Question

A super heterodyne receiver operates in the frequency range of 58 MHz − 68 MHz. The

intermediate frequency f_IF and local oscillator frequency f_LO are chosen such that f_IF ≤ f_LO. It is required that the image frequencies fall outside the 58 M Hz − 68 MHz band. The minimum required f_IF (in MHz) is ________.

Answer 1

Concept: 

The relation between image frequency and intermediate frequency is given by

\({{\rm{F}}_{{\rm{si}}}}{\rm{\;}} = {\rm{\;}}{{\rm{f}}_{\rm{s}}}{\rm{\;}} + {\rm{\;}}2{\rm{IF}}\)

Application: As \({{\rm{F}}_{{\rm{si}}}}{\rm{\;}} = {\rm{\;}}{{\rm{f}}_{\rm{s}}}{\rm{\;}} + {\rm{\;}}2{\rm{IF}}\)

IF the IF is to be chosen such that image frequency is outside the band 58 MHz - 68 MHz then

58 MHz + 2 IF ≥ 68 MHz

⇒ IF ≥ 5 MHz