The requirements of modern communications systems and their exotic modulation formats place greater demands on the performance of microwave frequency synthesizers.
Bandwidth is everything in communications. It is vital to the spread of wireless multimedia, instant data, high voice quality, and other key services. But it is also a limited resource, requiring the use of advanced amplitudeand phase-based modulation formats to squeeze the maximum amount of information into a given portion of bandwidth. One of the most critical components in enabling maximum bandwidth efficiency is the microwave frequency synthesizer. Modern synthesizers have leveraged available digital techniques to reach the levels of noise, stability, and resolution needed for most modern communications systems.
Frequency synthesizers, of course, come in many shapes and sizes, from tiny system-on-a-chip (SoC) devices and compact modules to rugged military-grade rack-mount systems and bench-top instruments. Available technologies are almost as diverse as the number of package options, using analog methods, digital techniques, and often a combination of the two approach-es. Frequency synthesizers have traditionally relied on a phase-locked-loop (PLL) architecture in which the phase of a tunable oscillator, such as a voltage- controlled oscillator (VCO) or a YIG-tuned oscillator, is locked to the phase of a reference source with inherently higher stability, such as a ovencontrolled crystal oscillator (OCXO).
Most early SoC synthesizer designs for wireless portable devices relied on PLL architectures, implementing a single RF or intermediate-frequency (IF) synthesizer to provide the local-oscillator (LO) frequencies needed in a wireless receiver or transmitter. More recently, leading PLL chip suppliers such as Fujitsu Microelectronics America (www.fujitsu.com) have combined multiple PLL circuits on a single chip, such as the firm’s model MB15U30SP which is a serial-input PLL synthesizer with 2.5- GHz and 510-MHz prescalers capable of generating output signals to those maximum frequencies. The dual PLL is ideal for mobile communications application requiring RF and IF signals for use as LOs, saving the circuit board space of an additional IC.
Similarly, National Semiconductor (www.national.com) offers the LMX2364 dual frequency synthesizer for digital cellular applications. It is based on two alternative frequency-synthesis technologies: fractional-N and integer-N synthesis. The IC combines a 2.6-GHz fractional- N synthesizer with a low-power 850-MHz integer-N synthesizer. Fractional-N synthesizer technology is also the basis for the model ADF4154 IC from Analog Devices (www.analog.com). The single frequency synthesizer produces output signals from 0.5 to 4.0 GHz using a combination of integer and fractional registers to generate low-noise signals with high resolution.
Analog Devices is also a supplier of one of the fastest-growing frequency synthesis technologies, direct digital synthesis (DDS), in which digital commands are essentially stored on frequency and phase accumulators and converted into CW and modulated waveforms by means of high-performance digital-to-analog converters (DACs). A general rule of thumb for a DDS is that it can reliably generate output frequencies at about 40 percent of the clock frequency, based on a conservative application of Nyquist theorem (in which a signal must be sampled at a rate of at least twice the signal frequency to reliably represent the original signal as a digital code).
Analog Devices’ model AD9858, for example, is a complete DDS with integral 10-b DAC. Since it operates at clock rates to 1 GSamples/s, it is capable of output frequencies to 400 MHz or higher. Using 32-b frequency tuning words, the synthesizer achieves sub-1- Hz frequency resolution. Its on-board DAC boasts phase noise of less than –130 dBc/Hz offset 1 kHz from the carrier.The frequency tuning and control words are loaded into the AD9858 via parallel 8-b or serial loading formats. The AD9858 can actually work with clock rates to 2 GHz, since it also includes a divide-by-two circuit on the clock input port.
Sciteq Electronics, one of the pioneering firms in DDS technology, is now a part of Meret Optical Systems (www.meretoptical.com). Meret Optical still supports many of the Sciteq DDS products, including the GaAsbased ADS-63x, which operates at maximum clock frequencies to 500 MHz. The module measures 5 × 7 × 1.125 in. and can be supplied with an internal clock or operate with an external clock. The DDS achieves better than 0.12-Hz frequency resolution by merit of its 32-b frequency words and boasts 230-MHz bandwidth. The frequency switching speed is better than 65 ns. Phase noise is better than –105 dBc/Hz offset 1 kHz from a 100-MHz carrier while spurious levels are less than –55 dBc at 180 MHz.
In addition to supplying PLL synthesizers and units that combine PLL and DDS technologies, the company also supplies the DCP-1 DDS with a dual-accumulator architecture for generating linear frequency modulation (linear FM or chirp) waveforms. Operating at clock rates to 500 MHz, it generates output signals to 230 MHz in a module measuring 1 × 5 × 7 in. The compact module includes a power supply, reference source, clock generator, output filter, control interface, and cooling fan. Suitable for applications in electronic warfare (EW), missile seekers, compressive receivers, and synthetic aperture radars (SARs), it is also available in a 5.25-in.-high rack-mount enclosure.
The WaveCor line of modular frequency synthesizers from ITT Microwave (www.ittmicrowave.com) is also based on DDS technology. As with Meret Optical, ITT Microwave derives its DDS lineage from an earlier company, in this case Stanford Telecom. ITT’s model STEL-2375B is a DDS chirp synthesizer that operates at clock frequencies to 1 GHz to generate output frequencies from DC to 400 MHz. Its 32-b frequency resolution also results in steps as fine as 0.23 Hz while its precision DAC exhibits spurious products of less than –50 dBc. The modular synthesizer measures just 2.33 × 1.14×0.21 in.
Among the smallest of packaged frequency synthesizers, the FSW190410- 100 surface-mount unit from Synergy Microwave (www.synergymwave.com) operates from 1900 to 4100 MHz in 1- MHz steps in a package measuring just 0.940 × 0.940 × 0.300 in. With output power of +3 dBm and spurious levels of –70 dBc, the miniature synthesizer settles to a new frequency in typically 10 ms. The phase noise is –85 dBc/Hz offset 1 kHz from the carrier and –110 dBc/Hz offset 100 kHz from the carrier.
Modular frequency synthesizers from dBm (www.dbmcorp.com) include broadband units in the FSS line with coverage from 6 to 18 GHz. With +13 dBm output power and standard frequency resolution of 1 MHz, the compact unit exhibits spurious content of –50 dBc. The phase noise is –65 dBc/Hz offset 100 Hz from the carrier and –90 dBc/Hz offset 100 kHz from the carrier. The modular synthesizer measures 8.75 × 4.76 × 4.75 in. and has better than 200 µs switching speed with 14- b binary control.
Although many of the previously mentioned sources have leveraged DDS technology for high resolution and fast switching speed, direct analog techniques have been used for many years longer to generate clean and stable output signals with extremely fast switching speed. One of the best known suppliers of this genre of synthesizer, Aeroflex (www.aeroflex.com), supplies its FS5000 direct-analog frequency synthesizer into a wide range of applications, including automatic test equipment (ATE), electronic-warfare (EW) simulators, radar, and radar-cross-section (RCS) measurements. Available in frequency bands from 300 MHz to 26.5 GHz, the frequency synthesizers employ multiplication and division of reference sources to create a comb of frequencies from which the desired output is selected by means of switches and filters. Since all output frequencies are"on" and all times and desired signals need only be selected, this architecture yields blazing settling times. Although the approach has a high component count, it leads to state-of-the-art switching speed with relatively good spectral purity (depending upon the quality of the reference source).
The FS5000, for example, achieves frequency switching speed of 200 ns or better across its frequency range. It delivers +10 dBm across the frequency range, and switches by means of parallel BCD or GPIB control. Supplied in a rack-mount chassis, the synthesizer features standard frequency resolution of 1 MHz with higher resolution available as an option.
The DS series of wideband direct frequency synthesizers from Herley- CTI (www.herley.com) provide coverage from 10 MHz to 20.48 GHz in a single unit with frequency switching speed of 30 ns to 1 s. Frequency resolution is as good a 1 Hz while phase noise is a low 120 dBc/Hz offset 10 kHz from a 10-GHz carrier. For applications requiring less bandwidth, the company also offers the DSX series of direct frequency synthesizers with 25-percent bandwidths from 0.5 to 18.0 GHz.
The firm also provides a miniature frequency synthesizer for military and commercial applications called the VSS series sources. Covering bandwidths of 5 to 17 percent at frequencies from 500 mHz to 14 GHz, these compact synthesizers measure just 2.25 2.64 0.5 in. but achieve 115 dBc/Hz phase noise offset 10 kHz from a 900-MHz carrier and 137 dBc/Hz phase noise offset 100 kHz from the same carrier.
The SLS series of fast-tuning synthesizers from MITEQ (www.miteq.com) cover tuning ranges to one-half octave from 1 to 15 GHz. The PLL-based synthesizers require just 500 s to acquire phase lock for a new frequency and achieve in-band spurious performance of 70 dBc. Frequency step sizes range from 200 kHz to 10 MHz (depending upon frequency) while minimum output power is +10 dBm.
Although best known for their lownoise YIG oscillators, Micro Lambda Wireless (www.microlambdawireless.com) also supplies frequency synthesizers based on its YIG technology. The MLSE series of wideband synthesizers, for example, is available in three variations covering 2 to 20 GHz, 1 to 22 GHz, and 2 to 22 GHz, each with 1-Hz frequency resolution. The sources provide as much as +20 dBm output power with full-band switching speed of 31 ms or better and switching speed of 10 ms for a 100-MHz jump. Spurious content is 60 dBc while phase noise is 78 dBc/Hz offset 100 Hz from a 10-GHz carrier and 117 dBc/Hz offset 100 kHz from the same carrier. The MLSE synthesizers measure 7 5 2 in.
The company has also developed the MLSL line of miniature frequency synthesizers based on permanent-magnet YIG-tuned oscillators (PMYTOs). The MLSL series offers 2-GHz tuning ranges from2 to 12 GHz in a package measuring 2.5 2.5 1.0 in. The phase noise is typically 98 dBc/Hz offset 10 kHz from the carrier while spurious levels are a low 70 dBc. The synthesizers, which consume less than 6 W power, feature switching speed of 100 ms.
Wide Band Systems (www.widebandsystems.com), perhaps best known for their instantaneous-frequency-measurement (IFM) receivers and digital frequency discriminators (DFDs), also offers a wideband, fast-tuning synthesizer that operates from 2.25 to 18.0 GHz. Available in a two-space, rack-mount enclosure or a compact 6 6 1 in. module, the frequency synthesizer provides +13 dBm output power with 60 dBc maximum spurious levels. It achieves phase noise of typically 75 dBc/Hz offset 1 kHz from the carrier and typically 95 dBc/Hz offset 100 kHz from the carrier. The switching speed is typically 3 s and no longer than 5 _s to settle to a new frequency. Standard frequency resolution is 1 MHz, although resolution as fine as 5 kHz is available as an option.
One of the highest-power synthesizers is the indirect MG3690B series sources from Anritsu Co. (www.us.anritsu.com), with +17 dBm output power to 20 GHz in standard models and +23 dBm output power to 20 GHz as an option. The synthesizers, which are available from 0.1 Hz to 65 GHz in different band models, can switch across 1-GHz frequency steps in about 5 ms. With the addition of the company's 63850-xx series banded waveguide multipliers, the frequency range can be extended 325 GHz.
Another extremely wideband frequency synthesizer is the UFS-18 from Elcom Technologies (www.elcomtech.com), a direct analog synthesizer with 200-ns switching speed from 300 MHz to 18 GHz. It features 1-Hz standard frequency resolution with +10 dBm output power and spurious levels of 65 dBc. The phase noise is 55 dBc/Hz offset 10 Hz from a 10-GHz carrier, dropping to 85 dBc/Hz offset 100 Hz from the same carrier, 117 dBc/Hz offset 10 kHz from the same carrier, and reaching 140 dBc/Hz offset 10 MHz from the 10-GHz carrier. The synthesizer is equipped with an internal ovencontrolled crystal oscillator (OCXO), which delivers frequency accuracy of 1 PPM. It can be programmed remotely by means of 44-b parallel BCD command codes or optionally with GPIB.
Programmed Test Sources (www.programmedtest.com) is one of the longestrunning suppliers of direct analog frequency synthesizers. Their highest-frequency instrument, the PTS 6400, tunes from 1 to 6400 MHz with 20 s or better switching speed. Although its basic architecture is analog, it also incorporates a DDS to achieve phasecontinuous switching with frequency resolution as fine as 1 Hz. Output power ranges from 3 to +7 dBm with 1 dB flatness. The PTS 6400 exhibits phase noise of 99 dBc/Hz offset 100 Hz from the carrier and 116 dBc/Hz offset 10 kHz from the carrier, with a noise floor of 136 dBc/Hz. Like the FS5000, it is remotely programmable by BDC or GPIB .
The FS5000 and PTS 6400 are examples of full-sized, rack-mountable instruments. But some customers, especially for military test applications, have sought more compact sources with similar performance. In support of this trend, Aeroflex developed their 3020 series of PXI synthesized signal generators that occupy three slots in a 3U PXI chassis. The 3020 series includes three models with frequency ranges of 250 MHz to 2.5 GHz, 250 MHz to 2.7 GHz, and 86 MHz to 6 GHz, with frequency switching speed of typically 250 s. The sources feature wide modulation bandwidth of 28 MHz in support of complex modulation formats as used in digital radios, with adjustable level control of 120 to +5 dBm and impressive level accuracy of 0.3 dB.
The PXI-5670 from National Instruments (www.ni.com) is another synthesized signal generator in the PXI format, capable of vector output signals from 250 kHz to 2.7 GHz with 22- MHz real-time bandwidth. It's 16-b digital waveform generation capabilities make the three-slot, 3U PXI source (Fig. 1) ideal for creating a wide range of modulated waveforms, from quadrature amplitude modulation (QAM) to frequency-shift-keying (FSK) modulation. It can control power levels from 145 to +13 dBm with typical tuning speed of 35 ms. The typical phase noise is 87 dBc/Hz offset 1 kHz from the carrier while spurious content is typically80 dBc.
To meet the rugged requirements of military applications, Elcom Technologies has developed the VMESG frequency synthesizer in the VME module format. With a tuning range of 0.5 to 18.0 GHz and frequency resolution as good as 1 Hz, the frequency synthesizer (Fig. 2) occupies 3 or 4 slots in a VME mainframe and uses the VME 64X interface for communication to the host processor and other connected VME equipment. At 10 GHz, the phase noise is 75 dBc/Hz offset 100 Hz from the carrier and 95 dBc/Hz offset 10 kHz from the carrier. The phase noise drops to 1450 dBc/Hz offset 10 MHz from the carrier. Harmonics are55 dBc while spurious levels are a respectable 60 dBc.
Across its wide frequency range, the VMESG frequency synthesizer delivers +11 dBm output power with 1 dB amplitude flatness. What sets this source apart from other VME units is its frequency agility, with the capability of settling to a new frequency in 200 s or less. The synthesizer can also settle within 1 dB of a new amplitude level is less than 50 s. Switching speed is remotely controlled by means of commands sent on a 44-b parallel binarycoded-decimal (BCD) VME Standard bus. As an option, the company also offers a version with GPIB or Ethernet port.
Because some synthesizers are required to generate digital modulation formats, some include inputs for external inphase (I) and quadrature (Q) modulation inputs or feature internal I/Q sources for generating wideband modulation. The newest of these is the 2910 series of vector signal generators from Keithley Instruments (www.keithley.com). With a modulation bandwidth of 40 MHz across a continuous frequency range of 400 MHz to 2.5 GHz, the generator can also achieve more than 100 MHz modulation bandwidth using external I and Q sources. The half-rack instrument is geared for high-frequency testing requiring complex modulation formats and includes a built-in arbitrary waveform generator and waveform library.
Even wider modulation bandwidth can be found on the SMU200A vector signal generator (Fig. 3) from Rohde & Schwarz (www.rohdeschwarz.com) which boasts an I/Q modulation bandwidth of 200 MHz at frequencies from 100 kHz to 2.2 or 3.0 GHz. The synthesizer achieves phase noise of 135 dBc/Hz offset 20 kHz from a 1-GHz carrier and, like the 2910, has a built-in arbitrary waveform generator.
The merging of synthesis and digital-signal-processing (DSP) technologies can be found in a growing number of arbitrary waveform generators that define output waveforms by means of digital input signals. These include the N8241A dual-channel synthetic-instrument (SI) module from Agilent Technologies (www.agilent.com). Designed in the LAN eXtension for Instrumentation (LXI) module format to fill military test needs for software-definable instrumentation (see Microwaves & RF, August 2005, p. 78), the N8241A samples to 1.25 GSamples/s with 15-b resolution to produce single-ended or differential output signals with 500- MHz bandwidth per channel or 1-GHz bandwidth in I/Q format. The phase noise is 95 dBc/Hz offset 1 kHz from the carrier and 115 dBc/Hz offset 10 kHz from the carrier. Spurious performance is 75 dBc/Hz.
LeCroy Corp. (www.lecroy.com), although associated with high-performance oscilloscopes, offers the PXA125 arbitrary waveform generator in a oneslot 3U PXI format. Operating with 14-b resolution at clock rates to 125 MSamples/s, the source can generate frequency-agile and modulated signals for modulating higher-frequency carriers from a microwave source, for example. Following the 40-percent convention, it yields output signals to about 50 MHz with 1 Hz resolution.
Tektronix (www.tek.com) also offers a series of arbitrary waveform generators with 4.2 GSamples/s sampling rate and 16-b vertical resolution, including the model AWG710B which operates at sampling rates to 4.2 GSamples/s and generates outputs to 2.1 GHz. It features a real-time sequencer that allows the creating of infinite waveform loops, frequency-hopped patterns, and arbitrary IF and I/Q signals.