Frequency synthesizers account for many of the RF and microwave signals in communications, radar, and test systems. Synthesizers come in many forms and performance levels, from tiny phase-locked-loop (PLL) integrated circuits (ICs) to rack-mountable subsystems and precision test instruments with programmable control. What follows is a brief sampling of synthesizer suppliers and their latest products.

Instrument-grade frequency synthesizers offer some of the highest performance levels among synthesizer products since they must provide reference-like signals for testing and equipment evaluation. Instrument synthesizers can generally be divided into two categories: moderate-speed and fast-switching synthesizers. Moderate-speed synthesizers are characterized by frequency switching speeds in the millisecond range, usually based on a stabilized YIG oscillator as the frequency source. This speed is adequate for the majority of test applications in which a limited amount of data must be captured during a sweep of frequency from one band edge to another. The list of suppliers for this category of test synthesizer includes Agilent Technologies (www.agilent.com), Anritsu Co. (www.us.anritsu.com), Giga-tronics (www.gigatronics.com), and Rohde & Schwarz (www.rohde-schwarz.com).

The modern stand-alone test synthesizer has evolved a great deal from the 19-in. rack-mount instruments of 20 years ago. Current-day frequency synthesizers such as the new MG3690B series from Anritsu Co. provide extensive feedback to a user through large display screens with computer-like pull-down menus (see figure). The MG3690B series includes models operating as high as 325 GHz (with waveguide connections). Coaxial models cover 0.1 Hz to 65 GHz (and usable to 67 GHz), with +17 dB standard output power to 20 GHz and as much as +23 dBm output power at that frequency available as an option. As much as +19 dBm output power is available at 40 GHz, with the sources generating as much as +13 dBm output power at 50 GHz. The typical switching time is 5 ms for a 1-GHz step.

The E8257D PSG series of analog frequency synthesizers from Agilent Technologies also provides coaxial coverage as high as 67 GHz, with as much as +23 dBm output power at 20 GHz. The phase noise is a mere —115 dBc/Hz offset 10 kHz from a 10-GHz carrier. These synthesizers feature dual internal function generators to produce a wide range of analog modulation, including amplitude modulation (AM), frequency modulation (FM), phase modulation, and pulse modulation with sine, square, triangle, ramp, and noise waveforms.

Anritsu's MS3690B series includes analog modulation, such as AM and FM, although instrument-grade synthesizers designed for modern communications testing typically include digital modulation formats. The company's MG3681A digital modulation signal generator operates at carrier frequencies from 250 kHz to 3 GHz with wideband vector modulation from DC to 30 MHz. The source can generate the amplitude- and phase-based digital modulation formats used in cellular and PCS systems, including amplitude-shift keying (ASK), phase-shift keying (PSK), quadrature AM (QAM), and quadrature PSK (QPSK).

Rohde & Schwarz (www.rohde-schwarz.com) also offers measurement-grade frequency synthesizers with analog or digital modulation. The company's SMR series of analog-modulation instruments, for example, provide frequency resolution as fine as 0.1 Hz from 10 MHz through 60 GHz. In addition to AM, FM, and pulse modulation, these sources can generate ASK and FSK formats.

Giga-tronics includes extensive analog modulation capabilities in its 2400M series of synthesizers. Available at frequencies to 40 GHz, the 2400M series includes AM, FM and pulse modulation and among the fastest frequency switching speed for an indirect frequency synthesizer, at 2.5 ms.

The other category of instrument frequency synthesizers achieve fast switching between frequencies by means of direct analog synthesis techniques. Sometimes aided by direct digital synthesis (DDS) for fine-step resolution, these sources can settle on a new frequency in a matter of microseconds or less. One of the best-known suppliers in this part of the market is Aeroflex (www.aeroflex.com). The company's FS2000 frequency synthesizer, which is available as a module for integration in large systems and as a rack-mount instrument, was first introduced in 1986 and continues to serve as a microwave reference source for high-speed signals. Covering a range of 10 MHz to 18.4 GHz, the synthesizer can step from any one frequency in the range to any other is typically less than 1 µs. Its parallel binary-coded-decimal (BCD) control interface supports high-speed remote programming, while an additional GPIB interface simplifies computer control in automatic-test-equipment (ATE) systems. The FS-2000 is a modular design that derives signals through multiplication, division, and filtering of a 100-MHz reference signal that is in turn generated from a 5- or 10-MHz reference oscillator. Two basic modules generate signals from 10 MHz to 2.3 GHz, while additional modules increase the frequency range through scaling. The phase noise is —73 dBc/Hz offset 10 Hz from a 2.4-GHz carrier, dropping to —128 dBc/Hz offset 10 MHz from the same carrier.

Programmed Test Sources (www.programmedtest.com) also offers direct analog synthesizers with outstanding frequency switching speeds, albeit at lower frequencies than Aeroflex. The PTS6400, for example, is the company's highest-frequency synthesizer, with a frequency range of 1 to 6400 MHz and frequency resolution of 1 Hz. With switching times ranging from 1 to 20 µs depending upon the step size, these sources feature a noise floor of —136 dBc/Hz and phase noise of —108 dBc/Hz offset 1 kHz from any carrier from 1600 to 3200 MHz.

For system-level applications, a number of suppliers offer modular or rack-mount frequency synthesizers with a variety of interface options, including dBm Corp. (www.dbmcorp.com), Herley-CTI (www.herley-cti.com), Elcom Technologies (www.elcom-tech.com), Endwave, and ITT Industries Microwave Systems (www.ittmicrowave.com), and Micro Lambda Wireless (www.microlambdawireless.com). As with the instrument-grade sources, these are available as indirect (moderate switching speed) and direct (fast frequency switching speeds) units with numerous functions and options.

Frequency synthesizers from dBm Corp. provide broadband coverage from 6 to 19 GHz with frequency switching times of less than 200 µs. Standard units feature 1-MHz step size and +13 dBm minimum output power. The phase noise is —65 dBc/Hz offset 100 Hz from an 18-GHz carrier and —90 dBc/Hz offset 100 kHz from the same carrier. The compact module measures just 6.9 × 4 in.