Frequency synthesizers are used throughout commercial and military systems, in designs as large as complete radar systems and as small as cellular telephones. Based on the number of companies competing for the many markets served by frequency synthesizers (a general search of the Internet will reveal more than 40 suppliers), the demand for synthesized sources with a wide range of performance levels is growing, as tuning in high-frequency systems is now dominated by digital approaches (tuning frequencies in discrete steps) rather than earlier analog (continuous tuning) methods.

Synthesizers are available in physical configurations ranging from packaged integrated circuits (ICs) to moderate-sized modules and hybrid circuits to larger rack-mountable system-type synthesizers complete with power supplies and supporting digital monitoring and communications circuitry. Because of the limited scope of this article, it will focus on modules, racks, and instrument-grade frequency synthesizers, with a future article providing details on available IC-level synthesizers.

Various technologies are used in modern frequency synthesizers, including traditional sources based on phase-locked-loop (PLL) technology to lock the phase of a voltage-controlled oscillator (VCO) to that of an inherently more stable reference source, such as a temperature-compensated crystal oscillator (TCXO) or an oven-controlled crystal oscillator (OCXO). Such synthesizers can be designed with a single loop for optimal frequency switching speed, or with multiple loops when lower noise performance is required. In essence, they can be called "integer-N" synthesizers where N is the multiplication factor used to determine the output frequency as a multiple of the reference source frequency.

In recent years, newer synthesizer technologies have gained in popularity, including fractional-N frequency synthesizers, which use non-integer values for N, and direct-digital synthesizers (DDS), which rely on the conversion of 32-to-48-b phase/frequency/amplitude digital data into analog output signals through the use of precision accumulators and digital-to-analog converters (DACs). Fractional-N synthesizers can achieve phase-noise levels that are very close to the reference source, although they tend to be limited in bandwidth. A DDS features nanosecond frequency switching speed, but is traditionally limited in spurious performance by the bit resolution of certain of its digital components, and limited in frequency by the clock rates of available digital components.

A DDS is an example of a "direct synthesis" technique, in which an output signal is created as a one-to-one function of an input digital word. A large number of digital words that define signal phase (frequency) and amplitude can be stored in memory and pipelined to a DDS, allowing high-speed frequency switching and execution of such functions as frequency hopping and generation of complex chirp signals. Direct synthesizers can also be realized by means of analog circuitry by generating, for example, a comb of frequencies and then filtering to select the desired output frequency. While this approach offers switching speeds similar to that of a DDS, the amount of filtering needed for high-frequency and broadband coverage leads to a design that is complex and expensive.

In some cases, such as the MTS2000-DS multiloop frequency synthesizer from Synergy Microwave Corp. (Paterson, NJ), several technologies are combined in one package. This multiloop PLL frequency synthesizer that also employs DDS technology to achieve extremely small step sizes with relatively fast switching speed. This compact module (10.16 × 10.16 × 2.54 cm) tunes from 1 to 2 GHz in step sizes as small as 1 Hz and with phase noise of −94.97 dBc/Hz offset 1 kHz from the carrier (see Microwaves & RF, August 2003, p. 92).

Another company that combines analog and digital frequency-synthesis techniques is Elcom Technologies (Rockleigh, NJ), with their UFS series of products. These larger, rack-mount synthesizers are available in narrowband and wideband models through 18 GHz suitable for radar, surveillance, electronic-warfare (EW), and ATE applications. For example, the company's model UFS-15 synthesizer tunes from 1.2 to 3.6 GHz and from 9.6 to 15.0 GHz (two separate output ports per a customer's request) with 1-Hz frequency resolution and 200-ns switching speed. Although DDS sources are traditionally guilty of high levels of spurious content, this synthesizer achieves spurious levels of −67 dBc from 1.2 to 3.6 GHz and −70 dBc from 9.6 to 15.0 GHz. Harmonics are as low as −80 dBc, and single-sideband (SSB) phase noise is a mere −110 dBc/Hz offset 100 Hz from a 12-GHz carrier, −116 dBc/Hz offset 1 kHz from the same carrier, and dropping to −142 dBc/Hz offset 10 MHz from the 12-GHz carrier (a more complete review of the UFS-15 will be available in the November issue).

A long-time supplier of DDS sources, ITT Industries, Microwave Systems (Lowell, MA), which has built upon technology developed by Stanford Telecom during the 1980s and 1990s, offers several lines of DDS-based frequency synthesizers. The firm's WaveCor synthesizers, for example, features sources operating in bands from 50 MHz to 20 GHz with spurious levels of less than −80 dBc and phase noise of −140 dBc/Hz offset 10 kHz from the carrier. Capable of switching frequencies in less than 200 ns, these high-performance sources are housed in a compact, six-inch cube.