Using a Spectrum Analyzer to measure FM Deviation of remote transmitters

[KD0RC]

I have a Rigol DSA 815-TG Spectrum Analyzer, and want to use it to measure FM deviation of remote transmitters. They will typically be voice or AX.25 (packet) signals and will typically transmit in short bursts.  Is there a reasonably easy way to do this?  Does the fact that most of the transmission will be normal voice (perhaps with a subaudible tone thrown in...) make it more difficult?  I don't need extreme accuracy, I just want to be able to identify signals that are badly over or under deviating.

Thanks,
Len

[babysitter]

Hi Len,

indeed, you will see a difference on your SA, but it is also very informative to look at the demodulator output signal of  vanilla type receiver with a o-scope, too. Watch out for clipped waveform, and never underestimate the good old eye diagram in the case ok FSK!

Best results when your receiver is very good aligned, e.g. the carrier signal is smack dab on the true center frequency of your IF filter. In case of FSK one should always consider a group delay optimized IF filter, i think there was a D suffix...

In case the remote transmitter is cooperative you can even measure deviation remotely:

If deviation divided by modulation frequency is 2.4, the carrier will be suppressed (first carrier zero).

If e.g. your partner transmits a slowly rising audio tone, and the center frequency of the spectrum is furthest down on 1 kHz, your partner has 2.4 kHz deviation at the AF level of the modulation. This will be a bit M-like shape of the spectrum, but you could also get a CW receiver listening with small bandwidth on the center frequency.

(Former packet addict, Co-Sysop of DB0HOM 1k2 AFSK and 9k6 FSK loserport here.)

[unicornio]

If deviation divided by modulation frequency is 2.4, the carrier will be suppressed (first carrier zero).

very good explanation of the 'zero of bessel' technique, the beter measure for deviation testing!

good day, mates!

[Melt-O-Tronic]

One of W2AEW's excellent videos is "How to measure FM Frequency Deviation without special equipment using Carrier Null or Bessel Null" ().

From this, it should be clear how you can use the Rigol to do the measurement and will shed some light on babysitter's post if you're a bit thick in the skull like me. 

[KD0RC]

Yep, I have seen Alan's video, and used the spectrum analyzer to find the null and make the calculation.  What I am wondering though, is if rough measurements of deviation can be made from quickly changing voice or packet transmissions, not carefully controlled single tone measurements.

[w2aew]

Yep, I have seen Alan's video, and used the spectrum analyzer to find the null and make the calculation.  What I am wondering though, is if rough measurements of deviation can be made from quickly changing voice or packet transmissions, not carefully controlled single tone measurements.

One way to get a good estimate of the deviation, if you don't have a means to perform the Bessel Null, is to open up the RBW wide enough to swallow up the major sidebands (out to about 20-30dB down).  Then, add a Max hold and Min hold trace.  Then, you measure the delta-f between the Min and Max hold traces at a given power level (say 20dB down from peak).  This delta-f will be roughly 2x the peak deviation. 

This is explained pretty well in the one of the old HP AN-150 appnotes.  See pages 13-14 on this:
http://www.hpmemory.org/an/pdf/an_150-1a.pdf

[KD0RC]

One way to get a good estimate of the deviation, if you don't have a means to perform the Bessel Null, is to open up the RBW wide enough to swallow up the major sideband (out to about 20-30dB down).  Then, add a Max hold and Min hold trace.  Then, you measure the delta-f between the Min and Max hold traces at a given power level (say 20dB down from peak).  This delta-f will be roughly 2x the peak deviation. 

This is explained pretty well in the one of the old HP AN-150 appnotes.  See pages 13-14 on this:
http://www.hpmemory.org/an/pdf/an_150-1a.pdf

Thanks Alan, this is exactly what I was looking for!  As always, you have the info I need.

Len

[w2aew]

One way to get a good estimate of the deviation, if you don't have a means to perform the Bessel Null, is to open up the RBW wide enough to swallow up the major sideband (out to about 20-30dB down).  Then, add a Max hold and Min hold trace.  Then, you measure the delta-f between the Min and Max hold traces at a given power level (say 20dB down from peak).  This delta-f will be roughly 2x the peak deviation. 

This is explained pretty well in the one of the old HP AN-150 appnotes.  See pages 13-14 on this:
http://www.hpmemory.org/an/pdf/an_150-1a.pdf

Thanks Alan, this is exactly what I was looking for!  As always, you have the info I need.

Len

Len - just in case you wanted to see this - here's a quick video...

[KD0RC]

Nice video Alan, thanks!  I was able to get similar results with my DSA 815.  I did have to narrow the RBW to 3 KHz.  At 10 KHz, the waveform is very jagged and cannot be easily measured.  The 815 has a 1, 3, 10 scheme, so a 5 KHz RBW is not available. 

[jrgandara]

Rigol have a technical note for measuring deviation available at:

Measuring deviation with Rigol DSA

I have been using this method and works pretty well. I just want to figure out how to activate a delta marker between traces. That way I can have a readout of the deviation instead visually count screen divisions. I did it by mistake twice, but I´m not having luck to reproduce this setup. I found nothing about delta traces in the user´s manual.

Any suggestions?

[jrgandara]

I figured out how I did a delta marker between two traces:

1) I set the trace 1 to max and hold, turned marker 1 on, put the transmitter on the air without modulation, move marker 1 20dB below and freeze trace 1; Marker 1 stay bond to trace 1.

2) I set the trace 2 to max and hold, put the transmitter on the air with modulation and pressed delta on marker 1.

Now I have marker 1 on trace 1 and delta marker 1R on trace 2 and the deviation readout. But if I freeze trace 2 the delta marker jumps to trace 1.

Someone know the right way to perform this?

[Melt-O-Tronic]

Clever!  I didn't think there was a way to do something like that.