SQUEEZING A 2U RACK MOUNT microwave frequency synthesizer into a two-slot VME card takes design innovation and experience. Delivering leading edge spectral purity and high speed switching is a challenge for even the best microwave designers. As a leader in high speed switching synthesizers and low phase noise designs, Elcom Technologies appears to have reached the right balance in their creation of the commercial-off-the-shelf (COTS) VMESG 1-to-20-GHz VME-form-factor frequency synthesizer. Its fast switching speed and spectral purity meets or exceeds the high levels established in the company's much-larger IBS Series rack-mount frequency synthesizers.
The VMESG frequency synthesizer (Fig. 1) was designed into the modular VME format as a "synthetic instrument" for larger flight-line test systems. Its modular design allows for numerous functions to be added as optionsincluding a wide range of interfaces, such as binary-codeddecimal (BCD), Ethernet, and GPIB. It can also be equipped with frequency modulation (FM) as an option and can even be supplied in a 19-in.-rack-mount format or as an original-equipment-manufacturer (OEM) module.
In spite of its small size, the VMESG (Fig. 2) incorporates several frequency synthesizer technologies to achieve its combination of fast switching speed and excellent spectral purity, including indirect-analog and direct-digital-synthesis (DDS) technologies. Direct-analog frequency synthesizerswhich select output frequencies by means of switching and filtering from signals created by multiplying and dividing signals from a low-noise reference oscillator, such as an oven-controlled crystal oscillator (OCXO)deliver the fastest performance among frequency synthesizer technologies. But they tend to be complex and expensive because of the number of component required. DDS frequency synthesizers have improved in recent years in terms of switching speed and overall performance, and Elcom has leveraged these advances into high performance levels within their own synthesizer designs, including the VMESG frequency synthesizer (see table).
The switching speed for the VMESG is specified as a maximum of 10 s for a fullband frequency step, with less time required for a smaller sized frequency step (Fig. 3). Synthesizer switching speed is critical for a number of commercial and military applications. By way of example, agile synthesizers are required for cellular communications technologies such as code-division-multiple-access (CDMA) communications and in radios based on spread-spectrum techniques, as well as in tactical military radios requiring encryption and security for the battlefield. Fast-switching frequency synthesizers are also commonly used as signal sources in automatic-test-equipment (ATE) systems. Some measurements, such as characterization of an antenna, require a massive number of data points, and a fast-switching frequency synthesizer can make a significant difference in the total test time required.
Some years ago, Elcom Founder and Chief Technology Officer (CTO), Uri Yaniv, explained the importance of frequency switching speed to Microwaves & RF. He noted that a synthesizer's frequency switching speed is a function of circuit topology and synthesizer technology, with direct-analog technology offering the fastest synthesizer solutions, and DDS technology improving. He also pointed out that the tuning speed of indirect frequency synthesizers was limited to tens of microseconds, but more typically in the millisecond range, with Elcom's IBS Series frequency synthesizers among the fastest of indirect frequency synthesizers, with switching speeds from 10 to 100 m at frequencies to 20 GHz.
The VMESG two-slot VME frequency synthesizer works with an external frequency reference at 100 MHz, such as an oven-controlled crystal oscillator (OCXO) or temperature-controlled crystal oscillator (TCXO), capable of 0 dBm signal power within 2 dB. By using an external reference source, an operator can choose their desired level of frequency stability. The synthesizer tunes across its 2-to-20-GHz frequency range with standard frequency resolution of 10 kHz, with an option (option 101) for selectable frequency resolution of 1 Hz. It generates typical output power of +23 dBm. The output power remains flat with frequency within a narrow 1-dB window (Fig. 4). This high level of output power results from Elcom's extensive use of high-frequency monolithic-microwave-integrated-circuit (MMIC) amplification.
The VMESG is actually designed as two smaller circuit assemblies that fit within the two-slot VME module. One of these circuit assemblies contains the frequency synthesizer circuits (including a DDS), while the other assembly combines broadband amplification, custom filters, fast pulse modulator with 70-dB on/off ratio, and attenuation to reduce harmonic levels and provide an output signal level control over a 30-dB dynamic range. In addition to controlling harmonics to −35 dBc or less, the VME synthesizer suppresses spurious levels to −60 dBc. The company designs and fabricates its own filters and other microwave components for the VMESG and other frequency synthesizers. It uses a ProtoMat laser-driven prototyping machine from LPKF to etch its own printed-circuit boards (PCBs) from commercial highfrequency microwave laminates such as RT/duroid 5880 material from Rogers Corp.
Elcom also performs its own environmental testing, using an in-house environmental chamber from Qualmark. Not only is the chamber used to screen for failures in production units, but it has also been invaluable in searching for weaknesses in prototype designs. In addition to its control of a wide temperature range, it can also change temperatures quickly for conducting thermal shock testing. As Yaniv notes, "We want to understand the limits of a design, not just the specifications." Having the in-house environmental chamber arms the company to perform advanced highly accelerated life testing (HALT), as well as highly accelerated stress screening (HASS) on its products for achieving high reliability.
The VMESG exhibits low phase noise across its frequency range, characterized by an E5052B signal source analyzer from Agilent Technologies with top measurement frequency of 26.5 GHz. The frequency synthesizer is evaluated for absolute rather than residual phase noise. A residual phase-noise measurement cancels the effects of external sources, such as power supplies and clock generators (often referred to as "additive phase noise"). An absolute phase-noise measurement includes the noise from the external sources and, in many ways, is more representative of the operation of the frequency synthesizer in an actual system, such as a receiver. The phase-noise floor of the VMESG is specified as −145 dBc/Hz, although the actual performance is more like −150 dBc/Hz.
To demonstrate the spectral purity of the VMESG frequency synthesizer, phase-noise measurements were performed at carrier frequencies of 9.990 and 19.990 GHz (Fig. 5 and Fig. 6, respectively). As Fig. 5 shows, the phase noise around a 9.99-GHz carrier is −109 dBc/Hz offset 1 kHz from the carrier, −122 dBc/Hz offset 10 kHz, −124 dBc/Hz offset 100 kHz, −125 dBc/Hz offset 1 MHz, −140 dBc/Hz offset at 10 MHz, and −145 dBc/Hz offset 40 MHz. In Fig. 6, the measured phase noise around a 19.99-GHz carrier is −104 dBc/ Hz offset 1 kHz from the carrier, −113 dBc/Hz offset 10 kHz, −118 dBc/Hz offset 100 kHz, −119 dBc/Hz offset 1 MHz, −135 dBc/Hz offset 10 MHz, and −148 dBc/Hz offset 40 MHz.
The VMESG frequency synthesizer is a good fit for many of the company's own receiver designs, including the SIR-4000 series of microwave receivers developed for demanding signal monitoring applications. SIR-4000 models cover 0.5 to 18 GHz, 0.5 to 26.5 GHz, and 0.5 to 40 GHz with 10-Hz tuning resolution. The receivers employ high-performance frequency- conversion circuitry combined with digital-signal-processing (DSP) techniques to provide eight selectable intermediate-frequency (IF) bandwidths from 0.5 to 40.0 MHz around a 70-MHz IF center frequency. The receivers can be controlled remotely by means of Ethernet, RS-232, or RS-422 interfaces.
Similarly, the VMESG frequency synthesizers are available with numerous optional control interfaces. Standard units are equipped with a VME 64x interface for automatic use in a VME chassis. But the frequency synthesizer can also be equipped with an Ethernet interface (option 102), a GPIB interface (option 103), a BCD interface (option 105), or an RS-232 or RS-485 interface (option 104). The VMESG 2-to-20-GHz VME frequency synthesizer is outfit with SMA female connectors at the reference-oscillator input port and RF output port. It weighs less than 5 lbs. and is ideal for integration in an automated system, whether for signal reception or test purposes, including in EW, SIGINT, ELINT, and COMINT receivers. The compact frequency synthesizer includes pulse-modulation capability with 70-dB on/off ratio, and is designed for operating temperatures from −20 to +60C.