Frequency synthesizers can be found in most systems where RF/microwave signals are generated and processed. Synthesizers have evolved as the needs for stable, high-frequency signals have grown. They come in many forms and technologies designed to fit myriad electrical and mechanical requirements.
Frequency synthesizers range in size from integrated circuits (ICs) to complete systems with power supplies and associated circuitry in rack-mount enclosures. Some are designed as modules for use as frequency sources in a system, such as a local oscillator (LO) for a heterodyne receiver. Some are meant for applications in test systems, with programmability that supports frequency and amplitude control, such as frequency and power sweeps. Essentially, synthesizers exist because RF/microwave oscillators do not provide the frequency stability necessary for many applications on their ownthat is, unless they are synchronized to a more stable lower-frequency source, such as an oven-controlled crystal oscillator (OCXO).
Selecting a synthesizer from among the vast array of products offered by high-frequency suppliers can be daunting, unless some of the key requirements for an application are used to guide the selection process. For example, a frequency synthesizer for a mobile application (such as a smartphone) must be small and consume very little power, which most likely will be an IC-based frequency synthesizer.
Modern synthesizer specifiers have a wide array of technologies from which to chose, including indirect analog (including phase-lock-loop, integer-N, and fractional-N types), direct analog, direct digital, and hybrid configurations. Each has advantages and disadvantages. In general, an indirect analog synthesizer such as a PLL type can provide excellent spectral purity over wide bandwidths, but will be limited in tuning speed. A direct-analog frequency synthesizer can achieve the fastest switching speed of all synthesizer types, but tends to be large and complex (and therefore expensive). A direct-digital synthesizer (DDS) provides fast switching speed, but is limited in spectral purity by the bit resolution of the digital-to-analog converter (DAC) used to produce the synthesizer's analog output signals.
Sorting synthesizers in terms of physical size and power consumption can eliminate a large number of products from consideration, especially as the numbers for size and power get smaller. Otherwise, performance parameters can be used to sort through different frequency synthesizers, since they are readily compared by a number of common specifications. These include frequency range, step size, switching speed, phase noise, spurious levels, harmonics, output power, and output-power flatness versus frequency and temperature. Frequency synthesizers function by means of frequency division, multiplication, and mixing of signals to produce a desired output frequency. The most common form of frequency synthesizer is still likely the indirect analog type based on a PLL.
A frequency synthesizer based on a PLL relies on comparing the phase of a high-frequency oscillator with that of a more stable, lower-frequency reference oscillator. There are three basic circuit blocks in a PLL synthesizer design: a phase comparator, a tunable oscillator , and a loop filter see figure (a)>. The phase comparator is used to compare the phase of signals from the tunable oscillator and a reference oscillator, and produces a signal equal to the difference of the two at its output port. The phase difference signal is processed by the loop filter to remove unwanted signal components, and then applied to the control terminal of the tunable oscillator as a correction signal. This signal loop will serve to maintain the phase of the tunable RF/microwave oscillator at the same phase as the reference oscillator, so that frequency remains constant.
This simple representation of a PLL oscillator will provide only stabilized, fixed-frequency output signals. To provide some degree of tuning, additional components must be added to the basic PLL architecture, such as a programmable counter or divider see figure (b)>.
Additional frequency synthesizer types include direct analog synthesizers and direct digital synthesizers (DDSs). Both offer faster tuning speeds than indirect PLL-based frequency synthesizers, but with other tradeoffs. A direct analog frequency synthesizer typically provides the fastest frequency switching speed of all synthesizers, but also tends to be the most complex and most expensive type. This architecture uses a limited number of frequencies from a reference oscillator, and then performs arithmetic operations on those signals to derive the full range of output signals for the synthesizer. It consists of frequency multipliers, dividers, high-speed switches, and fixed-frequency filters.
In its simplest form, a DDS consists of a phase accumulator (a clock dividing frequency counter) which generates a digitized ramp waveform. This, in turn, is transformed into an analog sine wave output waveform by means of a high-frequency digital-to-analog converter (DAC). In addition, programmable read-only memory (PROM) is used to store discrete values of the desired output sine wave functions for the synthesizer. The operating frequency of a DDS will be limited by the clock frequency feeding the phase accumulator, with practical output frequencies approaching one-half the frequency of the clock source. The spectral purity of a DDS will be limited by the bit resolution of the DAC, with lower-resolution DACs yielding higher levels of spurious signal products.
When sorting through different frequency synthesizers, the requirements of an application will establish the performance levels required. It is the combination of synthesizer performance requirementsincluding frequency tuning range, frequency resolution, switching speed, phase noise, output power, power consumption, size, and weightthat will dictate the type of technology needed. For a more extensive review of frequency synthesizers, don't miss the January 2012 Defense Electronics supplement.