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Thread: Measurement of average power

  1. #1

    Default Measurement of average power

    I am trying to understand if the Wi-Spy would work in a project I am involved with. Over what period of time can the measurement of average power be determined? What is the longest period of time that can be programmed for the Wi-Spy to average over? I am interested in measuring the duty cycle of emissions from SmartMeters (electric power meters that contain 1 W 902-912 MHz unlicensed transmitters) that, typically, have very low duty cycles. Could I get meaningful measures of duty cycles that might be at least -20 dB (i.e., about 1 percent?).

    Thanks for your help.

  2. #2

    Talking

    Try using the Spectral View (the "Waterfall" chart). The predominant color bursts will indicate peak amplitude (power), and the time scale will provide duration. Peak power over time will provide your average (-47dbm over say, 3 seconds). If the signal is really bursty (I suspect it is), you may want to use very short duration time intervals, or break the intervals into groups and average them seperately. With the power meters, due to their mostly dormant duty cycles, you will probably get an average power close to zero if you don't shorten your time sample significantly.
    Or, if you are a math guru, you could apply some statistics to your sample points...but that kind of math makes my head hurt.

  3. #3

    Default Average power NOW done correctly!

    Ryan Woodings, the Chief Geek at MetaGeek, was kind enough to take my inquiry, posted way back on March 3, 2010, on the issue of true average power seriously! I note that, as of today, there have been 361 views of my posting with only 1 response. Ryan has modified the code in the Channelizer software to, now, properly compute and display the true average power at any given frequency as displayed by the software (version 3.4)! I checked this out by acquiring a series of spectral scans using the 2.4 GHz analyzer and then opening up the .wsr file, sucking it into Excel, and independently computing the average power over the number of scans and confirming that the numbers given in the Inspector feature of the software are exactly correct. Very nice work Ryan and thank you for your interest in making this revision.

    Also, very nicely, Ryan displays the duty cycle of the measured fields at each frequency for which you create a marker. This is one of the things that I felt could be very useful to folks who are exploring highly intermittent sources. The only remaining item on my wish list would be an ability to export the displayed peak and average and duty cycle values as a file that could be conveniently subjected to further analysis with other applications, such as Excel.

    Thanks Ryan!

    ...Ric Tell

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