Consider an AM station broadcasting on a carrier frequency o

Consider an AM station broadcasting on a carrier frequency of 500 kHz. The radio has a 10 KHz wide fixed IF filter centered at 800 KHz. Find the two possible frequencies for the local oscillator and for each of these two frequencies find the corresponding image frequency.

Solution

local oscillator frequencies=>Frf-Fif = 800-10 = 790Khz

                                           Frf+Fif= 800+10 =810kHz

With the LO at 790kHz and the frequency wanted at 10kHz the image would be at 1590kHz

with the LO at 810kHz and the wanted frequency at 10kHz the image would be at 1610kHz

another way of looking at this is :-

Frequency + (2 x IF) = image frequency

thus 10khz + (2 x 800kHz) = 1610kHz

and

Frequency - (2 x IF) = image frequency

thus 10khz - (2 x 800kHz) = 1590kHz