The signal to noise ratio (S/N) of an FM data link can be improved by increasing the modulation index (up to a certain point).

• *Frequency Deviation - Fd - is the change in carrier frequency due to the peak voltage applied to the modulator Modulation Index** - Mi is the ratio of the frequency deviation (Fd) to maximum frequency (Fm) present in the modulating signal.

  Mi = Fd / Fm

An approximate rule for the bandwidth (Bw) of a FM signal is given by:

   Bw = 2(Fd + Fm)     - (this is called Carson's rule).

As the modulation index grows (Fd » Fm) the bandwidth becomes proportional to the frequency deviation.

Increasing the modulation index increases the output voltage of the de-modulated signal at the receiver. At large values doubling Mi doubles the channel bandwidth but also doubles the output voltage. Doubling the output voltage gives a 6db (power) inprovement - but doubling the Bandwidth also doubles the received noise power (i.e. 3db). Thus with an FM signal a 3db improvement in S/N can be achieved for each doubling of FM bandwidth.

This is called the FM improvement factor.

For this to work practically the carrier peaks must be more than the noise peaks (where the RMS power of the carrier is 10dB greater than the channel noise).