Digital RF memories (DRFMs) are critical components in many electronic-countermeasures (ECM) defense-electronics systems. They provide the means of harvesting and reusing RF/microwave signal waveforms via almost instant analog-to-digital conversion and digital signal processing, using powerful processors, mixed-signal electronics, and dedicated software packed into compact modules.
Of course, as with other markets, military specifiers are seeking smaller, lighter, and more reliable DRFMs. Design efforts embraced by DRFM designers that employ goals based on reduced size, weight, and power (SWaP) are delivering a new generation of devices that are smaller and more powerful than before and come with innovative modularity for improved reliability and ease of use.
Smaller, lighter DRFMs are important not so much for legacy military/aerospace electronic systems, but more for their expanding roles in unmanned aerial vehicles (UAVs) and unmanned marine vehicles (UMVs) used for intelligence, surveillance, and reconnaissance (ISR) missions. In unfamiliar territory, for example, U.S. Army troops often refer to surveillance drones and their DRFM-based systems as “eyes in the skies.” Some of these military drones are unmanned aerial systems (UAS), capable of carrying multiple missiles and jammers in addition to cameras, receivers, and transmitters to perform many different electronic-warfare (EW) operations under remote control from a distance.
The DRFMs within these systems are typically part of instantaneous-frequency-measurement (IFM) subsystems. DRFMs also find use in commercial and civilian applications, such as police-radar jammers and for cellular-telephone test equipment.
For ease of integration into larger systems with IFM functionality, DRFMs have traditionally been designed as printed circuit boards (PCBs). They are then installed into plug-in enclosures with board-mounted RF and digital/control connectors to facilitate interconnections with other systems.
Board-level DRFMs (Fig. 1) from Mercury Systems, for example, are available with analog-to-digital and digital-to-analog converters (ADCs and DACs) operating at sampling rates to 2.2 Gsamples/s, plus powerful Virtex-5 microprocessors from Xilinx. Such microprocessors rely on software-based programming to modify the delay times between the ADCs and DACs so that signals received from a threat radar system can be delayed an amount of time corresponding to a different target location, and then transmitted back to the threat radar so that the apparent location of its target is different than the actual target.
1. Board-level DRFMs like that shown are powered by Virtex-5 processors/FPGAs and dedicated data converters. (Courtesy of Mercury Systems)
These compact board-level DRFMs rely on other system functions, such as RF/microwave frequency downconversion (receivers) and upconversion (transmitters), filtering, and amplification. However, they are available as part of IFM systems with frequency ranges from 0.5 to 2.5 GHz or 2 to 18 GHz, or as even more compact modules for airborne use.
The demand for lower SWaP has been aided by using system-on-chip (SoC) semiconductor devices such as microprocessors combined with ADCs and DACs. This allows the design of extremely dense and powerful DRFM architectures into standard module formats such as OpenVPX.
For example, the first member of the Quartz line of products, the Model 5950 eight-channel signal processor from Pentek, is based on a highly integrated Zynq Ultrascale+ RFSoC FPGA from Xilinx, which features eight high-speed ADCs and DACs integrated into the semiconductor circuitry with the microprocessor. As a result, the data converters are already available and do not need to be added to a board-level circuit solution.
2. A highly integrated FPGA/processor with integrated data converters is at the heart of this compact programmable eight-channel signal processor. (Courtesy of Pentek)
By packing so much power into a single chip, even a compact module such as a 3U OpenVPX circuit board (Fig. 2) can carry much greater functionality—a GPS receiver, a large amount (18 GB) of DDR4 SDRAM memory, a PCIe interface, a gigabit serial optical interface that supports 100 Gigabit-Ethernet connections, and an on-board timing bus generator. Because of the flexibility and programmability of having an RFSoC with FPGA at the heart of the design, the Quartz Model 5950 can serve any number of applications, including as a chirp generator, a data-acquisition system, and a DRFM. In fact, Pentek offers intellectual-property (IP) programming that has been developed and refined to turn this RFSoC module into a DRFM with programmable delays from 1 to 32768 data samples.
The same company’s Talon RTX 2590 small-form-factor (SFF) module is a 250-Msample/s RF/IF signal recorder that’s built for harsh environments—and for ease of data exchange after a mission (Fig. 3). This SFF signal recorder provides eight phase-coherent channels of 250-MHz, 16-b ADCs, allowing users to capture as much as 100 MHz of RF/IF signal bandwidth per channel with wide dynamic range. It measures just 7.688 × 4.880 × 14.125 in., supplied in a 1/2 MIL Air Transport Rack (ATR) enclosure that can also hold as much as 30 TB of solid-state-data (SSD) memory.
3. This RF/IF signal recorder can hold as much as 30 TB of SSD memory in a 1/2 ATR MIL enclosure, with the unique QuickPac packaging technology. (Courtesy of Pentek)
The signal recorder records speeds to 4 Gb/s and can be equipped with a GPS receiver for time-stamping and positioning of recordings plus optical I/O rear-panel connections as options. The programmable recorder features the convenience of modular, removable SSD memory as well as a removing operating system (OS) drive. It employs Pentek QuickPac drive packs to make it possible to quickly remove all data storage from a recorder via the front panel. Circular connectors on the rear panel include power and computer interconnections, as well as bulkhead-mounted SMA connectors for all RF/microwave signals, GPS, clocks, and triggers. All are sealed for RF emissions and moisture protection.
In general, DRFM suppliers are facing the need to develop smaller, modular units that can be used with a great deal of flexibility in different applications. Long-time DRFM supplier Curtiss-Wright employs commercial-off-the-shelf (COTS) devices and components in its modular DRFMs and frequency-conversion units. It also offers software-defined-radio (SDR) programming flexibility in support of UAVs for the Army and UMVs for the Navy. The firm has adopted the use of removable data storage for convenience, and in recognition of the growing amount of sensor data from both military and commercial applications, such as IoT devices.
Additional suppliers of DRFMs include Herley Whippany/Ultra Electronics, Israel Aerospace Industries, Mistral Solutions, and SA Photonics. Also in the mix are major defense contractors, such as BAE Systems, Elbit Systems, Northrop Grumman Corp., Raytheon Co., and the Thales Group.