Hello Chris Maxwell, I have seen these things at several IEEE/Zurich EMC Symposia. I have listened to the sales people tell me how it works and how well it works, but I have to admit I have always been a skeptic. I have used various TEM devices in the past, mostly with good success. I have used the Military Standard stripline, the Crawford TEM cell, the GTEM, and a couple of other variations on the stripline, and I have used the OWL Chamber (open-wire line in a shielded room). I have experienced the limitations of these older methods, and I must say that the Crawford TEM and the GTEM have behaved the best.
All that being said, I think you have an excellent opportunity to help the rest of us understand the advantages and limitations of the device you are using. I believe that you said you have much comparitive EM emission data in the device that you can compare to the OATS data you have. If I have not misread your note, and what I believe you said is true, you should be able to create a good comparison and, while you might not be able to pin down the real accuracy of the method, you could certainly pin down the difference between OATS and this Cell you have. At least as important, you could say if there was always some offset in one direction, i.e., higher or lower. It seems to me this would be a great service to yourself and to others; if you come out with bad looking comparisons, the vendor of your device may not like it. Regards, EdB -----Original Message----- From: Maxwell, Chris [mailto:chr...@gnlp.com] Sent: Wednesday, September 20, 2000 7:34 AM To: 'EMC-PSTC Internet Forum' Subject: Stripline Cell Use for Radiated Emissions All, We have been using a Comtest G310 0.7m x 0.7m x 0.7m Stripline cell for a few years now. For those unfamiliar with a stripline cell, it is a cubic chamber with a type-N coax connection on each side. The coax center conductor is routed through the bottom of the chamber under a false floor. The coax shield is connected to the outside of the chamber which is metal. The chamber is split in half with hinges and latches so that it opens/closes in clam shell fashion. The seam is gasketed and the entire outer shell of the chamber is metal so that it forms a faraday cage when closed. The inside of the chamber is lined with ferrite loaded foam to prevent reflections. I believe that there is a crude description of a stripline chamber in EN 61000-4-3 (IEC 1000-4-3). We purchased this system with a signal generator, an RF amplifier and software which allows us to perform radiated immunity testing in the chamber. The chamber is specified to be able to perform fully compliant radiated immunity testing on DUT's that fall within a 23cm x 23cm x23cm imaginary cube centered in the chamber. We have used the chamber for this purpose with good results. However, I have also used the chamber for radiated emissions measurements. This is where my question comes up. The chamber is not "calibrated" for emissions. However, over the last few years, every time I have taken a DUT to a 10meter OATS (at an outside test lab), I put the same DUT in the chamber and connect the chamber's coax line to our spectrum analyzer. I connect a 50 Ohm load to the coax connector on the other side of the chamber. I then take spectrum analyzer readings from the DUT. I repeat this process for all orientations of the DUT that I can set up. (i.e. face up, face down , on its side ...) I use a software program to collect and graph these readings. I have saved these spectrum analyzer readings as an unofficial archive in my files. I have used these archives as a baseline for many comparisons. I have used the archives to evaluate design changes on previously tested DUT's. (I have even included the graphs as updates to our Compliance Folders for DUT's that have been tested this way.) I have also used these archives to estimate how completely new DUT's would perform at a 10m OATS. To me it seems almost too easy. There are no ambient emissions in the chamber to cloud the measurements. I just get a little uptight when things seem too easy. I start wondering if I'm missing something. After having said the "easy" statement above, I should say that it is a little bit of a pain monitoring the DUT through the window, and sometimes I have trouble trying to loop the DUT's cabling into the "compliant" area of the chamber. I am also limited by size and cabling interfaces. I can only bring power and a couple of signal cables in through the filtered connectors in the chamber wall. Is there anybody in the group that would have an idea how accurate this type of testing is? I'm not looking for somebody to calculate the +/-dB uncertainty. I'm asking if there are basic physical properties (i.e. near field vs. far field, inaccurate measurement of cable emissions, fringing effects ...) that would make these types of comparisons risky? Is there anybody who has tried to correlate these types of measurements to an OATS? Are there ways to improve this test method (i.e. certain ways to route the DUT's cables) that would make this type of measurement more accurate. P.S. I do have an antenna to go with my spectrum analyzer, but our factory is in the middle of a city. And I'm cramped for room. I have tried some 1m prescan testing with my antenna with some success. The ambients are very hard to work through. Well there's some food for thought. Anybody care to bite? Chris Maxwell, Design Engineer GN Nettest Optical Division 6 Rhoads Drive, Building 4 Utica, NY 13502 PH: 315-797-4449 FAX: 315-797-8024 EMAIL: chr...@gnlp.com ------------------------------------------- This message is from the IEEE EMC Society Product Safety Technical Committee emc-pstc discussion list. To cancel your subscription, send mail to: majord...@ieee.org with the single line: unsubscribe emc-pstc For help, send mail to the list administrators: Jim Bacher: jim_bac...@mail.monarch.com Michael Garretson: pstc_ad...@garretson.org For policy questions, send mail to: Richard Nute: ri...@ieee.org ------------------------------------------- This message is from the IEEE EMC Society Product Safety Technical Committee emc-pstc discussion list. To cancel your subscription, send mail to: majord...@ieee.org with the single line: unsubscribe emc-pstc For help, send mail to the list administrators: Jim Bacher: jim_bac...@mail.monarch.com Michael Garretson: pstc_ad...@garretson.org For policy questions, send mail to: Richard Nute: ri...@ieee.org