Jim, I have experience using current probes on cables carrying differential digital signals at 500 MHz and above.
I find that standing waves are ALWAYS present on the shield of the cable, even when the signal circuits are matched (at least as well as practical). Most of the following experience is based on a repetitive signal. Life may be more difficult with a transient like an ESD pulse. So, I agree that if you are looking for maximum current, you must search along the length of the cable. I also have used this technique of putting the spectrum analyzer on Max Hold while moving the current probe (with an insulating rod to avoid touching the probe with my body, since this always has some effect on the reading). If you are using a single cable and plugging it into different loads, then it is important to always measure at the same position on the cable. Jim Dr. Jim Knighten e-mail: jim.knigh...@sandiego.ncr.com Senior Consulting Engineer NCR 17095 Via del Campo San Diego, CA 92127 http://www.ncr.com <http://www.ncr.com> Tel: 858-485-2537 Fax: 858-485-3788 -----Original Message----- From: Jim Eichner [SMTP:jeich...@statpower.com] Sent: Monday, June 21, 1999 10:10 AM To: EMC-PSTC Subject: RE: Current probe problem In his write up of the problem, Doug writes "If possible, current probes should be positioned on the measured cable so as to minimize the potential between the cable and the current probe body. This usually means locating the probe near the ground/chassis end of a circuit." Following a tip from someone on this forum quite a while ago, I have observed significant variation in the spectrum of noise along the length of a cable. I have started doing my cable current measurements by sliding the probe over the length of the cable, with my spectrum analyzer in max hold mode. I assume I get a "worst case sweep" by doing this, as I pick up the worst of everything at any position on the cable. Following Doug's advice above, or any instance of using a current probe in a single fixed position, could result in the probe being positioned right at a null for a frequency of interest, sending you on a wild goose chase looking for some other source. Does Doug or anyone else have similar experience or a better way to overcome the problem? Regards, Jim Eichner > Senior Regulatory Compliance Engineer Statpower Technologies Corporation jeich...@statpower.com http://www.statpower.com Any opinions expressed are those of my invisible friend, who really exists, but can only be seen by my dog. Honest. > -----Original Message----- > From: ed.pr...@cubic.com [SMTP:ed.pr...@cubic.com] > Sent: Monday, June 21, 1999 9:48 AM > To: EMC-PSTC > Subject: Fw: Current probe problem > > > Posted for Doug Smith: > > > > > > > > ------------------------ > From: d...@dsmith.org > Subject: Current probe problem > Date: 18 Jun 1999 20:38:04 -0700 > To: emc-p...@ieee.org > > > > I have posted an interesting current probe measurement > configuration/problem on my website at http://emcesd.com. The picture > is at the bottom of the main index page. Click on the picture to see > the answer. > > > > Has anyone else run into problems involving parasitic capacitance > between a current probe and the circuit being measured? > > > > Doug > > > > -------------------------------------------------------------------- > > ___ _ Douglas C. Smith > > \ / ) P. O. Box 1457 > > ========= Los Gatos, CA 95031 > > _ / \ / \ _ TEL/FAX: 408-356-4186/358-3799 > > / /\ \ ] / /\ \ Email1: d...@dsmith.org > > | q-----( ) | o | Email2: d.sm...@usa.net > > \ _ / ] \ _ / Email3: d.c.sm...@ieee.org > > -------------------------------------------------------------------- > > > > > > ---------------End of Original Message----------------- > > -------------------------- > Ed Price > ed.pr...@cubic.com > Electromagnetic Compatibility Lab > Cubic Defense Systems > San Diego, CA. USA > 619-505-2780 > Date: 06/21/1999 > Time: 08:47:46 > Military & Avionics EMC Services Our Specialty > Also Environmental / Metrology / Reliability > -------------------------- > > > > --------- > This message is coming from the emc-pstc discussion list. > To cancel your subscription, send mail to majord...@ieee.org > with the single line: "unsubscribe emc-pstc" (without the > quotes). For help, send mail to ed.pr...@cubic.com, > jim_bac...@monarch.com, ri...@sdd.hp.com, or > roger.volgst...@compaq.com (the list administrators). > --------- This message is coming from the emc-pstc discussion list. To cancel your subscription, send mail to majord...@ieee.org with the single line: "unsubscribe emc-pstc" (without the quotes). For help, send mail to ed.pr...@cubic.com, jim_bac...@monarch.com, ri...@sdd.hp.com, or roger.volgst...@compaq.com (the list administrators). --------- This message is coming from the emc-pstc discussion list. To cancel your subscription, send mail to majord...@ieee.org with the single line: "unsubscribe emc-pstc" (without the quotes). For help, send mail to ed.pr...@cubic.com, jim_bac...@monarch.com, ri...@sdd.hp.com, or roger.volgst...@compaq.com (the list administrators).
