# Why should local oscillator frequency be greater than signal frequency in AM receiver? Also explain why IF has constant value?

Why should local oscillator frequency be greater than signal frequency in AM receiver? Also explain why IF has constant value?

Reason for LO frequency to be greater than signal frequency

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.

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