Transmitters: Architecture, Stages, Keying and Output Systems
Signal chain
A transmitter typically comprises an oscillator, buffer, frequency multipliers or PLL synthesiser, driver, power amplifier, low-pass filter and antenna matching. Know that buffers isolate the oscillator from load changes, multipliers produce harmonics deliberately, and the final low-pass filter removes harmonics before the antenna. Traps include confusing the purpose of a driver with that of a buffer, and forgetting that a mistuned PA can generate spurious products.
Modes of amplification and keying
CW keying must avoid clicks: shaped keying (raised cosine) limits keying sidebands. SSB generation uses filter or phasing methods, and linear amplification is mandatory to preserve the envelope. FM can use class C because the envelope is constant. Understand ALC: it reduces drive to prevent overdrive and splatter, but excessive ALC action can distort. Know the meaning of PEP, average power and duty cycle for different modes.
Output and protection
SWR protection, thermal shutdown and drain/grid current limits protect the PA. A mismatched load raises dissipation and can cause failure. Know how to interpret plate current, grid current and output power readings, and the significance of neutralising a triode amplifier to prevent self-oscillation.
Worked example
A 100 W PEP SSB transmitter with 12 dB gain ahead of the final needs 100/16 ≈ 6.3 W of drive, since 12 dB is a power ratio of about 16. If drive is doubled to 12.6 W, the PA saturates and splatter results; ALC should limit this.