Bill- Seeing as how I am seldom at a loss for words in technical matters, even when I am dead wrong, let me take a whack at this.
So far as I know, the averaging is a 1 Hz post-detection filter (1 Hz video bandwidth), single pole, i.e., an RC filter with a 1 Hz corner frequency (or the digital equivalent). To do it right, the averaging should be done on a linear signal (e.g., on the detected signal BEFORE going through the "log amp", not after. You could still display the results in a log display, so the "log amp" could be applied to the signal after the average filter. Side note: When measuring the average amplitude of Gaussian white noise, the narrow video filter needs to be before the "log amp". If the filtering is done after the "log amp", the noise power will read low by about 2.3 dB (if I remember correctly). This is because the statistics (distribution) of Gaussian noise are different from the statistics of the log of Gaussian noise. I don't recall if the "meter" is specified for average measurement, as it is for the quasi-peak measurement. Even if it is, I would think that the average signal is slow enough that the funky dynamics of the "meter" don't really come into play. Note: I use the phrase "log amp" because classical log amps are seldom used now. The log display in modern spectrum analyzers is generated using data from A/D converters, with the log function done in computer code. I suspect that your "EMI average detector" is a preset that gives you the 1 Hz post detection filter and then runs the signal to the "log amp" The multiple sweep average would probably give the same answer if the average were of the signal before the "log amp", assuming that there are no amplitude variations in the signal with frequencies approaching 1 Hz from the high side. Of course, this is not how the standard is written. And if the signal does have amplitude variations approaching 1 Hz, then you will get different readings with the conventional average measurement. Also, one would need to carefully specify how the multisweep average is done. The usual algorithm is an exponentially decaying contribution from older sweeps, with the number of sweeps in the decay sequence usually set-able by the user. I am not exactly sure how one would go about selecting this to result in a response equivalent to a 1 Hz video filter. The sweep rate will be generally be reduced when the 1 Hz video filter is selected. Sweep rates are set by the spectrum analyzer to make sure that ALL the filters, both IF and post detection, have time to respond fully when sweeping through a signal. Donald Borowski Schweitzer Engineering Labs Pullman, Washington, USA From: Bill Owsley <wdows...@yahoo.com> To: "EMC-PSTC@LISTSERV.IEEE.ORG" <EMC-PSTC@LISTSERV.IEEE.ORG> Date: 12/06/2011 09:17 AM Subject: Average detector/s Sent by: emc-p...@ieee.org 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 message is from the IEEE Product Safety Engineering Society emc-pstc discussion list. To post a message to the list, send your e-mail to <emc-p...@ieee.org> All emc-pstc postings are archived and searchable on the web at: http://www.ieee-pses.org/emc-pstc.html Attachments are not permitted but the IEEE PSES Online Communities site at http://product-compliance.oc.ieee.org/ can be used for graphics (in well-used formats), large files, etc. Website: http://www.ieee-pses.org/ Instructions: http://listserv.ieee.org/request/user-guide.html List rules: http://www.ieee-pses.org/listrules.html For help, send mail to the list administrators: Scott Douglas <emcp...@radiusnorth.net> Mike Cantwell <mcantw...@ieee.org> For policy questions, send mail to: Jim Bacher: <j.bac...@ieee.org> David Heald: <dhe...@gmail.com>
