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).
