Frequency Synthesizer – Features, Operation & Applications

Frequency synthesis is a technique of producing a range of frequencies with crystal-controlled stability. A frequency synthesizer uses phase-locked loop (PLL) to generate output frequencies over a wide range. The typical range is from 1 MHz to 160 MHz.

As illustrated in the block diagram above, the key components of the frequency synthesizer are:

• Crystal oscillator
• Phase detector
• Low-pass filter
• Voltage controlled-oscillator (VCO)
• Square-wave circuit
• Divide-by-N counter

The Operation of the Frequency Synthesizer

The output of the crystal oscillator, i.e. the reference frequency f_r, is fed into one input of a phase detector. The other input of a phase detector has another square-wave applied as illustrated in the diagram above. The frequencies of these two square waves are identical but there is a phase difference (Φ) between them.

The output of the phase detector is a pulse waveform with pulse width controlled by the phase difference. The output of the phase detector is applied to the low-pass filter which converts it into a dc voltage, E. This dc voltage, E is utilized as the control voltage for the voltage-controlled oscillator (VCO) and it defines the output frequency of VCO. The output of VCO is fed to a circuit that converts it into a square wave for triggering a digital divide-by-N counter, which divides the VCO frequency by a number set by a bank of switches. These switches may be push buttons with digital readouts or they may be thumb-wheel type which indicates the position numerically. The switches are connected in such a manner that the displayed number is the factor N by which the output frequency is divided before being applied to the phase detector. The switches allow the user to obtain frequency which is any integer multiple of the crystal oscillator frequency.

Applications of Frequency Synthesizers

Frequency synthesizers are commonly used in test equipment, communication systems and radio channel selection.

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