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

receiver.

 

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.

 

Estimating FM Deviation

 

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

"Calculate menu".

 

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.