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A superheterodyne receiver receives a 900 MHz signal by setting its local oscillator (LO) to 800 MHz and converting to a 100 MHz intermediate frequency (IF). Which is the image frequency that would also be converted to the same 100 MHz IF?

  1. A700 MHz
  2. B1000 MHz
  3. C1700 MHz
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Answer: A. 700 MHz

The difference frequency of the mixer output is used, so any signal that differs from the LO by 100 MHz is converted to the 100 MHz IF. This is satisfied by the desired 900 MHz and by 700 MHz on the opposite side of the LO, and the latter is called the image frequency. A signal at the image frequency cannot be distinguished after conversion to IF, so it is removed by a filter before the mixer (an image-reject filter) or by an image-reject mixer. A higher IF moves the image frequency farther from the desired signal, making it easier to remove with the filter before the mixer.

  • A. Correct. 800 − 100 = 700 MHz, on the opposite side of the LO from the desired signal.
  • B. This adds the IF to the desired signal (900 MHz). A 1000 MHz signal differs from the LO by 200 MHz and is not converted to the 100 MHz IF.
  • C. This is the sum of the received and LO frequencies (900 + 800), an unwanted component that appears at the mixer output. It is not the image frequency.
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