What is frequency synthesizer?

Frequency synthesizer

• A frequency synthesizer is a circuit that produces a range of frequencies from a single reference frequency.  A frequency synthesizer can use the techniques of frequency multiplication, frequency division, direct digital synthesis, frequency mixing, and phase-locked loops to get its frequency.

• The synthesizer is used for developing all the signals used by the transmitter and receiver. It uses the PLL (phase-locked loop) circuits and a mixer.
• The crystal oscillator or reference oscillator is used to provide a reference for the two PLLs. The output of VCO-2 is used as the local oscillator frequency for the first mixer in the receiver.
• The outputs of the two VCOs are added together, to produce the transmitter output frequency. The frequency divider section receives the divide by numbers from the logic section.

• These numbers are given by the MTSO computer. The divide by numbers will set the transmit and receive channel frequencies.
• The two outputs produced by the frequency synthesizer are applied to the modulator box in the transmitter and the first mixer in the receiver respectively.
• The frequency synthesizer thus acts as a local oscillator that can produce a wide range of frequencies with high stability.

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