This has generated some interesting discussion. Having operated an OATS for several years I can appreciate the difficulty of dealing with this problem.
1. It is always best to measure the EUT first in a chamber. while the results may, or may not correlate it does give you a list of frequencies to work from. 2. Correlation often depends on the EUT size. I found smaller units tend to correlate better. This of course depends on precise duplication of the test setup. 3. When measuring a signal that is buried by an ambient signal, it often depends on the exact nature of the signal and the type of ambient. There is no set rule that will work every time. 4. Some techniques that I have used are: Moving the antenna closer will give you a stronger signal without a comparable change in ambient level. This works if the ambient is not a very strong signal like you would get from a local radio or TV station. Using a narrower bandwidth to make the measurement. This can work depending on the position of the signal relative to the ambient signal. There can work well if the signal being measured is CW in nature. If it is broadband, or modulated, it creates problems. Signal substitution, note - this technique requires a receiver with a display. First identify the signal from the EUT on the OATS. Use a signal generator to match the level of the EUT signal on the visual display. The signal generator level can then be used to represent the EUT signal. This is a simple description, it is a bit more complicated in practice. 5. The issue of noise cancellation of the ambient on the OATS is a fairly complex one. The real issue is getting cancellation without changing the EUT signal. In theory it could be done I guess, but in practice I don't think so. I would think it might work with a CW ambient signal, but the real ambient problems are radio stations, TV stations, and intermittent transmitters. This type of technique will not work very well for them. Several years ago we had a demo of a system being sold by ETS specifically to do noise cancellation. It was a dual channel receiver that used 2 antennas separated by a distance, then digitally performed the cancellation. I forget what they called the system. It worked to some extent, but not very well with radio stations, TV stations, and intermittent transmitters. It might be better today if they kept developing the technology. Anyone else look at this system? Tom Donnelly From: Tsang Steven [mailto:[email protected]] Sent: Tuesday, January 27, 2004 11:41 PM To: [email protected] Subject: OATS Measurement Dear all, I have a question regarding on OATS measurement (3m site, 30MHz to 1000MHz), I hope someone could help me on this issue. Currently my company is undergoing a accreditation porcess and the accreditor is asking on the method of testing to overcome the issue of the noise floor covering the signal generate from the EUT (i.e. Radio station signal ). We have propose to measure the signal from EUT in an aechoic chamber and use a signal generator to simulate the signal with a change of frequency (i.e. 10kHz) to avoid the signal being block by the noise floor and re-measure the signal on OATS. This method is rejected by the assessor and asking to provide an alternative method. Could someone provide the solution, thanks!!! Best Regards, emc_st _________________________________________________________________ 靈格風 : 學英語, 免費學日語 http://go.msnserver.com/HK/34698.asp This message is from the IEEE EMC Society Product Safety Technical Committee emc-pstc discussion list. Visit our web site at: http://www.ieee-pses.org/ To cancel your subscription, send mail to: [email protected] with the single line: unsubscribe emc-pstc For help, send mail to the list administrators: Ron Pickard: [email protected] Dave Heald: [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 EMC Society Product Safety Technical Committee emc-pstc discussion list. Visit our web site at: http://www.ieee-pses.org/ To cancel your subscription, send mail to: [email protected] with the single line: unsubscribe emc-pstc For help, send mail to the list administrators: Ron Pickard: [email protected] Dave Heald: [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
