A dual conversion superhet has an input frequency of 62 MHz

A dual conversion superhet has an input frequency of 62 MHz and local oscillators of 71 and 8.6 MHz. What are the two IFs? Please show work.

Solution

The products of the mixers can be the sum or the difference between the local oscillators and the signal

the first IF is 71-62 = 9MHz

the second IF is 9-8.6 = 400kHz.

The first IF is a higher frequency so the LO is spaced away from the signal and a good image rejection can be achieved i.e.,71 + 9 = 80MHz so BPF filter for 62MHz signal can reject it properly.

If the 400kHz IF was used for the first IF, the image would be only 800kHz from the desired channel.