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Thread: Techniques for Calibrating Deviation Meter

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    Default Techniques for Calibrating Deviation Meter

    Hello to the group,
    I have a task I am working on, and am stuck.
    I am trying to figure out how to precisely measure deviation with the only standard available being a 10 MHz GPS disciplined oscillator.
    I have access to a oscilloscope, and a signal generator and a spectrum analyzer that is lockable to the standard.
    But my deviation meter is not calibrated. Nor is my FM signal generator.

    Is there a way to use just this equipment to measure deviation?

    Thanks in advance for your comments.

    John


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    Look up Bessel function or Bessel node.

    Why does this sound like a school assignment?

    Sent from my SM-T350 using Tapatalk

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    Practically, the easiest way would be to take a random talkie or mobile, and use a reliable calibrated Service Monitor/Communications Analyzer to measure peak deviation. Then, using the Spectrum Analyzer, if it's very accurate you might be able to measure the actual spreading of the carrier at peak deviation levels. If you can see it move when you shout into the mic, you can probably take an approximate reading.

    1. Set up or pick a channel with no PL or DPL, analog, or digital of some flavor.
    2. Whistle loudly, directly into mic while keying if analog. If digital, just key it.
    3. Measure deviation for reference level.
    4. Use same radio with similar technique to step #2, and measure horizontal waveform spreading out with Spectrum Analyzer.

    Directly whistling into the mic should peg the deviation to whatever MAX it's set to, so that level is easily repeatable by pegging it again with the device being tested/calibrated. A digital channel should have a fairly constant deviation level pretty much regardless of audio modulating signal, if any. It's not super accurate or scientific, but it should do for most cases. There is no doubt a proper, accurate way to measure the deviation as RFI-EMI-GUY referred to above.
    Last edited by Alpha; Mar 11, 2019 at 05:57 AM.

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    +1 for the Bessel null method.

    Its very reliable, especially on a modern digital spec an.

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    Bessel as well ....need a SA to look at the gen output ...has to have a low scan rate like 1khz per div or 10Khz span.
    Even an old IFR 1200 would do that and the SuperS had that built into the firmware to do a Bessel deviation check.