Here is an excerpt from an old (~1983) H.P. RF measurement presentation by Siegfried Linkwitz and Al Wilcox. // Average detection is provided in the spectrum analyzer by use of the video bandwidth filters. 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, averaging occurs. Narrowband (e.g., CW) signals amplitudes are not affected by video filtering. For a true average, the video BW must be less than the lowest pulse repetition frequency (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. When the analyzer is in the log amplitude display mode, video filtering greatly reduces the amplitude of impulsive and random broadband signals. This is useful for measuring lower level narrowband signals in the presence of higher level impulsive signals. The amplitude of the narrowband signal is unaffected and it will show on the display well above the broadband signals. // Perhaps another piece of the puzzle falls into place? -Bob Sykes
________________________________ From: emc-p...@ieee.org [mailto:emc-p...@ieee.org] On Behalf Of Bill Owsley Sent: Tuesday, December 06, 2011 5:23 PM To: Ken Javor; EMC-PSTC@LISTSERV.IEEE.ORG Subject: Re: [PSES] Average detector/s Indeed it does. The measured signal in this particular case is broadband stuff of a modulated carrier. So far I can get a 20 dB spread when measuring the signal with all the different buttons and techniques available for average. And you know the limit falls right in the middle !! And the fingers start pointing... The instrument manual says the average detector finds the average of everything in each measuring window and reports that. "instantaneous" average ?!?! And then do a trace max hold on that, and it begins to look similar to peak. Now if I turn on averaging over time, for instance a running average of a 100 runs, The reported value settles down nicely. But, I suspect any interference has already happened. Or, If I slow the sweep time down, then the same settling occurs as each window is averaged during the measurement dwell and that number is reported. I have not found a reference to an averaging time. Is there one? ________________________________ From: Ken Javor <ken.ja...@emccompliance.com> To: EMC-PSTC@LISTSERV.IEEE.ORG Sent: Tuesday, December 6, 2011 4:43 PM Subject: Re: [PSES] Average detector/s If video averaging is done with a log amplifier instead of linear, then a geometric mean is taken rather than an arithmetic mean, the latter being an average detector. Both means naturally yield the same output with a cw input, but as the variation in the averaged quantities gets greater (standard deviation increases) the difference between the two types of means gets larger and larger. Hence, for an intermittent broadband signal of high amplitude but very low duty cycle and duration, one would expect very different outputs when using the log display vs. linear. Ken Javor Phone: (256) 650-5261 ________________________________ From: <ralph.mcdiar...@schneider-electric.com> Date: Tue, 6 Dec 2011 11:15:40 -0800 To: Bill Owsley <wdows...@yahoo.com> Cc: <EMC-PSTC@LISTSERV.IEEE.ORG> Subject: Re: [PSES] Average detector/s For in-house pre-compliance testing, we use a VBW of 10Hz to approximate the time response of an AVG detector. Seems to work quite well. ________________________________________________________________________ _____________ Ralph McDiarmid | Schneider Electric | Renewable Energies Business | CANADA | Regulatory Compliance Engineering From: Bill Owsley <wdows...@yahoo.com> To: EMC-PSTC@LISTSERV.IEEE.ORG Date: 12/06/2011 09:15 AM Subject: [PSES] Average detector/s ________________________________ Quasi-peak was fun, so now let's have fun with the Average detector/s, each and all of them ! For reference, EN 302 208-1, Sec. 8.3, 8.4, 8.5. Detection mode: Averaging Which averaging would this be? On the SA that I'm using, There is video averaging by reducing the VBW to something a lot smaller than RBW. There is EMI average detector, and There is average detector which has two modes, power and video. (one is the log of the average of power, the other is the average of the log of power.) And there is a multiple sweep averaging of the above. And there is adjusting the sweep time while averaging is turned on. And adjusting the span to be measured has an effect on the average measurement. Each can give a different result. So starting with the always correct answer in EMC, "it depends," what are these dependencies? ______________________________________________________________________ This email has been scanned by the Symantec Email Security.cloud service. ______________________________________________________________________ - ---------------------------------------------------------------- This message is from the IEEE Product Safety Engineering Society emc-pstc discussion list. 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