FM Deviation (Video Deviation)
FM deviation is a measure of the modulator sensitivity, in units of MHz/V but is often quoted in MHz. This assumes the peak-to-peak value of the video signal is one
volt including syncronization pulses.
In a link budget calculation, we need the peak-to-peak deviation value by the video signal, in MHz, in order to calculate the S/N value after demodulation in the
If the peak value is quoted, remember to double it to the peak-to-peak value before entering it into the link budget parameter form. Watch out for this one, or a 6dB
error may be introduced into your S/N calculations.
FM deviation (p-p) = 2 x peak value.
With satellites operating on half transponder format the FM deviation value may be reduced (halved) to simulate the effect of reduced S/N since signals from two
channels are modulated onto the same carrier.
If the FM deviation is not known, but you know the bandwidth of the required channel you can use a transposition of "Carsons Rule" to arrive at a reasonable
estimate of peak frequency deviation & FM deviation.
Peak freq deviation = (RF bandwidth - 2 (maximum video freq) ) / 2 MHz
FM deviation (peak-to-peak) = 2 x peak freq deviation MHz
Example 1: Astra 1a (Europe) uses 26MHz bandwidth channels for 5MHz video.
Peak freq deviation (MHz) = ( 26 - 2(5) ) / 2 = (26-10) / 2 = 8MHz.
FM deviation (peak-to-peak) = 2 x 8 = 16MHz..
The FM deviation value quoted by the Astra operators is 16MHz/V. Satmaster incorporates a facility to calculate this if needed. Select "Carsons Rule" under the
Example 2: The Eutelsat II series uses 36MHz bandwidth transponders for 5 MHz video
Peak freq deviation (MHz) = ( 36 - 2(5) ) / 2 = (36-10) / 2 = 13MHz.
FM deviation (peak-to-peak) = 2 x 13 = 26MHz..
The quoted figure is 25MHz/V so, as you can see, a reasonable estimate is always obtained by using Carsons Rule.