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In an AM system receiver. the mixer translates the carrier frequency fc to a fixed intermediate frequency of 450 kHz. Given a broadcast signal of frequency 900 kHz. What is the corresponding image frequency when a superhetrodyne receiver is employed with local oscillator frequency greater than the broadcast signal frequency?

In an AM system receiver. the mixer translates the carrier frequency fc to a fixed intermediate frequency of 450 kHz. Given a broadcast signal of frequency 900 kHz. What is the corresponding image frequency when a superhetrodyne receiver is employed with local oscillator frequency greater than the broadcast signal frequency? Correct Answer 1800 kHz

Concept:

Image Frequency

  • It is an undesired input frequency at the receiver end which can also be demodulated by the superheterodyne receivers along with the desired incoming signal. This results in two stations being received at the same time, resulting in interference.
  • Image frequency is given by fi = fRF + 2fIF­, where fRF = frequency of desired incoming signal and

           fIF = Intermediate frequency.

           Graphically,

           [ alt="F2 S.B Madhu 11.10.19 D 12" src="//storage.googleapis.com/tb-img/production/19/10/F2_S.B_Madhu_11.10.19_D%2012.png" style="width: 259px; height: 115px;">

Calculation:

fi = fRF + 2fIF­

fi = 900 + 2(450) = 1800 kHz

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