Some standards have asked LISN’s to be used up through 150 MHz. While you are modeling the impedance at the EUT port of the LISN, you should also think about what a meter or two of cable between the LISN and the EUT does.
True, most standards have ignored this, simply defining what impedance the LISN shall present to the EUT. A few years ago, the USA MIL-STD-461 limited the upper end of conducted emission testing to 10 MHz, primarily due to the “de-coupling” effects of the length of the power cables between the LISN and EUT. I’m greatly in favor of modeling whatever I can, but I always believe the network analyzer a bit more. You can learn a lot by modeling an LISN’s performance; you can learn a lot more by measuring the port impedance of four “identical” LISN’s. Ed Price ed.pr...@cubic.com <blocked::mailto:ed.pr...@cubic.com> WB6WSN NARTE Certified EMC Engineer Electromagnetic Compatibility Lab Cubic Defense Applications San Diego, CA USA 858-505-2780 Military & Avionics EMC Is Our Specialty From: emc-p...@ieee.org [mailto:emc-p...@ieee.org] On Behalf Of americo...@aol.com Sent: Thursday, April 08, 2010 6:05 PM To: EMC-PSTC@LISTSERV.IEEE.ORG Subject: Re: [PSES] 50 ohm LISN (AMN) In a message dated 4/8/2010 5:57:05 P.M. Pacific Daylight Time, americo...@aol.com writes: If you use EE101 on the LISN it can be seen that at 10 KHz it has an impedance of about 5 ohms. At 30 MHz its impedance is about 9.42 Kohms in that the 250 micro henry inductor is essentially shorted out by the 4 and/or 8 micro farad capacitors. Therefore, the 50 micro henry inductor at 30 MHz provides the 9.42Kohms and LISN turns into a low pass filter. The majority of the impedance error consists of two factors, namely the extreme changes of the capacitors impedance due to temp and the capacitors initial value ( an AE can be 80% different than what marked on the device. Don't panic its much better than that, i.e. that's the worst case) along with the parasitic coupling to the structures, etc. Some companies build their own LISNs to control these error sources. BTW, the inductors are also changing their characteristics with temp and frequency because in its simplest form the inductor is a L in parallel with a C the combination is in series with the ESR. Robert Hanson In a message dated 4/8/2010 5:27:29 P.M. Pacific Daylight Time, ken.ja...@emccompliance.com writes: Umm, no, you give me too much credit. I was merely pointing out that above 150 kHz, the LISN impedance curve is indeed modeled to a very high degree of accuracy by 50 uH in parallel with 50 Ohms, as a previous post stated. The degree of accuracy being to the nearest 0.1 Ohm at 150 kHz, and getting better with increasing frequency. Whereas if you need 0.01 Ohm accuracy at at 150 kHz, then you have to do what you suggested, and take into account the entire LISN circuit. Which was meant to be tongue-in-cheek, because the tolerance on LISN impedance is typically 10 or 20%. So that in practice you never need take into account, above 150 kHz, that the LISN is anything but 50 uH bypassed by 50 Ohms. Ken Javor Phone: (256) 650-5261 > From: <m...@sfo.com> > Date: Thu, 8 Apr 2010 14:35:17 -0700 > To: <EMC-PSTC@LISTSERV.IEEE.ORG> > Subject: Re: [PSES] 50 ohm LISN (AMN) > > Thanks, Ken. > > You picked up on the effect that using different LISNs may have on the > measurement of conducted noise as seen at the spectrum analyzer. That is, > measurement of noise generated by the internals of the SMPS 'kicking' back > onto the lines. > > Regards, > Robert Macy, PE > > AJM Electronics > 101 E San Fernando St., Ste 402 > San Jose, CA 95112 > tel: 408 286 3985 > fax: 408 297 9121 > m...@sfo.com > > >> Mr. Macy is exactly right if you need LISN impedance correct to 0.01 Ohm. >> For the rest of us, calculating 50 Ohms in parallel with 50 uH is within >> 0.1 >> Ohm of the correct value from 150 kHz up. And you don't need >> sophisticated >> modeling. You can use a simple spreadsheet. >> >> Ken Javor >> >> Phone: (256) 650-5261 > ..snip... > > - - 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.ieeecommunities.org/emc-pstc Graphics (in well-used formats), large files, etc. can be posted to that URL. Website: http://www.ieee-pses.org/ 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 <emcp...@socal.rr.com> Mike Cantwell <mcantw...@ieee.org> For policy questions, send mail to: Jim Bacher <j.bac...@ieee.org> David Heald <dhe...@gmail.com>
