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.