As I said, I am not an expert on this test, but the CDNs used for 61000-4-6 are built to provide 150 Ohm source impedance, assuming a 50 Ohm signal source impedance. They do this by providing an additional 100 Ohm series impedance between signal source and cable-under-test. I don't understand why this would be a proper model for the 61000-4-6 test (field-to-cable coupling) but not the EFT requirement.
Also, if you used a 0.1 uF blocking cap, at 150 kHz that increases a 50 Ohm source impedance to 51 Ohms, which is hardly a significant increase. At the power frequency, 0.1 uF is 26.5 kOhms, which provides 54.5 dB of attenuation protecting the signal source from the power line. That brings the power frequency potential at the 50 Ohm signal source down to 226 mV (0 dBm) for a 120 Vrms mains which should prevent harm. For a 220 Volt mains, the numbers are 450 mV or 6 dBm, which also shouldn't cause harm. > From: Doug Smith <[email protected]> > Organization: D. C. Smith Consultants > Reply-To: [email protected] > Date: Sat, 13 Aug 2005 22:17:39 -0700 > To: Ken Javor <[email protected]> > Cc: EMC-PSTC - Forum <[email protected]> > Subject: Re: Capacitive Clamp insertion loss > > Hi Ken and the group, > > Yes, the blocking caps are low impedance. But the generator is only 50 > Ohms not 150 Ohms so the total source impedance is still about 50 Ohms > including the blocking caps. > > Doug > > Ken Javor wrote: > >> Isn't the point of CDN blocking caps to be transparent at the rf or >> transient spectrum and only block the power line frequency? I think the >> blocking cap impedance should be negligible relative to 150 Ohms over the >> spectrum which makes up the EFT. >> >> >>> From: Doug Smith <[email protected]> >>> Organization: D. C. Smith Consultants >>> Reply-To: [email protected] >>> Date: Sat, 13 Aug 2005 09:19:18 -0700 >>> To: Ken Javor <[email protected]> >>> Cc: EMC-PSTC - Forum <[email protected]> >>> Subject: Re: Capacitive Clamp insertion loss >>> >>> Hi Ken and the group, >>> >>> The higher level for the power lines is because EFT only occurs on >>> power lines and couples to I/O lines. The capacitive clamp is meant to >>> simulate the coupling mechanism, but is, in my experience, nowhere >>> near what really happens. The clamp also has an inductive component >>> and combined with its dimensions is why is is (unintenionally) >>> directive which in turn sends more energy in the wrong direction. >>> >>> Still not sure what is meant by 6 dB insertion loss of the capacitive >>> clamp. I would like to see how the lab thinks that is to be >>> defined/measured. >>> >>> The generator impedance is 50 Ohms and is fed into the clamp directly. >>> The CDN couples the generator onto powerlines dirctly through >>> capacitors so the source impedance is not 150 Ohms. The load of course >>> is not anything like 150 Ohms most of the time for this broad band >>> stress (bandwidth about a few hundred MHz). >>> >>> Doug >>> >>> Ken Javor wrote: >>> >>>> I haven't watched these tests performed, nor performed them, so I am not in >>>> any way an expert. But I see a flaw in the logic. When any of the >>>> requirements implemented with a CDN call out a test potential, it is cited >>>> as an open circuit potential with a 150 Ohm source impedance. Since good >>>> rf >>>> measurement practice (at least at high frequencies) would be to calibrate >>>> the CDN-injected potential into 150 Ohms, the matched potential would >>>> indeed >>>> be precisely 6 dB down from the open-circuit limit. This is borne out by >>>> the fact that the AE side of the CDN is supposed to be an impedance much >>>> higher than 150 Ohms, so it wouldn't load the shunt-injected signal. >>>> >>>> So if the capacitive clamp truly has a 6 dB insertion loss, then it should >>>> inject precisely the same value as the CDN, when both devices are fed with >>>> the same potential, and loaded by 150 Ohms. >>>> >>>> My guess (and it is just that) is that the rationale for a higher test >>>> level >>>> on the ac lines vs. the signal lines is that the ac lines are generally >>>> longer and are more efficient pickups. >>>> >>>> > > -- > ------------------------------------------------------- > ___ _ 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] > \ _ / ] \ _ / Website: 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/listserv/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: > > Richard Nute: [email protected] > Jim Bacher: [email protected] > > All emc-pstc postings are archived and searchable on the web at: > > http://www.ieeecommunities.org/emc-pstc > 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/listserv/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: Richard Nute: [email protected] Jim Bacher: [email protected] All emc-pstc postings are archived and searchable on the web at: http://www.ieeecommunities.org/emc-pstc
