Jim, I too have played with a sliding probe to find peaks and nulls on cables. Typically this is for diagnostics purposes only. In order to minimize some of the uncertainty I will typically place one or two ferrite toroidal cores on the coaxial cable leaving the current probe as a common mode choke. This, in my feeling will help minimize and induced RF which is common to the shield and center conductor and leave the normal mode currents unaffected. My thought is that the RF being measured source is using the instrument power and ground connections as a path.
I use lossy ferrite and put on only a couple of turns only as the interwinding capacitance eventually overcomes the common-mode inductance at high frequencies. Positioning the ferrite is sometimes a challenge but generally I begin at the instrument and work up to the probe. I also may place a sleeve behind the probe and a toroid in front of the analyzer. Naturally if you plan to use this idea at a certified test site, you may need to make an argument about non-interference in the measurements. -doug ======================================= Douglas E. Powell, Compliance Engineer Advanced Energy Industries, Inc. Fort Collins, Colorado 80525 USA --------------------------------------- 970-407-6410 (phone) 970-407-5410 (e-fax) mailto:[email protected] http://www.advanced-energy.com ======================================= -----Original Message----- From: Jim Eichner [mailto:[email protected]] Sent: Monday, June 21, 1999 11:10 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 [email protected] 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: [email protected] [SMTP:[email protected]] > Sent: Monday, June 21, 1999 9:48 AM > To: EMC-PSTC > Subject: Fw: Current probe problem > > > Posted for Doug Smith: > > > > > > > > ------------------------ > From: [email protected] > Subject: Current probe problem > Date: 18 Jun 1999 20:38:04 -0700 > To: [email protected] > > > > 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: [email protected] > > | q-----( ) | o | Email2: [email protected] > > \ _ / ] \ _ / Email3: [email protected] > > -------------------------------------------------------------------- > > > > > > ---------------End of Original Message----------------- > > -------------------------- > Ed Price > [email protected] > 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 [email protected] > with the single line: "unsubscribe emc-pstc" (without the > quotes). For help, send mail to [email protected], > [email protected], [email protected], or > [email protected] (the list administrators). > --------- This message is coming from the emc-pstc discussion list. To cancel your subscription, send mail to [email protected] with the single line: "unsubscribe emc-pstc" (without the quotes). For help, send mail to [email protected], [email protected], [email protected], or [email protected] (the list administrators). --------- This message is coming from the emc-pstc discussion list. To cancel your subscription, send mail to [email protected] with the single line: "unsubscribe emc-pstc" (without the quotes). For help, send mail to [email protected], [email protected], [email protected], or [email protected] (the list administrators).
