Smaller is often better for many military/aerospace applications, although miniaturization often comes at a cost. Take RF/microwave frequency synthesizers, for example. Smaller signal sources, such as modules developed for portable use, may lack the output power or noise performance of a larger unit, such as a rack-mount frequency synthesizer intended for test-and-measurement applications.
Fortunately, at least one frequency synthesizer supplier, FEI-Elcom Tech, has accepted the challenges of delivering large performance in smaller packages. The firm recently announced a series of dual frequency synthesizers designed into standardized VPX Open System Architecture (OSA) modules as well as in ruggedized and customized modules.
More RF/microwave frequency synthesizers are being designed as modules that can be combined with similar component-sized assemblies, such as frequency upconverters and downconverters, local-oscillator (LO) sources, modulators, and even clock timing oscillators, to create compact receivers, transmitters, and transceivers for airborne or portable applications. Due to the small size of the modules, a multitude of functions can be integrated into one VPX equipment chassis, while the efficient bus control architecture allows for high-speed communications among the modules in a VPX chassis.
Packing a Pair
A high degree of integration can be found in FEI-Elcom Tech’s most-recent synthesizer line. The firm’s VPXMS-2500 dual synthesizer VPX modules (Fig. 1) leverage the latest monolithic-microwave-integrated-circuit (MMIC), field-programmable-gate-array (FPGA), and digital-signal-processing (DSP) technologies to create fast-switching (microsecond or faster) dual frequency synthesizers that fit within 6U VPX OSA modules. The performance provided by these compact frequency-synthesizer modules once required 6U spaces in a 19-in. rack.
1. The VPXMS-2500 synthesizers fit two frequency sources within standard VPX modules with modulation bandwidths as wide as 2 GHz. (Courtesy of FEI-Elcom Tech)
Dual-synthesizer models have been designed in the VPX format for standard frequency ranges of 5 to 10 GHz, 10 to 20 GHz, 5 to 20 GHz, and as wide as 1 to 20 GHz, with other frequency ranges to 67 GHz available as options. The synthesizer modules come with real-time modulation bandwidths as wide as 2 GHz for applications requiring advanced signal generation. They can also be supplied with phase-coherent switching between channels as an option.
Output signals for the dual frequency synthesizer are provided at a robust +15 dBm, flat within ±3 dB. The device features a wideband noise floor of better than −150 dBc/Hz. Its single-sideband (SSB) phase noise is quite low, whether measured close or far from the carrier. Phase noise drops steadily with distance from the carrier. For a 10-GHz carrier, phase noise is −126 dBc/Hz offset 100 kHz from the carrier, −138 dBc/Hz offset 1 MHz from the carrier, −148 dBc/Hz offset 10 MHz from the carrier, and −154 dBc/Hz offset 100 MHz from the carrier.
When One is Enough
For applications that only need a single frequency source, FEI-Elcom Tech’s DFS2000 series of single frequency synthesizers offer outputs in the range from 1 to 20 GHz, also in compact modules (Fig. 2). Based on traditional phase-locked-loop (PLL) indirect frequency synthesizer technology and advanced direct-digital-synthesizer (DDS) architectures, these miniature modules provide excellent spectral purity with fast switching speeds from a package measuring just 5.95 × 3.55 × 0.75 in.
2. The DFS2000 series features frequency synthesizers housed in compact modules for space-saving applications from 1 to 20 GHz. (Courtesy of FEI-Elcom Tech)
The DFS2000 series of single-frequency-synthesizer modules are well-suited for test systems and for signal-intelligence (SIGINT) receivers. The modules also provide a fixed low-noise LO signal at L band for receiver applications. The fast-tuning DDS-based synthesizers operate from 1 to 20 GHz with a standard tuning step size of 1 MHz (tuning steps as small as 1 Hz and as large as 10 MHz are available as options). High-speed tuning speeds of 10 µs or better can be achieved by using list or trigger sweep modes. The synthesizers excel in terms of noise performance, with harmonics of −20 dBc or better and spurious content of better than −70 dBc within ±100 MHz of the carrier.
Phase noise for the DFS2000 series single synthesizers, as measured with a commercial phase-noise test set from Agilent Technologies (now Keysight Technologies), is quite low, dropping to a noise floor of −150 dBc (Fig. 3). For a 20-GHz carrier, the SSB phase noise is −103 dBc/Hz offset 1 kHz from the carrier, −112 dBc/Hz offset 10 kHz from the carrier, −115 dBc/Hz offset 100 kHz from the carrier, −118 dBc/Hz offset 1 MHz from the carrier, and −133 dBc/Hz offset 10 MHz from the carrier.
3. The low phase noise of the DFS2000 series dual frequency synthesizers combines with low harmonics and low spurious noise to deliver excellent spectral purity to 20 GHz. (Courtesy of FEI-Elcom Tech)
The frequency synthesizers, which run with typical dc power consumption of 12 W at operating temperatures of −20 to +70°C, are well-suited for radar test, as well as EW and SIGINT receivers. They are designed for use with a reference/clock oscillator at 100 MHz, although synthesizers are available for other reference frequencies as an option.
Frequency synthesizers such as the DFS2000 series sources provide clean signals that can be mixed, multiplied, or divided although they lack the modulation needed for many system applications. The firm offers lines of single frequency synthesizers (without modulation) and signal generators (with modulation) at carrier frequencies from 625 MHz through 67 GHz in a variety of module formats, including standardized modules such as VME and VPX modules.