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"Proper" FM Modulation

posted Feb 19, 2012, 8:57 PM by Charles Boling
With a normal AM signal, it's very obvious that you're over-modulating if you look at the "envelope" on an oscilloscope and see it "bottom out" -- you can't reduce the power of your carrier by more than...the power of your carrier!  With FM, there's not such an obvious limitation.  The further the carrier's frequency deviates from its center, the louder the sound coming out of the receiver, but what's the proper amount of deviation?  The answer: Whatever the receiver wants!

The receiver is designed & adjusted to consider a particular amount of deviation to represent the loudest sound it'll put out.  At the present, Amateur radio in the U.S. usually uses a 5kHz maximum deviation; in the old days, it was 10kHz.  Most commercial radio services are now legally limited to 2.5kHz in the U.S. and elsewhere.  Broadcast FM uses 75kHz, being designed for quality rather than efficiency.

What happens if you transmit with your maximum deviation set too low (e.g. 2.5kHz on amateur bands)?  Nothing big -- you will simply sound softer than most other people, and if your signal is weak, it will be harder to hear you above the sound of the noise.  It's roughly the same effect as backing away from the microphone when you talk, but not quite as bad because you don't get the accompanying relative increase of echoes and room noise.

And if you have your deviation set too wide when you transmit?  Like "kissing the mic", you will sound louder up to a point, but once you hit the maximum deviation for the receiver, the sound will simply become distorted and much less understandable.  If a lot of your energy is being "wasted" by flooding adjacent channels instead of the one the receiver's tuned to, your weak signal (to those a long ways away) will become even weaker to them, because much of its energy is being rejected by the tuner.  So...not a good thing to do.  While increasing the deviation above the norm can compensate for a weak microphone, unless you're careful to ensure that sounds are never allowed to over-deviate your transmitter, you will not only sound unpleasant, but you will cause greater interference to people on adjacent frequencies.  Get a new mic.

Of course, adjusting your deviation has the exact opposite affect on your reception; if your radio is set to 2.5kHz (some are by default these days!), you will find that many people on the local amateur radio repeater sound terrible, as if they're speaking too close to the mic, even as you're sounding weak to them.

Fortunately, once you ensure that the deviation is set correctly, most people don't have to worry about it again.  Unfortunately, for those that do, most radios don't make it terribly easy to change -- rather than being able to be stored in a memory like a repeater's offset, there's a single global setting for the radio, typically buried in the menus.  So, for example, if you have a GMRS radio that you are also operating on the 70cm amateur band, or a commercial Fire Dept. radio that you're also using on amateur 2m frequencies, there's no way to automatically switch back and forth; the best you can do is to either remember to change it back and forth, or to set it one way and remember when you speak to be careful; for example, leave it narrow but being especially careful that you're close to the mic when talking to hams, and tolerate their distortion with grace.