#159: How to measure FM frequency deviation with a spectrum analyzer

This short video shows a quick way to get a good estimate of the frequency deviation of a FM transmitter using a spectrum analyzer. This method isn't as accurate as the Bessel Null method shown in this video:
But is is faster and easier to setup, and doesn't require a single, adjustable modulation tone. The RBW is set wide enough to "swallow up" the sidebands, and then a min hold and max hold trace are used to capture the excursion. The frequency difference between the skirts of these two traces represents the peak-peak deviation. Note that the wattmeter is included for two reasons - to monitor the output power, and to provide a low-level RF sample of the transmitter output to send to the spectrum analyzer. This could also be done with any other form of RF sampler or tap, such as a directional coupler, etc. Just be sure to keep the power injected into the spectrum analyzer at a safe level.

[area]

Fascinating....i forgot what db/m means...have to go back and watch that other one you did.last week...I can see the value of that analyzer now WOW....powerful unit

[julesfromspace]

Thanks for the great videos! Quick question about deviation and packet work. I’ve bought and built a tnc-x and there is an internal deviation adjustment. What should I look for when adjusting within the tnc for correct deviation? Everything I’ve read or heard so far mentions just going by ear or until the packets go through. How could I be more precise when tuning?

[jakemcgrae]

i forgot how to connect a fax machine to a tds2014 (or related equipments such as Renselair boards) .

i need to have a look at the general fax modulation.
amnesia isn't serving me too well.
not sure if a tds2014 can store the entire a4 sheet of intensities.

would u know if i have global, selective, or transient amnesia?

[josegermanparra]

Thanks again! I was able to get an estimate of the deviation of an old signal generator with FM modulation using my basic Siglent 100MHz oscilloscope. But not in real time (the scope only takes 1024 points, not enough frequency resolution) but saving the signal values (about 20000 points) and doing DFT via software.

[andrewferg]

Video Request: Can you PLEASE (x10) do a benchtop build of an FM broadcast band receiver using discrete components? There is very little quality information online. I have had success w Alan Yates' regen receiver, but it's quite tricky and I'm not sure if that architecture is suitable for heterodyning. FM circuits generally are quite difficult for amateurs like me, especially without an expensive VHF / UHF signal generator.

[chasbader]

I have that radio! Nice spectrum analyzer. Love your videos. AL60

[AliMirjamali]

Thanks for the video. Would it be advisable to measure WiFi 2.4GHz frequency deviation and output power of Access points & NICs with a similar config? This would require 3GHz Spectrum Analyzer upgrade for MDO3000 I suppose?

[tinasilalahi]

Do you have any application suggestions for measuring radio frequencies?

[jimlanigan]

Could you do a quick video showing how to measure the bandwidth while talking in different tones (maybe a whistle to reach the maximum audio range). I see others like me were getting confused since FM bandwidth is usually about 12.5kHz. Thanks again

[andrewferg]

Is there a rule of thumb for how much AF power you need to inject at the oscillator to achieve sufficient deviation?

[j.t.johnston]

I always thought deviation would be measured +/- from the center frequency. I would have measured from the center to the max marker. Perhaps I have been confusing +/- deviation with signal bandwidth?

[basbas]

Why deviation are not 10 khz ? 2khz are noise rumor ? The standard measure is delta from F carrier to -3dB of carrier ?

[yuanchenzhu]

I am confused with your way of using menu markers to get peak-to-peak frequency deviation. I kind of suspect that the 8 khz p-p frequency deviation reading in your case is actually obtained by SA itself, not delta value between markers along a common level of max and min hold. Can you justify your measurement a bit?since I also find the delta value at different power levels are not a constant, Tbh I even doubt that delta value way is correct to make this measurement. Hope you can shed some light on my concern, thanks!

[styvoll]

Hi I'm looking for a software that can retrieve music information from radio.. 100k songs a day. Kindly help. thanks

[Ibelieve]

how do you tell if radio is legal as far as the narrow band?

[waynegram]

How do you know what is the maximum deviation for the carrier frequency? because their must be regulations on each carrier frequency on how much FM deviation the carrier deviation SWING can be but not sure who sets these regulations or how they come up with this Deviation swing.

[mpexkm]

can this method be used with a rtl-sdr???? THANK YOU VERY MUCH FOR THE NICE VIDEO. Kostantinos SV5FRW

[PerchEagle]

But what this example related to ? WiFi signal, audio signal over phone, audio signal captured by antenna .. or what exactly ?

[gizmo]

Why isn't min-hold going down to the noise floor? thats the way it reacts at my spectrum analyzer. I cant reproduce this, unfortunately :(

[thalb]

nice oscilloscope, did you buy that at walmart?