Mumbai University > Electronics > Sem 4 > Priniciples of Communication Engineering

• The local oscillator frequency (f0) is made greater than signal frequency (Fs) in radio receiver:

  Local oscillator frequency range is 995 KHz to 2105 KHz for MW band.

  Fmax/Fmin = 2105/995 = 2.2

• If local oscillator has been designed to be below signal frequency, the range would be 85 to 1195 KHz and frequency ratio is,

  Fmax/Fmin = 1195/85 =14.0

  The normal tunable capacitance ratio is, Cmax/Cmin = 10

  So this capacitance ratio easily gives the frequency ratio of 2.2:1.

  Hence, the 2.2:1 ratio required for the local oscillator operating above signal frequency is well within range whereas the other system has a frequency ratio of 14:1 whose capacitance are not practically available.