Transmitter basics

What it is

A transmitter makes a clean radio-frequency signal at the frequency you chose, puts your message onto it (modulation), and amplifies it to the power you need. Its stages run in the opposite order to a receiver's: from low-level, low-power circuits at the start to a powerful final amplifier at the end.

Microphone to antenna

1. Audio amplifier. Lifts the tiny microphone voltage to a usable level. Too much gain overdrives the later stages (see Mic gain, compression and ALC).
2. Modulator. Combines the audio with a carrier from an oscillator. For SSB a balanced modulator cancels the carrier and leaves two sidebands (see Single sideband (SSB)).
3. Filter. Removes one sideband. SSB also works by phasing methods in DSP.
4. Mixer. Moves the finished signal from the fixed frequency where it was made (here 9 MHz) to the operating frequency. The VFO sets where it lands: 9.0 + 5.2 = 14.2 MHz. The mixer also makes the difference, 3.8 MHz, which a filter removes.
5. Driver and power amplifier. Raise power in steps. The amplifiers must be linear for SSB, so the signal's shape is not distorted; see Amplifiers and classes of operation.
6. Low-pass filter. Real amplifiers also make harmonics (multiples of the operating frequency). The filter removes them so they are not radiated (see Harmonics and spurious emissions).

Other modes use the same blocks

• CW: an oscillator is switched on and off by the key, with the edges shaped so the signal doesn't click. Often no modulator at all.
• FM: the audio changes the frequency of an oscillator directly, or a phase modulator does it. A chain of multiplier stages can raise the frequency, which also multiplies the deviation. The amplifiers can be non-linear (Class C), which is more efficient.
• Digital modes: the computer provides audio tones that enter the same SSB chain at the microphone input (see Computer and sound-card interfaces).

What makes a transmitter good

Quality	What it means
Linearity	the output is a faithful, scaled-up copy of the input; poor linearity splatters beyond the signal's width
Spectral purity	few harmonics and spurious signals
Frequency stability	stays on frequency; oscillator drift is the common cause of drifting signals
Efficiency	how much DC power becomes RF; the rest is heat

Overdriving any stage (excessive mic gain, too much drive to an amplifier) makes the output wider and dirtier. An ALC circuit watches the output and turns the drive down to avoid it, which is why you watch the ALC meter and not the power meter when setting mic gain.

Related

• The transceiver, end to endHow one box does both jobs.
• Single sideband (SSB)Dropping the carrier and one sideband for efficient HF voice.
• What modulation isPutting information onto a carrier wave.
• Amplifiers and classes of operationGain, linearity and efficiency: classes A to C.
• Linear amplifiersAdding power: tubes, solid state and the limits.
• Harmonics and spurious emissionsCleaning up your transmitter.
• Mic gain, compression and ALCGetting a clean SSB signal.