Steven (emc-st), For measuring near OATS ambients, the following have been accepted in the past:
- filter out the ambient using bandstop, band pass, low or high pass filters while still passing the frequency you wish to measure - use a lower RES BW and a smaller span to separate signals near the ambient. This is generally acceptable esp. if the emission is CW (CW amplitude not dependent on REW BW) - move in closer, apply 20 log (Meas dist/spec dist) correction - cautions re being in the near field for lower frequencies - try using a receiver instead of an analyzer - front end pre-selection gives better rejection of unwanted signals - make the test at a time when the ambient station is off the air, if possible - Try switching relative position of EUT and search antenna, this works well for directional antennas (the directional antenna is now pointing away from the ambient source). Nulling out the ambient signal is good in theory, but you have to be careful and document it well to make sure you have complete phase cancellation, and so that the agency understands what you are doing. This method is time consuming, especially if you have more than one ambient frequency. Hope this helps. Good luck. best regards Tom Cokenias T.N. Cokenias Consulting www.tncokenias.org P.O. Box 1086 El Granada CA 94018 tel 650 726 1263 fax 650 726 1252 cell 650 302 0887 > Isn't this allowed? Measure the EUT in the chamber to see if the signal > at this particular frequency is an issue. Say it's 10dB below the limit- > then why bother getting an accurate OATS measurement? But if it is 3dB > below the limit then you have probably do have to obtain an accurate > OATS measurement. > > How to work around the radio station short of taking it off the air for > a few minutes? You could lower radio station signal at the OATS antenna > by 10 dB or so by cancellation. Receive the radio station on an antenna > 30 meters from the OATS. Amplify it and re-radiate it 5 meters from the > OATS. Adjust the amplitude (attenuator) and phase (insert lengths of > coax if you don't have a phase shifter)until the radio station is > largely nulled out. You would want the OATS antenna at a fixed height or > else you will have to adjust amplitude and phase for each change in > antenna height. Is this allowed? > > Dave Cuthbert > Micron Technology > > -----Original Message----- > From: [email protected] > [mailto:[email protected]]On Behalf Of Ken Javor > Sent: Wednesday, January 28, 2004 10:50 AM > To: Grasso, Charles; 'Tsang Steven'; '[email protected]' > Subject: Re: OATS Measurement > > > > Mr. Grasso and I disagree. He comes from a point of strength (his > knowledge and expertise in modeling), and I, from a position of weakness > (little knowledge and less interest in predictive techniques). > > If I were concerned whether the EUT were emitting at a particular > frequency, and if I had a screened or anechoic chamber, I would do the > following very quick test (realizing this doesn't answer the original > post's question, except in a basic operational sense): > > I would put the EUT and a radio tuned to the offending ambient station > in the chamber. I would pull the door to until radio reception was just > marginal. Then I would energize the EUT and monitor effect on radio > reception. I would move the radio as close to the EUT as I could to see > if there was any response, but I would use the response at roughly three > meters as a gauge of whether there would be a real problem. > > > >> From: "Grasso, Charles" <[email protected]> >> Date: Wed, 28 Jan 2004 10:28:18 -0700 >> To: "'Ken Javor'" <[email protected]>, "'Tsang >> Steven'"<[email protected]>, >> "'[email protected]'"<[email protected]> >> Subject: RE: OATS Measurement >> >> I must disagree. I would cite Mr Leferinks paper from the Boston EMC >> Symposium. In it he concluded that >> a) correlation was possible for tuned dipoles to +/- 4db - an 8dB >> swing.and b) for test objects the variation in field strength can be >> as high as 40dB for "actual EUTs" and therefore correlation between >> different radiated emissions test methods is impossible. >> >> I think its time we use predictive techniques rather than correlation >> techniques between test facilities. There will be a paper in the Santa >> Clara Symposium this year addressing just this issue. >> Best Regards >> Charles Grasso >> Senior Compliance Engineer >> Echostar Communications Corp. >> Tel: 303-706-5467 >> Fax: 303-799-6222 >> Cell: 303-204-2974 >> Email: [email protected]; >> Email Alternate: [email protected] >> >> >> -----Original Message----- >> From: Ken Javor [mailto:[email protected]] >> Sent: Wednesday, January 28, 2004 7:25 AM >> To: Tsang Steven; [email protected] >> Subject: Re: OATS Measurement >> >> >> Is the (semi?) anechoic measurement a 3 meter measurement? If it is >> and it meets the NSA curve, you ought to be able to correlate your >> anechoic and OATS measurements reasonably well. If they correlate >> where you don't have ambients, you ought to be able to use the >> anechoic measurement in lieu of the OATS measurement where the ambient >> intervenes. >> >>> From: "Tsang Steven" <[email protected]> >>> Reply-To: "Tsang Steven" <[email protected]> >>> Date: Wed, 28 Jan 2004 12:41:22 +0800 >>> 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 >>> >>> _________________________________________________________________ ??? >>> : ???, ????? 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 >> > > > ------------------------------------------- > 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 This message is from the IEEE EMC Society Product Safety Technical Committee emc-pstc discussion list. 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