GATE 2016 EC – Question 32
A superheterodyne 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 $f_{IF} \leq f_{LO}$. It is required that the image frequencies fall outside the 58 MHz - 68 MHz band. The minimum required $f_{IF}$ (in MHz) is ________
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Correct answer: 4.9 to 5.1
Explanation
With $f_{LO} = f_s + f_{IF}$, the image frequency is $f_s + 2f_{IF}$. For every signal in the band to have its image outside it, even the lowest signal at 58 MHz needs its image above 68 MHz. So $58 + 2f_{IF} > 68$, which gives $f_{IF} > 5$, and the minimum is 5 MHz.