT output. It is not just like a spectrum analyzer output and it will be difficult if not impossible to identify specific frequencies of problems like you can with a spectrum analyzer. You will be able to identify the envelope of the energy easily, but where under that envelope the specific frequencies lie will be a problem. I think there will be too much preamp noise and scope noise in this setup anyway. Doug Ken Javor wrote: > The sensitivity issue could be addressed via a pre-amplifier, or possible > cascade of a couple pre-amplifiers. So that isn't a show-stopper. > > The F-65 current probe transfer impedance is flat with respect to frequency > over its 1 M - 1 GHz range, so there is no problem there, either. > > But the o'scope gives you time domain info, and your 40 dBuA limit is in the > frequency domain. So you need to get from the time domain to the frequency > domain. > > I'm not familiar with the o'scope models cited but some scopes have built-in > FFT capability, and even if that doesn't exist, there should be some way to > get the scope data into a PC and perform the FFT there. > > > > >>From: [email protected] >>Date: Fri, 22 Jun 2007 11:50:32 +0530 >>To: [email protected] >>Cc: [email protected] >>Subject: Using Common mode current probe with an oscilloscope >> >>Dear Experts, >> >>1. With a current probe (similar to the Fischer Custom Communication Inc. >>make model 'F - 65') connected to the oscilloscope (similar to the model >>'TDS 7145' & 'TDS 7404' of Tektronix) will it be possible to measure / >>identify common mode emissions in signal cables like UTP ethernet cable, >>USB cable used for high bus powered (about 500 mA) device, USB cable used >>for low bus powered (about 100 mA) device, D.C power cable (both shielded >>and unshielded) with about 1 amps etc. >> >>2. What are the issues / short comings that one can expect while using such >>a common mode current probe connected to a digital storage oscilloscope >>(1.5 / 4.0 GHz bandwidth). >> >>We have scopes & an impedance analyser but do not have a spectrum >>analyser. We have Agilent make '4294A' model precision impedance analyser. >>Is it possible to interface through some hardware the common mode current >>probes with this analyser and measure cable common mode emission? >> >>Bruce L. Harlacher of Fischer highlighted issues like poor >>sensitivity of scopes for the 0.1 milli volts output of F - 65 for 40 dBuA >>current and like things. I started discussing with Tek engineers and to >>start discussion with Agilent. I always found experts advise / inputs from >>this forum very useful. >> >>Sincerely >> >>K.Balasubramanian >>Manager - Hardware Support. >> -- ___ _ 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: [email protected] \ _ / ] \ _ / Web: http://www.dsmith.org - This message is from the IEEE Product Safety Engineering Society emc-pstc discussion list. Website: http://www.ieee-pses.org/ To post a message to the list, send your e-mail to [email protected] Instructions: http://listserv.ieee.org/request/user-guide.html List rules: http://www.ieee-pses.org/listrules.html For help, send mail to the list administrators: Scott Douglas [email protected] Mike Cantwell [email protected] For policy questions, send mail to: Jim Bacher: [email protected] David Heald: [email protected] All emc-pstc postings are archived and searchable on the web at: http://www.ieeecommunities.org/emc-pstc ______________________________________________________________________ This email has been scanned by the MessageLabs Email Security System. For more information please visit http://www.messagelabs.com/email ______________________________________________________________________
