What Is a Frequency Synthesizer?

A frequency synthesizer is a device that produces a waveform at a frequency determined by analog or digital circuits. The most common frequency synthesizer uses a voltage-controlled oscillator (VCO), which is controlled by a phase-locked loop (PLL) using a stable frequency reference. Frequency synthesizers are used in most telecommunications equipment designed to transmit or receive on a certain range of frequencies in a sub-band.

The operation is made possible by steering the frequency of a VCO to directly or indirectly lock onto the frequency reference. Steering the VCO is achieved by a variable direct current (DC) voltage that is able to continuously adjust the VCO frequency. If the VCO frequency output is too low, it is possible to apply frequency multiplication as needed. In similar applications, the digitally-controlled oscillator is an improved VCO with additional features for a wider range of frequencies and special features such as faster frequency locking.

Digital frequency division is the process of using up-down digital counters to produce a bit stream that is lower in frequency by an integral number to the input rate. The dual-modulus prescaler uses two divider stages for achieving the resolution needed in frequency step in the output. By setting the right divisor figure, the VCO output frequency may be scaled down in frequency and compared to the reference to re-establish PLL lock.

A simple frequency synthesizer will have a VCO, a frequency reference, a PLL, prescaler, divider, and associated circuits. For instance, to generate a 10-megahertz (MHz) sinusoidal output, the steering line is at a DC level so that the VCO output is 10 MHz. The VCO output is also converted into square waves at digital levels for frequency division by 10,000 to obtain a 1-kilohertz (kHz) sample, and this will be fed to a phase comparator together with a reference 1 kHz signal. The resulting DC error voltage controls the VCO steering line, and the PLL is locked in this condition. With any change in VCO output frequency or phase, the process repeats and correction is made at the VCO steering line until the output is back at 10 MHz and the PLL locked.

Frequency synthesizers are commonly used in radio transmitter and receiver circuits. In transmitters, the frequency synthesizer produces the carrier frequency that has to be accurate enough to pass legal emissions requirements. For receivers, the superheterodyne receiver makes use of frequency mixing of the local oscillator (LO) and the incoming radio frequency (RF) signal to obtain the intermediate frequency (IF). In the receiver, the VCO output is the local oscillator signal. It should be noted that the superheterodyne receiver requires an LO frequency that may be the receiver signal plus or minus the desired IF.