Deviation is a term used to describe the amount which a signal moves about a center frequency in an FM transmitter. Maximun Deviation +/- 75Kc simply means that is it's limits of usable MODULATION. You can, in most cases, drive it harder, but that usually results in noise being generated outside the desirable limits of the FM system you are using. And like anything overdriven it will become distorted.

Example: let's say you have a transmitter on 174.000 MHz (megahertz) if you properly modulate (drive) it, the signal would actually deviate 75Kc ABOVE 174.000 and 75Kc BELOW 174.000 (Kc = Kilocycles commonly refered today as kilohertz, same thing)

So we now have a signal that occupies a portion of radio spectrum from 173.925 to 174.075 MHz. This is considered wideband FM. This is the same deviation that commercial FM broadcast stations are suppose to limit themselves to. But that's another story...

Now, What can we do with this wideband FM signal? Well if you have a matching FM wideband receiver, you will be able to send some pretty decent audio wirelessly from point A to point B. However, try and listen to it on a narrowband receiver, (scanner etc...) it will sound like trash. Why? The narrowband receiver is typically limited by filters to a maximum ability to receive +/- 5Kc. +/- 5kc 0r +/- 2.5Kc which is normally used in Fm two-way radio. This is what is deemed needed for proper transfer of voice grade analog two-way radio communications.

But wait!, we want a HIGHER quality signal from our wireless microphone or instrument than just two-way radio quality right? How do we get it? Well like anything else, to get more takes more. Takes more what? BANDWIDTH. We have gone from +/- 5Kc of radio spectrum bandwidth to +/- 75Kc. Now we can transfer a much higher quality signal. Remember though, you gotta have a matching wideband receiver.

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