Frequency Synthesizer

a device for obtaining sinusoidal electrical oscillations at desired frequencies through the linear conversion of fixed-frequency oscillations produced by one or more reference generators. The conversion may involve multiplication by a constant coefficient, division by a constant coefficient, addition, or subtraction.

Synthesizers are used as sources of frequency-stable oscillations in radio transmitters, superheterodyne radio receivers, frequency meters, and other devices that must be tuned to various frequencies within the frequency range appropriate for the operation of the device. Frequency synthesis provides higher accuracy and stability than does frequency tuning by varying the inductance and capacitance of an oscillatory circuit.

Reference generators with thermostatically controlled quartz-crystal resonators (seeQUARTZ-CRYSTAL OSCILLATOR) have a very high frequency stability (10^–8 or higher). Consequently, the synthesized oscillations have an equally high frequency stability.

The operation of a frequency synthesizer is usually based on one of two methods. In the first method, individual harmonics of the oscillations of the reference generator are isolated by means of filters. The numbers of the chosen harmonics are determined by the digits in the various orders of the numerical value for the synthesized frequency. In the second method, which is preferred, synchronization of oscillations is carried out: the oscillations obtained when the fundamental frequency of the reference generator is divided by means of a frequency divider with a constant scaling factor are synchronized with the oscillations obtained when the frequency of another generator (with automatic phase control of the frequency) is divided by a digital divider with a variable scaling factor.

Discrete values of the frequency can be set up on a synthesizer (within the operating range) at definite, fairly small intervals, such as 1 kilohertz, 100 hertz, or 10 hertz. A frequency is generally set up with a decimal set of digits for its numerical value by means of disks, handles, or knobs; such a synthesizer is consequently called a decade synthesizer. In some cases, the value of the synthesized frequency is shown on a digital electronic display.