Modulation, Demodulation and Modes: CW, SSB, FM and Digital
Analogue modulation
AM varies carrier amplitude; total power includes carrier plus sidebands, and the modulation index must stay below 1 to avoid overmodulation. SSB transmits one sideband, saving power and bandwidth; it needs a carrier reinsertion (BFO) at the receiver. FM varies frequency; deviation and modulating frequency set the modulation index, and Carson's rule estimates bandwidth as 2(deviation + highest modulating frequency). Traps include confusing deviation with bandwidth and misreading sideband inversion on lower sideband.
CW and digital modes
CW is on-off keyed carrier, decoded by beat frequency oscillator; bandwidth is narrow (a few hundred hertz). Digital modes include FSK, PSK and multi-tone systems; they trade bandwidth for sensitivity. Know the concepts of symbol rate, baud versus bit rate, error correction and the need for accurate frequency and timing. A common trap is assuming a higher baud rate always means better weak-signal performance.
Demodulation
Understand envelope detectors for AM, product detectors for SSB/CW, discriminators and PLL detectors for FM, and DSP demodulation. Emphasise that the demodulator must match the mode and bandwidth.
Worked example
An FM signal with 5 kHz deviation and 3 kHz maximum audio needs roughly 2(5 + 3) = 16 kHz bandwidth by Carson's rule; a 12.5 kHz channel would be too narrow and cause distortion.