FM deviation measurement

[Steve]

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

 

[Andrew Holme]

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 cycles

5 * 9.7uS = 48.5us

 

[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 cycles

5 * 9.7uS = 48.5us

Makes perfect sense, I don't know why I couldn't visualize it. Thanks
for your help.

Best,
Steve

 

[Bob Masta]

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

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
www.daqarta.com
Scope, Spectrum, Spectrogram, FREE Signal Generator
Science with your sound card!