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Question

# A double conversion superheterodyne receiver is designed with fIF(1)=30MHz and fIF(2)=3MHz. LO frequency of each mixer stage is set at the higher of the two possible value. When the receiver is tuned to a carrier frequency of 300 MHz, insufficient filtering by the RF and first IF stages results in interference from three image frequencies. Those three image frequencies are

A
294 MHz, 360 MHz, 426 MHz
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B
354 MHz, 360 MHz, 366 MHz
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C
357 MHz, 360 MHz,363 MHz
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D
294 MHz, 360 MHz, 366 MHz
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Solution

## The correct option is D 294 MHz, 360 MHz, 366 MHzLet us consider a simple block diagram of a double conversion receiver as follows: fLO(1)=fc+fIF(1)=300+30=330 MHz fLO(2)=fa+fIF(2)=3+30=33 MHz When there is insufficient filtering by RF and IF stage -I, the possible value of fa at the input of IF stage for the selected fLO(2) are , Desired , fa=fLO(2)−fIF(2)=33−3=30 MHz Unwantedf′a=fLO(2)+fIF(2)=33+3=33 MHz The input frequencies that produce fa=30MHz are Desiredfc=fLO(1)−fa=330−30=300 MHz Unwantedf′c=fLO(1)+fa=330+30=360 MHz The input frequencies that produce f′a=36 MHz Unwantedfc′′=fLO(1)−f′a=330−36=294 MHz Unwantedfc′′=fLO(1)+f′a=330+36=336 MHz So, the three unwanted image frequencies are 294 MHz, 360 MHz , 366 MHz

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