Could someone please describe to me the calculation behind FM deviation measurement using an oscilloscope?
I've made the measurement thousands of times, but I've just followed the manual. The measurement I make is 30Hz FM with 480Hz deviation on
9960Hz carrier (VOR signal). Following the procedure, iirc, set main timebase to 10us/div, measure deviation at 6th rising edge, should be 48.5us. I've never understood how they came to this.Thanks for any help, Steve
Max freq = 9960 + 480 = 10440 Min freq = 9960 - 480 = 9480
Max period = 1/9480 Min period = 1/10440
Peak-to-peak period variation = 1/940 - 1/10440 = 9.7us
6th rising edge = after 5 cycles5 * 9.7uS = 48.5us
Makes perfect sense, I don't know why I couldn't visualize it. Thanks for your help.
Best, Steve
Steve:
I don't claim any VOR expertise, but if it's of any interest you can use a Windows sound card and my free Daqarta signal generator to synthesize VOR signals. To create the signal you describe, just click Waveform Controls, then FM and set your parameters. Note that due to the AC coupling capacitor on the sound card outputs, there will be a phase offset. I haven't worked out a simple way to measure that yet using Daqarta alone, but if you're interested I will give it more thought. The offset should be constant, so you can easily adjust the modulator phase to compensate, once you know the amount.
Daqarta also allows true phase modulation (PM instead of FM), and you can create up to 4 of these VOR signals per channel at various levels to test interference. Or you can create noisy, fading in and out signals to test fringe reception. Or add jitter, or lots of other tricks. Let me know if there is any particular case you'd like to synthesize, and I will work out a setup.
Best regards,
Bob Masta DAQARTA v3.50 Data AcQuisition And Real-Time Analysis
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