Frequency synthesizers generate the stable, low-noise signals for many microwave designs, including commercial communications systems, electronic warfare and intelligence systems, and test equipment. Synthesizers come in many shapes and sizes, from tiny integrated circuits (ICs) to modular systems in rugged rack-mount enclosures, and in many technologies, from direct and indirect analog types of fast-switching direct-digital synthesizers (DDS). Because of the breadth of this subject, this report will focus on microwave frequency synthesizers in modular or rack-mount forms.

Depending upon the application, a frequency synthesizer can be specified for extremely broadband, multi-octave operation, or for narrowband use as the local oscillator (LO) across a single communications band. Series DSX frequency synthesizers from Herley-CTI (www.herley.com), for example, have been designed to cover 25-pecent bandwidths from 0.5 to 18.0 GHz for radar and automatic-test-equipment (ATE) applications. Using a combination of direct analog and digital techniques (when small step sizes are needed), the compact frequency synthesizers offer switching speed of 1 microsecond or less and as fast as 300 ns. Designed for low power consumption, the modular synthesizers can be supplied in housings as small as 6 X 4 X 6 in. They achieve typical single-sideband (SSB) phase noise of -120 dBc/Hz offset 10 kHz from a 10-GHz carrier. The synthesizers provide +13 dBm output power with less than -20 dBc harmonic levels and less than -70 dBc nonharmonic spurious content. They are programmed via parallel binary-coded-decimal (BCD) control.

The firm also supplies Series BBS broadband synthesizers with frequency ranges as wide as 0.01 to 5.12 GHz and 1-Hz frequency resolution. Using a VXI interface, these card-type synthesizers supply output levels from +13 to +17 dBm with typical phase noise of -131 dBc/Hz offset 100 kHz from a 1-GHz carrier.

Another high-speed modular frequency synthesizer based on direct analog synthesis is the model FS5000 from Aeroflex (www.aeroflex.com). Available in a variety of configurations covering 0.5 to 18.0 GHz, including 4.8 to 8.0 GHz, 8.0 to 13.5 GHz, 4.8 to 13.5 GHz, and the full band from 0.5 to 18.0 GHz, the FS5000 switches frequencies in step sizes of 100 and 500 kHz, depending upon model, with typical switching speed of less than 200 ns. Output power is better than +10 dBm (and +13 dBm for some models), while amplitude accuracy is between ±1.5 and ±3.0 dB. The phase noise for a 4.8-to-8.0-GHz model is -110 dBc/Hz offset 1 kHz from the carrier, -120 dBc offset 10 kHz from the carrier, and -123 dBc/Hz offset 1 MHz from the carrier. Harmonic levels are typically -50 dBc while spurious levels are typically -70 dBc. The typical size for one of the FS5000 frequency-synthesizer modules is 4.5 X 8.5 X 11 in. with weight of 18 lbs. The FS5000 synthesizers are also available in larger, rack-mount configurations.

Elcom Technologies (www.elcom-tech.com) also offers fast-switching synthesizers in modular and rack-mount configurations, such as the broadband UFS series synthesizers (Fig. 1) . The UFS-0.3/18, for example, tunes from 0.3 to 18.0 GHz with 1-Hz frequency resolution and better than 250 ns switching speed. It achieves phase-coherent and random-access switching with that speed while delivering better than -105 dBc/Hz phase noise offset 1 kHz from all carriers, better than -112 dBc/Hz offset 10 kHz from all carriers, and better than -132 dBc/Hz offset 1 MHz from all carriers. In addition to switching frequency in less than 250 ns, it can also settle to a new amplitude level in that time. It provides as much as +10 dBm output power with ±2 dB amplitude accuracy. Harmonic levels are typically -50 dBc while nonharmonic spurious levels are typically -65 dBc.

Switching speed may not always be the primary specification in choosing a synthesizer. But when it is important, an article appearing earlier this year in Microwaves & RF (see "When Switching Speed Is Important," May 2006, p. 98), written by Uri Yaniv of Elcom Systems (www.elcom-tech.com) offers sound advice on when and where frequency synthesizer switching speed is important.

One of the newer synthesizer suppliers, Wide Band Systems (www.widebandsystems.com), comes to the market with a long history in instantaneous-frequency-measurement (IFM) receivers and other EW subsystems. The firm's 2-to-18-GHz fast-tuning frequency synthesizer delivers +10 dBm output power across that range and is available with single, dual, or dual-switched outputs with 1.25-dB power balanced between the multiple output ports. The synthesizer tunes in 1-MHz steps. Switching seed is no worse than 5 microsecond and typically 3 microseconds. Phase noise is typically -75 dBc/Hz offset 1 kHz from the carrier, -80 dBc/Hz offset 10 kHz from the carrier, and -121 dBc/Hz offset 1 MHz from the carrier. The agile frequency synthesizer measures just 6.5 X 6.25 X 1.050 in. and consumes just over 16 W power. It switches frequency under 14-b offset binary control.

Syntech Microwave, Inc. (www.syntechmicrowave.com) supplies compact synthesizer modules from 8 to 15 GHz with 125-kHz tuning resolution. These sources generate +18 dBm minimum output power with less than -60 dBc second harmonic levels and phase noise of -80 dBc/Hz offset 10 kHz from the carrier.

TRAK Microwave Corp. (www.trak.com) offers one of the smaller 2-to-18-GHz modular synthesizers with their model SYN111 indirect analog design. It tunes in 1-MHz steps with 6-ms switching speed. Designed for military airborne environments, it can be supplied in a package as small as 6 X 5 X 2 in. and weighing just 2 lbs. It exhibits SSB phase noise of -65 dBc/Hz offset 100 kHz from the carrier and -90 dBc/Hz offset 700 kHz from the carrier. Maximum harmonic levels are -25 dBc while maximum nonharmonic spurious levels are -60 dBc. When faster switching speed is needed, the company also offers the model SYN146, a direct frequency synthesizer with 500-ns switching speed from 9.25 to 19.00 GHz. Standard frequency steps are approximately 33 kHz, although steps of smaller than 1 Hz can be specified. The SSB phase noise is -90 dBc/Hz offset 1 kHz from the carrier and -112 Hz offset 10 kHz from the carrier. Maximum harmonic levels are -50 dBc while nonharmonic spurious levels are typically -60 dBc. The SYN146 delivers +11 dBm nominal output power with maximum power consumption of 43 W.

ITT Microwave Systems (www.ittmicrowave.com) incorporates DDS technology in their line of Wavecor synthesizers. The modular architecture is designed for applications from 50 MHz to 20 GHz with phase-continuous or phase-coherent outputs. Supplied in a standard 19-in. rack-mount enclosure, the broadband synthesizers weigh less than 40 lbs.