Dear all There is a UK EMC test equipment company called Laplace Instruments that claims to have done considerable work on this sort of issue, having found that A-B does not work.
They have special software that uses a calibrated Comparison Noise Emitter to compensate for site variations. It may also be useful against ambients. It is a low-cost pre-compliance system and not intended for full compliance tests. I only know of it through talking to them at the IEEE show in Denver. For more info: their site is www.laplace.co.uk or you could Email David Mawdsley at t...@laplace.co.uk. Hope this helps! Keith Armstrong Partner, Cherry Clough Consultants EMC-UK Associate phone: +44 1457 871 605 Fax: +44 1457 820 145 Email: karmstr...@iee.org Thomas N. Cokenias wrote: > > Muriel, > > It depends on how you are subtracting. > > When measuring with EUT on, you are looking at ambient + EUT signal - in a > logarithmic representation when using the analyzer log scale. Since > subtracting logs is equivalent to dividing numbers, you have to be > careful. > > A worst-case scenario is when you have an EUT signal at the same frequency > as an ambient, but at only slightly lower amplitude that the ambient. I > don't know of any way of extracting the EUT signal using conventional EMI > instrumentation. As I recall, using the subtraction function available on > most analyzers will cancel out the ambient+EUT, giving the impression there > is no EUT signal there. > > Your task is to reduce the ambients being received by your measuring system > to a level more than 6 dB below the spec limit you are measuring to, > hopefully by a lot more than 6 dB. Most specifications consider a > measurement valid only if signal+ambient is at least 6 dB greater than > ambient alone. This way, if the EUT signal is near the limit, you will see > it come and go as you turn the EUT on and off. > > For line conducted tests, try using an AC EMI line filter at the AC input > to the LISN, placed as close as you can to the LISN. The filter should > attenuate the radio signals being picked up by the mains (antenna) wires. > The LISN and the filter should be mounted on the same ground plane. I have > had good results in on-site tests with LISNs, filters, and a portable > ground plane. > > If you are still measuring strong ambients in the .150-30 MHz region, you > may need to take steps to shield your equipment and set-up. > > Best regards > > Tom Cokenias > Consultant, EMC & Radio Type Approvals > > --------- > This message is coming from the emc-pstc discussion list. > To cancel your subscription, send mail to majord...@ieee.org > with the single line: "unsubscribe emc-pstc" (without the > quotes). For help, send mail to ed.pr...@cubic.com, > j...@gwmail.monarch.com, ri...@sdd.hp.com, or > roger.volgst...@compaq.com (the list administrators). --------- This message is coming from the emc-pstc discussion list. To cancel your subscription, send mail to majord...@ieee.org with the single line: "unsubscribe emc-pstc" (without the quotes). For help, send mail to ed.pr...@cubic.com, j...@gwmail.monarch.com, ri...@sdd.hp.com, or roger.volgst...@compaq.com (the list administrators).
