Reason for LO frequency to be greater than signal frequency
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.
Why IF has constant value?
*The problem in the TRF receiver are solved in this receiver by converting every selected RF signal to a fixed lower frequency called as the Intermediate frequency(IF)
*This frequency contains the same modulation as the original carrier. The IF signal is then amplified and detected to get back the modulating signal.
*As the IF is lower than the lowest RF signal frequency the possibility of oscillations and instability is minimized.
*Also the required value of Q for constant BW does not depend on the frequency of desired signal because the IF is constant and same for all the incoming RF signal.