Talk:FM Frequency Deviation

Deviation versus Modulation (mike hot) non-conducting ceramic screwdriver

First located the deviation pot and adjust your maximum deviation to +/- 4.8 KHz on a service monitor, deviation monitor or adjust the FM dual basal display on a spectrum analyzer for +/- 4.8 KHz.

Do not set the deviation for more than +/- 4.8 KHz or you will deviate out of many of the new narrow band repeaters.

Now set the mic. gain so you are close talking the microphone, about 1 inch from your lips to microphone in a normal voice so your audio is about +/-4.8 KHz . Remember I said a normal voice not a wisper.

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If you have a scope and access to the discriminator test point (DC coupled), you can set deviation pretty easily:

Attach scope to DC coupled discriminator point in a suitable FM receiver (for the band you're using).

Transmit a carrier from another rig tuned to the same frequency as that receiver and look at the DC voltage created. It should be zero, or nearly zero.

Then, shift the TX carrier 5 kHz one way or the other and measure DC voltage again, using the scope. It should shift. Set the vertical sensitivity adjustment on the scope so that "shift" is a convenient amount to measure, like 1cm or 2cm on the scope display (scopes usually have display markings every 1cm).

Now go back to the original frequency (so TX and RX are both tuned to the same frequency) and talk into the microphone of the transmitter to modulate it.

That should create a demodulated display of your voice on the scope, and the "peak" deviation should be the same as the "shift" indicated earlier when you changed frequencies by 5 kHz; i.e., your peak deviation should be 5 kHz.

If it's grossly under or over that amount, it will be easy to see. You can make deviation adjustments to the transmitter to hit 5.00 kHz pretty closely if you want to; obviously, the more of the scope screen you use, the more closely you can adjust things.

However, "sounding good" and "having the right amount of deviation" really have nothing to do with each other. I have FM transmitters set to exactly 5.00 kHz peak deviation as they should be, and still sounding crappy because they're just not very good FM transmitters and would need a lot of work to sound better.

I make "mike gain" adjustments on FM transmitters by watching deviation and adjusting the gain so I can hit 5 kHz peak deviation just fine if I speak directly into the microphone, within 1/2" distance from mike to lips and then back gain down to the point where if I pull the mike back one inch, the modulation almost disappears. This is about the proper adjustment, because it allows normal deviation while close-talking and absolute minimum background noise modulation. Adjusting mike gain so that you can fully modulate while sitting back 6" or more from the microphone will usually sound awful no matter what kind of equipment you use.

Bessel Curves

Bessel Curves / Bessel Functions

Modulation Index = Peak Deviation / Modulation Frequency