A superheterodyne receiver is tuned to

PHYSICS · Multi-part · Fermi University Physics · Electronics, Instrumentation & Communication · Communication systems and RF engineering · Communication systems and RF engineering

Electronics for Measurement, Ch Modulation, receivers and the link that carries the measurement

Problem

f_{\rm RF} = 137.5\,\mathrm{MHz}

. Its local oscillator sits above the received frequency, at

f_{\rm LO} = 148.2\,\mathrm{MHz}

, and the intermediate-frequency filter is centred on

f_{\rm IF} = 10.7\,\mathrm{MHz}

. A technician reasons: "The image is a frequency that produces the same intermediate frequency after mixing, and the wanted signal and its image sit symmetrically about the carrier, one intermediate frequency either side. So the image must be at

f_{\rm RF} - f_{\rm IF} = 126.8\,\mathrm{MHz}

." ((a)) Find the error in this reasoning. ((b)) State the correct image frequency.

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