Frequency synthesizers come in many shapes and sizes, although the ultimate goal in any design is to generate stable output frequencies with minimal spurious and phase noise.
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.
Series DS direct synthesizers from Herley-CTI, for example provide coverage as wide as 0.01 to 20.48 GHz in a single unit, with standard bands ranging from 500 MHz to 18 GHz. Typical frequency switching speeds range from 300 ns (for large steps) to 1 µs for steps from 1 to 100 Hz), with available step sizes from 1 Hz to 1 MHz. As with the Aeroflex synthesizers, parallel BCD programming provides the fast control needed to coordinate such fast frequency switching. The phase noise is —109 dBc/Hz offset 1 kHz from a 10-GHz carrier and —111 dBc/Hz offset 1 kHz from a 1-GHz carrier. For an offset of 1 MHz (approaching the noise floor), the phase noise is —128 dBc/Hz at a 10-GHz carrier and —147 dBc/Hz at a 1-GHz carrier. As with the Aeroflex direct synthesizer, the phase noise is at a minimum close to the reference frequency (100 MHz).
For lower-frequency applications, the firm's Series BBS broadband frequency synthesizers provide coverage from 0.1 to 5.12 GHz in a single unit. Standard models feature 1-Hz resolution in bands that include 750 to 1000 MHz, 1500 to 2000 MHz, 10 to 2200 MHz, 10 to 4400 MHz, and 10 to 5120 MHz. The phase noise is —93 dBc/Hz offset 1 kHz from a 1-GHz carrier and —153 dBc/Hz offset 1 MHz from the same carrier.
Elcom Technologies has made great strides in recent years as a supplier of low-noise synthesizers for digital radios, satellite communications (satcom), and military applications. The company's MFS series of frequency synthesizers, for example, is based on a low-power CMOS application-specific integrated circuit (ASIC) and dedicated microcontroller. Available in bands from 1 to 23 GHz for satcom applications, the synthesizers are designed for power efficiency for fixed and mobile satcom systems.
The company's WMFS series of frequency synthesizers also covers a range of 1 to 23 GHz, in bands a wide as 6 GHz. Available with frequency step sizes of 1 Hz or less, these sources provide frequency switching of 1 ms or less (and an option for frequency switching of 250 µs or less) and power consumption of 10 W or less. The typical phase noise is —95 dBc/Hz offset 100 kHz from a 10-GHz carrier. For digital radios, the company's DFS Series synthesizers also cover 1 to 23 GHz in bands, while supporting complex modulation formats to 256QAM.
A long-time supplier of frequency synthesizers for satcom applications, MITEQ (www.miteq.com) offers several lines of sources, including the SLS series, which as based on a high-speed PLL architecture. Covering a frequency range of 1 to 15 GHz with steps from 200 kHz to 10 MHz, the SLS synthesizers require only 50 µs to acquire a new frequency, with an option that increases this speed to 25 µs.
Micro Lambda Wireless, a company well known for its YIG-based filters and oscillators, more recently added stabilization circuitry to its sources to create high-performance synthesizer modules, such as the MLSE series of sources. The MLSE series wideband synthesizers are available at frequencies from 2 to 20 GHz and 1 to 22 GHz with 1-Hz steps. With as much as +20 dBm output power to 20 GHz, these synthesizers achieve switching speeds of 18 ms or better and phase noise of —101 dBc/Hz offset 1 kHz from a 10-GHz carrier and —142 dBc/Hz offset 1 MHz from the same carrier.
EDO Electronic Systems Group (www.edocorp.com) also supplies broadband frequency synthesizers for optical and satellite communications, radar, and military applications. Designs to 26 GHz are based on analog sampling architectures and indirect digital approaches. Available with full MIL screening, sources in the line can be specified for fast-tuning or frequency-hopped use in ECM/ESM systems.
Based on DDS technology, the WaveCor synthesizer modules from ITT Industries Microwave Systems provide clean signals from 50 MHz to 20 GHz. Sporting spurious levels of better than —80 dBc and phase noise of —140 dBc/Hz offset 10 kHz from a 2.5-GHz carrier, the WaveCor synthesizers boast switching times of 200 ns or less due to the near instantaneous signal generation of the DDS technology. The unit synthesizers are housed in a six-inch cube weighing about 10 lbs.
For millimeter-wave frequencies, Insight Product Co. (www.insight-product.com) offers a line of frequency synthesizers in full waveguide bands from 120 to 180 GHz, with output-power levels of 30 mW or more. Ideal for measurement and astronomy applications, these synthesizers can be equipped with optional AM and FM capabilities and GPIB remote control.
When frequency synthesizer modules are needed for lower frequencies and more moderate switching speeds, YIG-based sources provide extremely low phase noise and high reliability. Endwave Corp. (www.endwave.com), for example, offers compact frequency synthesizer modules with tuning ranges as wide as 2 GHz from 4/5 to 14.0 GHz. The phase noise is as low as —100 dBc/Hz offset 10 kHz from the carrier and —128 dBc/Hz offset 100 kHz from the carrier. As an example of the line, model SYN-50B-00 operates from 7 to 10 GHz with 125-kHz step size and +10 dBm typical output power. The phase noise is —100 dBc/Hz offset 10 kHz from the carrier and —143 dBc/Hz offset 1 MHz from the carrier.
When even smaller frequency synthesizers are required, a number of firms offer sources in surface-mount packages or even at the chip level. Synergy Microwave (www.synergymwave.com), for example, offers both integer PLL and fractional-N frequency synthesizers through about 2.5 GHz. For example, the model FSPll950SA fractional-N frequency synthesizer operates from 950 to 1700 MHz with typical phase noise of 90 dBc/Hz offset 10 kHz from the carrier and —112 dBc/Hz offset 100 kHz from the carrier.
In addition, low-cost PLL ICs are available in packaged form from numerous suppliers, including Analog Devices (www.analog.com), Fujitsu Microelectronics America (www.fujitsu.com), National Semiconductor (www.national.com),and Zarlink Semiconductor (www.zarlink.com). These packaged ICs can be specified as single or dual PLLs for multimode, multiband wireless applications.
This has been a brief sampling of the many frequency synthesizer products currently available in different form factors. For a more comprehensive listing, please visit the Microwaves & RF Product Data Directory website at www.m-rf.com and search manufacturers by product category under the Oscillators/Synthesizers category. The online directory shows manufacturers under several frequency synthesizer categories, including agile, DDS, and modular synthesizers.