Frequency Synthesizers Yield Stable Signals

Analog and digital frequency synthesizers are available in a variety of shapes, sizes, and technologies, depending on the requirement.

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.

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In contrast, Advanced Radio Corp. (Reston, VA) is a newcomer on the list of synthesizer suppliers (see Microwaves & RF, April 2003, p. 94). The firm's ADV-3000S synthesizer module leverages DDS technology to achieve low phase noise and spurious performance of better than −90 dBc from 20 MHz to 3 GHz, with optional coverage to 18 GHz. The fast-switching synthesizer is well suited as a programmable LO for radar, signal-intelligence (SIGINT), and electronic-warfare (EW) applications. Through the use of innovative spurious-concealing circuitry, the company's engineers have managed to suppress DDS spurious noise by more than 30 dB compared to traditional DDS filtering methods.

Although advances occur quickly with digital synthesizer approaches, improvements are also being made with analog technologies. Micro Lambda Wireless (Fremont, CA), for example, bases its low-noise synthesizers on its building-block YIG oscillator technology, offering both wideband and narrowband frequency synthesizers. Its wideband models, for example, are suitable for use as LOs in communications equipment or a signal sources in test equipment. The MLSW wideband series includes the 0.6-to-3.0-GHz model MLSW-0603, the 2-to-8-GHz model MLSW-2080, and the 2-to-10-GHz model MLSW-2010. The synthesizes feature 1-Hz frequency resolution, power levels of +10 to +12 dBm, and spurious levels of −60 dBc. The phase noise is typically −100 dBc/Hz offset 1 kHz from the carrier and −108 dBc/Hz offset 10 kHz from the carrier. At a 1-MHz offset, the phase noise for the 3-GHz synthesizer is −140 dBc/Hz, with levels of −138 dBc/Hz from the 8-GHz synthesizer and −135 dBc/Hz for the 10-GHz unit. The synthesizers, which measure 7 × 5 × 1 in. (17.78 × 12.7 × 2.54 cm) and consume only 29 W, achieve full-band tuning in 13 to 18 ms, and tune across a 100-MHz step in only 10 ms.

The company's newly expanded MLSN series of narrowband synthesizers now includes models operating in 2-GHz bands from 2 to 16 GHz. These sources, which feature similar or improved phase-noise performance compared to the wideband MLSW models, also tune in 1-Hz steps and achieve better than +10 dBm output power through 9 GHz and better than +8 dBm output power through 16 GHz. As an example, the highest-frequency model, the 14-to-16-GHz MLSN-1416, features 1-Hz frequency resolution and 12-ms full-band tuning speed. Spurious noise is typically −60 dBc while phase noise is −94 dBc/Hz offset 1 kHz from the carrier, −101 dBc/Hz offset 10 kHz from the carrier, and −135 dBc/Hz offset 1 MHz from the carrier.

Another company that espouses the use of YIG technology in its frequency synthesizers is Endwave Corp. (Sunnyvale, CA). The firm offers compact modular frequency synthesizers in bands as wide as 2 GHz from 4.5 to 14.0 GHz with phase noise as low as −100 dBc/Hz offset 10 kHz from the carrier. For example, the company's 50 Series includes the 4.5-to-7.0-GHz model SYN-50A-00 and the 7-to-10-GHz model SYN-50B-00. Both tune over a 1500-MHz tuning range in 125-kHz steps with phase noise of −100 dBc/Hz offset 10 kHz from the carrier, dropping to −143 dBc/Hz offset 1 MHz from the carrier. The typical output power is better than +10 dBm for both models, and spurious content is typically less than −70 dBc. The 50 Series synthesizers measure 3.9 × 3.12 × 1.38 in. (9.9 × 7.9 × 3.5 cm).

Endwave's 20 Series includes the 7.9-to-8.4-GHz model SNY2018E and the 8.0-to-8.3-GHz model SYN2018F. These models tune with maximum step sizes of 27.5 and 20 MHz, respectively, achieving spurious levels of −70 dBc. The sources feature +10 dBm output power and have phase noise levels of −85 dBc/Hz offset 10 kHz from the carrier and −140 dBc/Hz offset 1 MHz from the carrier. They measure 6.25 ×2.98 × 1.1 in. (15.9 × 7.6 × 2.8 cm).

TRAK Microwave Corp. (Tampa, FL) is another well-known supplier of high-speed frequency synthesizers, employing both direct and indirect analog synthesis techniques. An example of a direct analog unit is a 7-to-9-GHz model with 1-MHz step size and 500 ns maximum switching speed. The synthesizer employs a bank of crystal oscillators, which undergo frequency conversion, switching, and filtering to generate the final output frequencies. This synthesizer generates relatively high output power of +19 dBm with spurious levels of −60 dBc, and phase noise of −110 dBc/Hz offset 10 kHz and −120 dBc/Hz offset 1 MHz from the carrier. The unit, which includes a power supply, measures 15 × 8.5 × 5 in. (38.1 × 21.6 × 12.7 cm).

The company's indirect analog synthesizers are based on selecting and filtering frequencies from a comb generator. The firm's lineup includes a 575-to-1075-MHz synthesizer capable of better than 100-ns switching speed. Although possessing large frequency steps (25 MHz), the source boasts good phase noise, with performance of −110 dBc/Hz at 1 kHz offset, −130 dBc/Hz at 10 kHz offset, and −140 dBc/Hz at 10 MHz offset from the carrier. The synthesizer measures 6.5 × 4.6 × 0.95 in. (16.51 × 11.7 × 2.4 cm).

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Communications Techniques, Inc. (Whippany, NJ) is a veteran supplier of microwave frequency synthesizers, offering one of the larger varieties of package styles of synthesizer topologies. For example, the company's Series DS synthesizers are rack-mount, instrument-grade synthesizers capable of achieving a full tuning range of 0.005 to 20.48 GHz in a single unit. Available as a rack-mount or modular unit, the DS synthesizers feature typical switching speeds of 0.3 to 1 µs, frequency steps as small as 1 Hz, and +13 dBm output power. The direct analog frequency synthesizers achieve spurious levels of −64 to −80 dBc, depending upon frequency. The SSB phase noise is −109 dBc offset 1 kHz from a 10-GHz carrier, −119 dBc/Hz offset 10 kHz from the same carrier, and −128 dBc/Hz offset 1 MHz from the same carrier. For a 1-GHz carrier, the phase noise is −127 dBc/Hz offset 1 kHz, −137 dBc/Hz offset 10 kHz, and −147 dBc/Hz offset 1 MHz.

The company's broadband Series BBS synthesizers includes models covering as wide as 0.01 to 5.12 GHz in a single unit with step sizes from 1 Hz to 10 MHz. The typical phase noise is −131 dBc/Hz offset 100 kHz from a 1-GHz carrier, while spurious levels range from −63 to −85 dBc, depending upon frequency. These compact synthesizers measure just 3.6 × 5.8 × 0.98 in. (9.1 × 14.7 × 2.5 cm), excluding connectors, and deliver output levels from +13 to +17 dBm.

Another long-time supplier of frequency synthesizers, MITEQ (Hauppauge, NY), offers frequency synthesizers to 40 GHz. The company's SLS Series is optimized for fast switching applications including wireless-communications and satellite-communications systems, while the MFS series provides extremely low phase noise for critical satellite communications applications. The firm's unique CFS series of synthesizers offers dual output signals for systems requiring dual upconversion or downconversion. These synthesizers provide output signals in Ku-band (12.710 to 13.280 GHz) and L-band. The synthesizers switch in 125-kHz steps with +13 dBm output power with −70 dBc spurious content.

A long-time supplier of instantaneous-frequency-measurement (IFM) receivers, and user of frequency synthesizers, is now also a supplier: Wide Band Systems (Rockaway, NJ). The company's model PS-070-180A tunes from 7 to 18 GHz in 1-MHz steps and offers at least +13 dBm output power. It settles to a new frequency in 100 µs or less and boasts spurious levels of −50 dBc or better. The SSB phase noise is typically −60 dBc/Hz offset 1 kHz from the carrier, −65 dBc/Hz offset 10 kHz from the carrier, and −110 dBc/Hz offset 1 MHz from the carrier. The synthesizer measures 4 × 4 × 8 in. (10.16 × 10.16 × 20.32 cm).

The frequency synthesizers mentioned above rely on digital commands from a control bus for executing frequency and amplitude changes, making many of them suitable for automatic-test-equipment (ATE) applications. Some test applications, however, require a more flexible local interface for control, as evidenced by another class of frequency synthesizer designed for instrumentation applications. Suppliers include Aeroflex (Plainview, NY), Agilent Technologies (Santa Rosa, CA), Anritsu (Morgan Hill, CA), Communications Techniques, Gigatronics (San Ramon, CA), Rohde & Schwarz (Munich, Germany), Programmed Test Sources (Littleton, MA), and Universal Microwave Corp. (Odessa, FL, see p. 55). For more information on these and other suppliers of frequency synthesizers, please refer to the Microwaves & RF Product Data Directory website at

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