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What are the advantages of 75 kHz deviation as allowed in the US versus the narrower 50 kHz (or less) deviation allowed in Europe and some other parts of the world?


Deviation is the measure of how far a frequency modulated RF carrier can change frequency in response to a signal such as audio. The amount of deviation is limited to a maximum value by regulatory agencies or it can be limited to a maximum bandwidth that the signal can occupy centered on the carrier frequency. For instance, the FCC specifies a 75 kHz peak deviation and a maximum occupied bandwidth of 200 kHz. 

FM is a form of spread spectrum modulation since the occupied bandwidth is greater than the bandwidth of the audio signal. For instance, at full modulation, a 1 kHz test tone broadcast by an AM station would occupy a little over 2 kHz of bandwidth but as wideband FM modulation it occupies more than 150 kHz of bandwidth. This additional occupied bandwidth has "process gain" just like any spread spectrum signal and suppresses interfering signals and noise. The greater the deviation, the greater the noise suppression effect. In general, 75 kHz deviation systems have over 3 dB better noise performance than 50 kHz systems, all other things being equal. With a compander in the system, the 3 dB RF link improvement due to the wider deviation sounds like a 6 dB improvement to the ear. There is a downside to the wider deviation and that is at very low levels of RF, the wider deviation loses its advantage over the narrower deviation systems and actually has a disadvantage. However, this occurs only when audio signal to noise ratios are at 12 dB or lower, which is effectively useless for wireless microphone purposes anyway.

Posted 1 year agoby LectroAdmin