June 2011

Digital and Microwave Technologies Collaborate in Next-Generation
EW Receivers
By Marc Couture Director, Product Management, Microwave and Digital Solutions, Mercury Computer Systems

If all the signals in the electromagnetic (EM) spectrum were visible, and each frequency was a different color, it would be impossible to see anything. “EM smog” would blanket every inch of our vision. Imagine trying to sort and identify the characteristics and location of each signal in this EM smog. Signals might hop from one frequency to another, be modulated to become almost undetectable, or appear for a fraction of a second and then reappear perhaps with a different modulation technique and at another frequency, adding to the difficulty. In essence, this describes the task of today’s electronic warfare (EW) receiver, which is more difficult with each passing year as threats become more complex and diverse. Effectively addressing this plethora of unpredictable signals requires innovative design and components that deliver unprecedented performance.

This challenge is compounded by the extraordinary density of the spectrum in many parts of the world. A standard frequency allocation chart demonstrates the problem graphically, as it shows the EM spectrum by the type of service allocated to each spectral slice. The chart is so densely packed with allocations for commercial entities such as wireless carriers, unlicensed services, government agencies, scientific organizations, and the military (some of which overlap), that it’s difficult make sense of it. This signal density makes the challenge for EW receivers especially hard in the urban environments where our forces are often deployed.

Solution Starts at the Antenna
In a “typical” EW application an antenna or several antennas capture signals over a given bandwidth. These signals are sent to a tuner/receiver, downconverted to a lower frequency and then converted from the analog to the digital domain. At this point manipulation of the signal varies based on the mission.

For example, in the case of radar jamming or other electronic countermeasures, a Digital RF Memory (DRFM) approach can be used. With a DRFM, a signal must be identified and a deceptive signal created using a techniques generator, amplified, and sent back through the transmission chain. All of this must occur at incredible speeds, as the target being “lit up” by a radar has only fractions of a seconds to jam the signal before a weapon is launched.

To equip EW receivers to successfully meet this type of sophisticated challenge, design engineers must focus on several fronts simultaneously:

• increasing tuning speed
• achieving broader instantaneous bandwidths and sensitivity, and
• ensuring coherency over multiple channels, among others

These are often conflicting goals as optimizing a system for one may have a degrading effect on the others. Fortunately, digital technology can supplement advanced microwave design techniques and technologies for addressing all these challenges simultaneously. FPGAs, very fast ADCs with high bit depth, high-performance CPUs and, in some cases, graphics processing units (GPUs) are valuable tools available for this purpose.

Tuning speed is critical, as this metric plays a major role in overall system agility. The use of direct digital synthesis (DDS) is a virtual necessity in current systems. When compared to PLL-based alternatives, DDS enables greater frequency agility, lower phase noise close to the carrier by essentially decoupling tuning speed from phase noise, less reference clock jitter, and much more precise control of phase during transitions.

“Ping-ponging” is another tried-and-true technique for improving switching speed in multi-Voltage Controlled Oscillator (VCO) systems. While one VCO operates at a lower frequency, others can be “staged” to operate at higher frequencies (or vice versa), resulting in significantly faster retuning that when using a single VCO.

Increasing instantaneous bandwidth and sensitivity is a multi-faceted challenge that starts at the device level and must be maintained throughout the receive chain. Ensuring coherency among multiple tuners in nanoseconds is an equally daunting task, as there may be two dozen channels that must be synchronized in time. That is, when capturing signals from multiple antennas, the system must tag each pulse as it arrives from the antenna along with its angle of arrival (among other metrics) in order to triangulate, and thus locate, it through directional finding (DF) techniques.

RMS error must be exceptionally low, and very fine spatial discrimination must be achieved through LO distribution, which means synchronization must be extremely precise. For example, when sampling at 1 or 2 Gsample/s, sample periods are in the hundreds of picoseconds and clock edges are in femtoseconds. This requires digitizers with the highest available number of bits – currently ranging from 10 to 12 – which must be optimized to ensure that the effective number of bits (ENOB) remains as close as possible to the “raw” number specified by the manufacturer. This is accompanied by the need to retain the greatest bit depth possible while compensating for roll-off that increases at higher frequencies and simultaneously maintaining low spurious levels.

Mercury Computer Systems is meeting these challenges with new modular components developed as building blocks for EW solutions. For example, Mercury’s RFM-1802RF tuner and synthesizer covers 500 MHz to 18 GHz with 80 MHz and 500 MHz outputs centered at 160 MHz and 1 GHz respectively. The IF outputs interface directly with an external ADC on one of the company’s FPGA-based digital receivers such as the DCM-V6-OVPX. The receiver employs three Virtex-6 FPGAs and uses FPGA Mezzanine Cards (FMCs) that ensure upward compatibility with advances in technology. The eight-channel version has 16-bit, 250 Ms/s channels.

At high sample frequencies rates and bit depths, clock jitter along with isolation between the clocks within each component is a thorny issue and must be accommodated effectively to ensure full ADC performance. While some applications require only the traditional 10-MHz reference clock, a stand-off jamming platform must isolate an emitter tens of kilometers away. In this case, even minute inaccuracies are compounded, so the ability to precisely synchronize sample clocks is essential. In short, time-tagging signals from many channels while achieving coherency in local oscillator (LO) distribution and tuning control, and distributing the resulting data to FPGAs and then across a high-speed fabric such as serial RapidIO is a complex system level problem.

Obviously, achieving very low latency throughout the system is critical, and one of the often-overlooked but vital elements in making all this “work” is clock distribution, which requires very sophisticated paths for the control and distribution of LOs. As the bit resolution of ADCs increases, so too does jitter that produces distortion – the bane of achieving higher dynamic range – which must be dramatically reduced as it can be cumulative throughout such a complex system, reducing overall accuracy.

The ability to achieve sensitive, wider instantaneous bandwidths requires attention to elements of the system. For example, Mercury has developed new superheterodyne architectures that build on traditional requirements such as filtering, reducing spurious signal content, careful frequency conversion design, and developing very-low-noise front ends. Clean and super agile LOs are synthesized via sophisticated DDS techniques that optimize tuning speed while preserving phase noise figures often associated with clean PLL implementations. Finally, digitizer-FPGA designs are optimized to maximize on SNR, SINAD, SFDR, and ENOB, all critical for preserving fidelity and crucial for optimal mission performance.

Summary
EW receiver designers are striving to address requirements that include both complex signal environments and deceptive techniques that simply did not exist in years past. Fortunately, there are tools to help them overcome the hurdles that range from the immense power of FPGAs and high-speed ADCs that continuously raise the frequency at which signals can be directly sampled, to design and fabrication techniques that allow greater functional integration. This “digitization of EW” and the application of innovative approaches to RF and microwave design, like those seen at Mercury, will usher in the next generation of EW receivers that far surpass their predecessors in all aspects of performance.

Mercury Computer Systems
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