The only *allowed* method to get over the problem of an interfering radio station is rotate the OATS set-up so the radio signal is attenuated by the directivity of the antenna.
In the current situation almost NO OATS will fulfil the requirements for radio-signals to be at least 6 dB below the limits. A workaround may use a shielded room to verify the absence of ANY frequency on the interfering frequency, and use a TCF to show compliance. Measuring the EUT in a chamber (FAR or SAR) is said to be equivalent to an OATS measurement in case of a PASS result only. Officially the OATS remains the reference to compliance. This means that no real straightforward compliant measurement method is available but for those lucky owners of an OATS in the middle of nowhere (where radio signal are virtually absent). (ignoring the problem how to get customers there ;<) Regarding FAR-SAR-OATS measurement correlation: After correcting for distance (3m instead of 10m :: +10dB) and absence of ground plane reflected wave in an anechoic room (::-5 dB), one must also consider that an OATS (and SAR) gets virtually blind (or deaf)for horizontally polarised signal below 200 MHz (in spite of height variations) and a FAR is not, so the signals in a FAR may be 10 times stronger (at 30 Mhz). Add to this the different behaviour of antennas regarding close field, viewing angle and any correlation is getting really difficult and EUT / Antenna dependent. Regarding the signal strength: A real tight uncertainty scheme will provide you with a measurement accuracy of +/- 4.6 dB (see CISPR 16-4) This is about the best one can get with standard methods. Valid only for a small EUT without cables, so using a noise emitter or comb generator. A quick test in "a chamber" will give you an uncertainty that will probably be much larger then 4.6 dB and probably much more than 10 dB. Building your compliance on that is really dangerous. The nulling method below seems perfectly legal to me as long as one can prove that the EUT signal won't be compromised. One may consider putting the re-radiating antenna at least 10 meters BEHIND EUT. As the METHOD is not according to CISPR 16 one will need to use a TCF to show the results remain CISPR compliant. The radio method : I won't use it for compliance testing, possibly not even for pre-compliance, but it is definitely an interesting experiment if you want to learn about the interference capabilities of your EUT for this radio on this frequency. I mean to solve specific problems. Gert Gremmen > Approvals manager > ====================================================== > ce-test, qualified testing > Member of EMC committee CENELEC/IEC > > + Independent Consultancy Services > + Compliance Testing and Design for CE > + Improvement of product quality > + and reliability > + Testing services according to: > Electro magnetic Compatibility 89/336/EC > Electrical Safety 73/23/EC > Medical Devices 93/42/EC > Radio & Telecommunication Terminal equipment 99/5/EC > > Website: www.cetest.nl (english) > www.ce-test.nl (dutch) > Phone : +31 10 415 24 26 > Fax : +31 10 415 49 53 > ====================================================== > From: [email protected] [mailto:[email protected]] On Behalf Of [email protected] Sent: woensdag 28 januari 2004 19:46 To: [email protected]; [email protected]; [email protected]; [email protected] Subject: RE: OATS Measurement 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 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. 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
