Good Day, Very interesting discussion about making average measurements. I brushed away a few cobwebs and dug out an old White Paper:
"Techniques For Measuring Narrowband And Broadband EMI Signals Using Spectrum Analyzers" by Siegfried Linkwitz & Al Wilcox of HP's Signal Analysis Division. I've no idea how old it is, (no date / I've had it for 12+ years) >From pp 21-22... "Average detection is obtained using video filtering in the linear amplitude display mode. Video (post detection) filtering provides averaging of the higher frequency components (such as noise) at the output of the envelope detector. When the video filter bandwidth is narrower than the resolution bandwidth filter, averaging occurs. Narrowband (e.g., CW) signal amplitudes are not affected by video filtering. For true average, the Video BW must be less than the lowers PRF, the frequency sweep must be slow enough to let the filters charge completely and the spectrum analyzer must be in the Linear amplitude display mode. With the analyzer in Log amplitude mode, video filtering greatly reduces the amplitude of the impulsive and random broadband signals. The amplitude of narrowband signals is unaffected as shown above." [sorry no diagram - Bob] "With the analyzer in Linear amplitude mode, video filtering provides the average value of a signal. In the log amplitude mode the analyzer's video filter smoothes the logarithmically distorted detector output signal. For BB impulsive signals the smoothed indication is considerably lower than the average value of the impulses." [Lot's of other interesting stuff in this paper too] To Steve Kuiper (who posted the original question): If you can still get the HP Product Note 85650A-1; (another ancient document) "A Guide to EMI Measurements Using the HP 85650A Quasi-Peak Adapter with the HP 8568A or 8566A Spectrum Analyzer" it has comprehensive instructions on making manual QP/AV measurements. FWIW I must have a light touch. It took me 11 years to break the keyboard on my (departed 8568B) SA. It was the only repair it ever needed. Bob Sykes EMC Engineer Gilbarco Inc. --------- This message is coming from the emc-pstc discussion list. To cancel your subscription, send mail to [email protected] with the single line: "unsubscribe emc-pstc" (without the quotes). For help, send mail to [email protected], [email protected], [email protected], or [email protected] (the list administrators).
