Low-power transmitter
A simple low-power transmitter may contain a frequency source, buffer, driver, modulator or keying stage, RF power amplifier, harmonic filter and antenna matching network. The exact design depends on the mode and frequency.
Oscillator and buffer
The oscillator sets the carrier frequency. A buffer isolates it from changes in the load. This reduces frequency pulling when later stages or antenna loads change.
RF driver and power amplifier
The driver supplies the signal level needed by the final stage. The RF power amplifier raises the signal to the required output power. Bias and cooling must suit the device and operating class.
Semiconductor transmitter
A modern transmitter often uses transistor oscillators or synthesizers, driver amplifiers, RF power transistors and switching devices. Modulation may take place at low level or in later stages, depending on the design.
Frequency stability
Stability is the ability to remain close to the intended frequency. Temperature, supply changes, component ageing, load variation and mechanical stress can cause drift. Crystal references and synthesizers improve stability.
Harmonics and spurious emissions
Harmonics occur at integer multiples of the fundamental frequency. Spurious emissions can also arise from mixing, oscillation or poor circuit layout. Low-pass and band-pass filters reduce unwanted output.
Modulation quality
Over-modulation in AM causes distortion and splatter. Excessive FM deviation uses too much bandwidth. Poor SSB audio processing can cause unwanted bandwidth. Correct drive and filtering are important.
Dummy load
A dummy load absorbs transmitter power during testing. It should have the correct impedance, power rating and frequency range. It allows tests without intentionally radiating a signal.