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9-3: Frequency Conversion
Local Oscillator and Frequency Synthesizers: Frequency Synthesizer
Most new receiver designs incorporate frequency synthesizers for the local oscillator, which provides some important benefits over simple VFO designs.
The synthesizer is usually of the phase-locked loop (PLL) design and the output is locked to a crystal oscillator reference which provides high stability.
Tuning is accomplished by changing the frequency division factor in the PLL, resulting in incremental rather than continuous frequency changes.