Radio transmitters and receivers
Radio transmitters and receivers tests block-diagram fluency. You should be able to trace a signal from oscillator to antenna, explain what each stage does, and recognise why a superheterodyne receiver needs an IF filter. Questions often ask about mixer products, oscillator stability, amplifier class, or filter selectivity.
Transmitter chain
A typical transmitter is oscillator → buffer → driver → power amplifier → low-pass filter → antenna. The oscillator sets frequency; the buffer isolates it; the driver raises level; the PA delivers power. The output filter reduces harmonics. For CW and FM, the PA can be Class C because the signal is constant-envelope. For SSB or AM, use Class A, AB or B with good linearity to avoid splatter.
Receiver chain
A superheterodyne receiver uses RF amplifier → mixer → IF filter → demodulator → audio amplifier. The mixer converts RF to a fixed IF, where sharp filters are easier to build. Selectivity is set mainly by the IF filter; sensitivity by noise figure and gain; dynamic range by overload and intermodulation performance. AGC holds audio output steady as signal strength varies.
Oscillators, mixers and filters
Oscillators use LC tanks (Colpitts, Hartley), crystals for stability, or PLLs for synthesis. A mixer multiplies two signals and produces sum and difference frequencies: fIF = |fLO − fRF| or fLO + fRF. Crystal, ceramic, LC and SAW filters trade bandwidth, loss, and cost. A product detector is used for SSB/CW; an envelope detector for AM; a discriminator for FM.
Worked example
A receiver has RF = 7.100 MHz and a high-side LO of 7.555 MHz. The difference is 0.455 MHz = 455 kHz, the IF. The image frequency is fLO + IF = 7.555 + 0.455 = 8.010 MHz, equivalently 2fLO − fRF = 15.110 − 7.100 = 8.010 MHz. A preselector or RF filter must reject 8.010 MHz before the mixer.
Traps that cost marks
- Using Class C for SSB: it is efficient but nonlinear and causes distortion.
- Forgetting that the mixer creates both sum and difference products and that one becomes the image.
- Confusing selectivity (adjacent-channel rejection) with sensitivity (weak-signal capability).
- Assuming a higher IF always improves image rejection; it also affects selectivity and spurious responses.
- Ignoring the detector type: envelope detection on SSB gives poor, distorted audio.