Differences

This shows you the differences between two versions of the page.

--- communication:fm_improvement_factor [2010/06/15 22:27] – rocketboy
+++ communication:fm_improvement_factor [2010/06/15 22:50] (current) – rocketboy
@@ Line 1: / Line 1: @@
 ====== FM Improvement Factor ======
-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).
+The signal to noise ratio (S/N) of an FM 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
+       * **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.
+  * **Modulation Index** - Mi - is the ratio of the frequency deviation (Fd) to maximum frequency (Fm) present in the modulating signal.
     Mi = Fd / Fm
@@ Line 13: / Line 13: @@
      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.
+As the modulation index grows (Fd >> Fm) the bandwidth becomes proportional to the frequency deviation. So at large values of Mi doubling the deviation doubles the channel bandwidth.
-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.
+Increasing the deviation increases the output voltage of the de-modulated signal at the receiver. Doubling the deviation doubles the output voltage and hence results in a 6db (power) improvement.
+However doubling the channel 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. I.e. channel bandwidth can be traided for S/N.
 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).
+For this to work practically the carrier peaks must be more than the noise peaks - where the RMS power of the carrier (C) is greater than the channel noise (N) by 10dB (C/N).
+This is called the FM improvement threshold.
+Thus the improvement for a given transmit power only holds while the received channel noise for the widened channel is 10dB less than the carrier power.