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.

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Micro Lambda Wireless (www.microlambdawireless.com), with a strong heritage in YIG-tuned oscillator technology, offers extensive lines of both narrowband and broadband modular frequency synthesizers. Bandwidths for the company's different synthesizers typically range from 2 to 3 GHz, although the firm most recently added the MLSE Series YIG-based frequency synthesizers with coverage of 1 to 22 GHz or 2 to 20 GHz with 1-Hz frequency resolution. These broadband synthesizers deliver output levels from -20 to +20 dBm with ±1 dB output power flatness and 0.1-dB resolution. They offer an option for a second LO output, from 0.5 to 4 GHz as well as an option for second RF output, from 4 to 11 GHz. The typical phase noise is -104 dBc/Hz offset 10 kHz from a 20-GHz carrier. Typical harmonic levels are -12 dBc while typical nonharmonic spurious levels are -60 dBc. The switching speed ranges from 10 to 18 ms, depending upon step size. The MLSE Series synthesizers weigh 3.5 lbs. and measure 7 X 5 X 2 in. not including the SMA female connectors.

Modular frequency synthesizers from dBm (www.dbmcorp.com) include the 6-to-18-GHz Frequency Synthesizer Series (FSS) sources with 1-MHz tuning resolution and +13 dBm minimum output power. These fast synthesizers settle within 1 MHz of a new frequency in less than 200 microsecond. They achieve phase noise of -65 dBc/Hz offset 100 Hz from an 18-GHz carrier, -72 dBc/Hz offset 10 kHz from the same carrier, and -90 dBc/Hz offset 100 kHz from the same carrier. Harmonic levels are -12 dBc or better while spurious levels are -50 dBc or better. The compact FSS sources measure 8.75 X 4.76 X 4.75 in. and are suitable for use as radar LOs and in wideband receivers.

As an alternative to high-priced test frequency synthesizers, the firm also offers the Synthesized Signal Generator (SSG) CW signal generator with frequency range of 10 MHz to 4 GHz. Standard models provide better than 2-ms switching speed over GPIB or LAN, while an option accelerates switching speed to better than 200 microseconds. Output power levels can be set from -30 to +10 dBm with 0.1-dB resolution. The phase noise is -58 dBc/Hz offset 10 Hz from a 1-GHz carrier, -81 dBc/Hz offset 100 Hz from the same carrier, -103 dBc/Hz offset 1 kHz from the same carrier, -109 dBc/Hz offset 100 kHz from the same carrier, and 128 dBc/Hz offset 1 MHz from the same carrier. Harmonics are less than -30 dBc while spurious levels are less than -55 dBc. The frequency resolution is 10 Hz to 1999.99999 MHz and 20 Hz from 2 to 4 GHz. The compact test source measures 10 X 10 X 3 in.

TESTING SYNTHESIZERS
Test-equipment suppliers offer frequency synthesizers tailored for measurement applications, usually with excellent amplitude accuracy and a variety of remote control options, modulation formats, and other features useful in manual and automatic-test-equipment (ATE) installations. Anritsu Co. (www.us.anritsu.com), for example, offers the MG3690B Series of RF/microwave signal generators (Fig. 2) with coverage as wide as 2 to 65 GHz (and to 325 GHz with waveguide extension modules) in a single model; options can extend the lower-frequency coverage to 10 MHz and 0.1 Hz.

Agilent Technologies (www.agilent.com) offers a number of variations on its high-performance PSG line of Performance Signal Generators, including models that cover frequency ranges of 250 kHz to 20 GHz, 31.8 GHz, 40 GHz, 50 GHz, and 67 GHz (this latter instrument usable through 70 GHz). With 0.001-Hz frequency resolution in CW mode and 0.01-Hz frequency resolution during a frequency sweep, the PSG synthesizers feature better than 11 ms typical switching speed and amplitude settling time of better than 5 ms. Numerous options are available for output power levels and control range, extending over a dynamic range as wide as -135 to +7 dBm through 40 GHz. The level accuracy is typically ±0.9 dB from 0 to +10 dBm through 40 GHz and ±1.5 dB from 0 to +10 dBm through 67 GHz. Harmonic levels are typically -50 dBc through 67 GHz while nonharmonic spurious levels are typically -70 dBc through 10 GHz, -64 dBc through 20 GHz, and -52 dBc beyond 40 GHz. The SSB phase noise is typically -128 dBc/Hz offset 20 kHz from a 2-GHz carrier and -102 dBc/Hz offset 20 kHz from a 40-GHz carrier. And for those who would prefer to pay for slightly less performance, the firm has launched its new line of MXG synthesized signal generators with an outstanding price/performance ratio.

Rohde & Schwarz (www.rohdeschwarz.com) has developed the SMP family of synthesized signal generators for test applications from 10 MHz to 20 GHz, 27 GHz, and 40 GHz with 0.1-Hz frequency resolution. Each provides better than +10 dBm output power across the full frequency range with level accuracy of better than ±0.9 dB at 0 dBm from 10 MHz to 40 GHz. The synthesizers offer switching times of 11 ms or less. Harmonic levels are typically -50 dBc while nonharmonic spurious levels are typically -54 dBc through 40 GHz. The SSB phase noise is better than -105 dBc/Hz offset 10 kHz from a 10-GHz carrier. The signal source offers a wide range of standard modulation formats (AM, FM, and pulse), and an optional phase modulator provides more complex forms of modulation, including ASK and FSK.

Giga-tronics (www.gigatronics.com) recently announced their Panther 2500 series signal generators with models covering 100 kHz to 8 GHz, 20 GHz, 26.5 GHz, and 40 GHz. Frequency resolution is 0.0001 Hz over each full band. The synthesizers are loaded with modulation capabilities, including AM, FM, and pulse/square-wave modulation.

Programmed Test Sources (www.programmedtest.com) is a long-time supplier of fast-switching direct-analog frequency synthesizers for test and other applications. With models operating through 6.4 GHz, the firm features instruments with straightforward operation and clean output signals. For example, the PTS 1600 (Fig. 2) operates from 1 to 1600 MHz with 1-Hz frequency resolution and power levels from +3 to +13 dBm. It provides phase-continuous switching to within 1 radian of a new frequency in less than 20 microsecond. The amplitude accuracy is ±0.7 dB. Harmonic levels are 30 dBc while nonharmonic spurious levels are -60 dBc. The SSB phase noise is -105 dBc/Hz offset 100 Hz from the carrier, -114 dBc/Hz offset 1 kHz from the carrier, and -122 dBc/Hz offset 10 kHz from the carrier. For higher frequencies, the firm's model PTS 6400 provides output signals from 1 to 6400 MHz with 1-Hz frequency resolution and similarly fast frequency switching speed.

As an alternative to traditional "rack-and-stack" measurement-grade frequency synthesizers, many test equipment suppliers are offering modular synthesized sources that can fit into a card rack or communicate by means of local-area network (LAN). Racal Instruments (www.racalinst.com), for example, offers the model 3271 2.4-GHz signal generator in a C-sized, dual-slot VXI format. Designed to work in an ATE system with VXI plug&play drivers, it tuned from 9 kHz to 2.4 GHz with 1-Hz frequency resolution. Output levels can be set from -137 to +25 dBm with 0.1-dB resolution. Model 3271 achieves level accuracy of ±1 dB through 1.2 GHz and ±2 dB at higher frequencies. It exhibits typical harmonics of -30 dBc, nonharmonic spurious of -60 dBc or better, and SSB phase noise of -121 dBc/Hz offset 20 kHz from a 1-GHz carrier. The VXI instrument features frequency and phase modulation, frequency-shift-keying (FSK) modulation, swept-frequency capability,

National Instruments (www.ni.com) also supplies numerous "instruments on a card," including the PXI-565x series of PXI format frequency synthesizers. The line contains three models, including a single unit capable of tuning from 500 kHz to 6.6 GHz. Employing DDS technology, the compact test source tunes with resolution of 1 Hz or less and provides an output power range of -100 to +10 dBm adjustable in steps as fine as 0.1 dB. The tuning speed is 2 ms while the SSB phase is -110 dBc/Hz offset 10 kHz from a 1-GHz carrier. Harmonics are less than -15 dBc while nonharmonic spurious content is less than 65 dBc. The PXI-565x series sources include FM, FSK, and on-off-keying (OOK) modulation formats. In addition, the company's model PXI-5671 PXI RF vector generator is another source based on digital synthesis with tuning range of 250 kHz to 2.7 GHz. Designed to create output signals with complex modulation, the PXI-5671 incorporates an arbitrary waveform generator with 100-MSamples/s sampling rate and 16-b resolution to produce a 20-MHz instantaneous signal bandwidth. It generates a wide range of modulation formats, including AM, FM, PM, ASK, FSK, MSK, GMSK, PSK, QPSK, PAM, and QAM.

Keithley Instruments (www.keithley.com) has packed the full performance of an RF vector signal generator into their half-rack model 2910. Ideal for production testing of mobile handsets, the signal source operates from 400 MHz to 2.5 GHz with a 40-MHz modulation bandwidth when running with internally generated modulation and 200 MHz when using external in-phase (I) and quadrature (Q) modulation sources. The model 2910 makes used of an arbitrary waveform generator and a software-defined-radio (SDR) architecture to create waveforms from stored files. It is shipped with numerous waveform files to create test signals for GSM, GPRS, EDGE, and WCDMA testing as well as conventional analog modulation formats such as AM and FM. It switches frequencies in less than 1.6 ms and achieves outstanding level of accuracy of better than ±0.3 dB for an amplitude control range of -120 to +13 dBm for CW signals. Model 2910 provides numerous connections for remote control and data transfers, using Ethernet, USB, or GPIB ports. It is LXI Class C compliant for full communication over a LAN with other LXI instruments.