Transmitters, Receivers, and Transceivers: Architecture, Performance, Interference Control
Superheterodyne architecture
Most receivers mix the incoming signal with a local oscillator to reach a fixed intermediate frequency, where filters and gain are cheap and stable. Image response is the classic weakness: the image sits on the opposite side of the local oscillator, so with high-side injection the image frequency = signal + 2 x IF. RF preselection and a high IF push the image away.
Performance measures
Sensitivity is the weakest signal giving usable output, often 12 dB SINAD. Selectivity is rejection of adjacent channels, set by IF filters: crystal, ceramic, mechanical, or SAW. Dynamic range and third-order intercept describe strong-signal handling; intermodulation between two strong signals creates products on your channel. Blocking and desensitization are the same story at close range.
Transmitters
A transmitter is oscillator, buffer, drivers, final amplifier, harmonic filter, antenna. Class C finals need a tuned load; a mismatched antenna raises dissipation. Spurious outputs come from harmonics, parasitics, and mixer products and must be suppressed by filtering, shielding, and bypassing.
Receivers and controls
AGC holds output constant as input varies; squelch mutes between transmissions; noise blankers and limiters handle impulse noise. Direct-conversion and software-defined receivers digitize earlier, trading analog filters for DSP.
Ships' gear
VHF channels may be simplex or semiduplex; Channel 70 is DSC only; DSC alerts on the calling channel and traffic then moves to a working channel.
Rule of thumb: image frequency = signal + 2 x IF when the local oscillator runs high.