An AM receiver is tuned to 855 kHz. If IF is 455 kHz, what is the local oscillator frequency?

1) The local oscillator frequency (f₀) is made greater than the signal frequency (f_s) in the radio receiver. 

2) The local oscillator frequency range is 995 kHz to 2105 kHz for the MW band.

f_max/f_min = 2105/995 = 2.2 

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

f_max/f_min = 1195/85 =14.0

4) The normal tunable capacitance ratio is, C_max/C_min = 10

5) This capacitance ratio easily gives the frequency ratio of  2.2:1.

6) The 2.2:1 ratio required for the local oscillator operating above signal frequency is well within the range,

7)  The other system has a frequency ratio of 14:1 whose capacitance is not practically available.

Concept:

In a standard AM superheterodyne receiver the local oscillator frequency (\({f_{Lo}}\)) is given by:

For high side tunning or up-conversion, we have:

\({f_{Uo}} = {f_s} + {f_{IF}}\)

For low side tunning or down-conversion, we have:

\({f_{Lo}} = {f_s} - {f_{IF}}\)

Where,

f_S : carrier frequency of the desired AM radio signal.

f_IF : intermediate frequency.

Calculation:

\({f_{Uo}} = {f_s} + {f_{IF}}\)

= (855 + 455) kHz

= 1310 kHz