Radio Receivers and Transmitters
Receiver architecture
The superheterodyne receiver converts incoming signals to a fixed intermediate frequency (IF) using a local oscillator and mixer, then filters and amplifies at the IF before demodulation. Selectivity is set mainly by IF filters; sensitivity by front-end noise figure and gain. The image frequency sits 2 × IF away from the wanted signal and must be rejected by front-end tuned circuits or a preselector. AGC holds audio output steady as signal strength varies; squelch mutes the receiver when no carrier is present.
Modulation and transmitters
AM varies carrier amplitude, with sidebands at carrier ± modulating frequency and a total bandwidth of twice the highest modulating frequency. FM varies carrier frequency; deviation and modulation index determine bandwidth, approximated by Carson's rule: BW ≈ 2(deviation + highest modulating frequency). SSB transmits one sideband with the carrier suppressed, halving bandwidth and concentrating power. A transmitter typically comprises oscillator, buffer, multiplier or driver, power amplifier and output filter; harmonic and spurious outputs must be suppressed.
Traps and rule of thumb
Classic traps: forgetting that the image is on the other side of the local oscillator, confusing selectivity with sensitivity, and miscalculating FM bandwidth. Remember that an IF filter does not fix an image response - that is a front-end job.
Worked example: a receiver with a 10.7 MHz IF tuned to 100 MHz has its local oscillator at 110.7 MHz and an image at 121.4 MHz.