New Signal-Generation Techniques Enhance Electronic Warfare Environment Simulation: The Keysight UXG Agile Signal Generator (N5193A)

1. Introduction
2. The Landscape of EW Solutions
3. Benefits of a DDS-Based Architecture for EW applications
4. Evolving Needs in EW Signal Generation
5. Keysight’s New Agile Signal Generator Gets Closer to Reality
1. DDS signal generation
2. New solid-state switches optimized for agile signal generation
3. Pulse descriptor words: Speaking the language of the EW engineer
6. Conclusion
7. Related Information

Major improvements in the bandwidth and purity of digital-to-analog converters (DACs) and digital signal processing (DSP) have brought direct digital synthesis (DDS) over the technology horizon to practicality for agile LO and electronic warfare (EW) threat simulation. Keysight has combined these improvements with new switching technology to create a signal generator ideally suited to the particular demands of agile local oscillators (LOs) and modern EW environment simulation.

In recent years, several different architectures have been used to produce the agile, wideband signal sources used to simulate EW threats and, more fundamentally, to produce local oscillators and RF pulse generators that can switch quickly over wide frequency and amplitude ranges. The new Keysight UXG agile signal generator represents a major technological and architectural advance that changes the landscape of tradeoffs and performance available to EW engineers.

The generation of EW signals, both agile continuous-wave (CW) elements and complex pulse environments, can be seen as a special—and very demanding—case of traditional RF and microwave signal generation. The signals are similar in fundamental ways but the performance demands tradeoffs and some special capabilities are different. Specifically, EW signal generation requires a high degree of agility in frequency and amplitude, along with the ability to generate complex environments of pulses with controlled timing, amplitude and phase relationships. EW signal generation also requires the generation of very short RF pulses and this demands very wide instantaneous bandwidth.

Generating individual signals is different from generating signal scenarios. Some signals can be generated in a sequential or repeating fashion, while other signals must be generated with parameters that change in a way that is commanded live or on-the-fly for purposes such as closed-loop or “hardware in the loop” testing.

Effective EW testing demands signal generation that is highly agile, highly accurate and extremely pure. Unfortunately, these characteristics are often in conflict with typical signal generator architectures and topologies:

• PLLs for indirect analog synthesis: Traditional high-purity approaches such as PLLs locked to a stable frequency reference can provide low phase noise, accurate amplitude and high dynamic range (low spurious and intermodulation distortion). PLL techniques such as fractional-N also offer very fine frequency resolution, often from a single PLL. Unfortunately, PLLs have multiple elements in the signal generation chain, many of which have inherent delays in filtering mechanisms that reduce agility.
• Direct analog synthesis: In this approach several frequency references, perhaps derived from a single crystal oscillator, are combined using arithmetic operations such as multiplication/division and addition/subtraction to create the desired output frequency. The arithmetic operations and their configuration can be switched rapidly to enable frequency agility. However, this approach is more complicated and costly than indirect synthesis and generally cannot provide phase-coherent or phase-continuous output signals.

Both indirect and direct analog synthesis approaches have limitations in the creation of complex multi-emitter environments. Generation of multiple simultaneous signals requires multiple synthesizers that may need to be synchronized and then combined and amplified to create the desired signal environment.

• Direct digital synthesis: This newer approach, based on high-speed DACs, can provide virtually instant changes in amplitude and frequency. DDS techniques can generate very complex signals such as multi-emitter environments, with the complexity and number of emitters limited only by the available memory and DSP capabilities. Despite their theoretical advantages, DDS approaches have had limited application in EW simulation due to inadequate signal quality and insufficient bandwidth.

The DAC-based DDS approach offers compelling advantages for EW applications, both agile LO generation and EW environment simulation:

• Very high agility in frequency, amplitude and phase. Signal output can be changed arbitrarily with a new stream of samples, and agility is limited only by bandwidth filtering.
• Support of multiple signals and complex scenarios. Sample sequences can represent multiple signals as easily as a single one.
• Phase repeatability. Signal-generation calculations can include phase accumulators programmed with any desired phase relationship.
• Good match to evolving threats. The agility and flexibility of DDS-based solutions allows them to accurately simulate the equivalent characteristics of the modern threat environment, and to adapt as these threats change.
• Simpler path from pulse requirements to actual signal output. DDS-based solutions can directly reproduce complex and dynamic pulsed signals as they are mathematically created from desired pulse characteristics.
• Signal quality. DDS architectures can generate signals with low spurious and without the phase noise pedestal that is characteristic of PLLs.

For quite some time, DDS techniques have been well understood as a solution to the challenges of frequency agility and complexity. However, they have not been available at the appropriate point on the DAC performance envelope. That is, they have not had the required performance for EW applications in terms of bandwidth and signal quality (primarily distortion and phase noise).

The EW environment is an ongoing contest between development and implementation of improved systems, and the analysis and countermeasures needed to deal with them.

Specifically, engineers developing EW systems need RF/microwave sources with the performance, bandwidth and flexibility to stand in for everything from agile LOs to scalable EW environment simulators. Engineers need these simulators early in the development and testing phase, to avoid late-stage surprises or failures, which can delay project development by requiring additional design cycles and repeated testing or verification.

In other words, aerospace and defense engineers need the kind of complete, fully-developed, integrated solutions that wireless engineers have enjoyed for complex signal generation. Such a solution allows them to focus on system development instead of becoming experts in signal generation.

The UXG agile signal generator brings more extensive and realistic testing to earlier design phases, allowing engineers to optimize and verify system performance before the expense, potential delays and poor repeatability of field testing. It will also significantly shorten the path from the gathering of signal intelligence to the creation of realistic simulated threats.

The performance and capability improvements of the UXG are based on three innovative and proprietary technologies:

• DDS-based signal generation with an unmatched combination of purity and bandwidth
• Solid-state switches that change output levels very quickly over large amplitude ranges and with accurate time alignment
• The ability to create complex and precise outputs using pulse descriptor words (PDWs), the natural language of EW engineers

DDS signal generation

The core of the DDS architecture is a unique DAC that covers a bandwidth of more than 1 GHz, providing signals of very high purity. In this DAC, both dynamic range and phase noise are dramatically improved. An example of the purity of a 10 GHz signal is shown in Figure 2.

Signal purity extends beyond dynamic range to include phase noise, which is vital to radar applications. For example, for a 10 GHz signal the UXG’s phase noise is -126 dBc/Hz at a 10 kHz offset. Phase noise for this agile signal source is comparable to high quality PLL-based signal generators.

This performance is available with industry-leading switching speed and very low latency for frequency-change commands. Basic frequency switching speed is 50 to 100 ns and the delay between external commands and frequency changes is 250 ns.

For some applications, it is essential to maintain specific phase and frequency relationships while signals are pulsed or frequency-hopped, and when different signals are interleaved in a sequence to simulate a threat environment. The DDS approach allows any phase or frequency trajectory to be produced, without limiting frequency or amplitude agility. In addition, multiple DDS sources can be linked together through clocks and triggers to provide phase coherence across multiple sources as a way to simulate steerable beam antennas or produce angle-of-arrival trajectories to evaluate direction-finding receivers. An example of phase continuity across transitions between a low frequency and higher frequency pulses is shown in Figure 3.

New solid-state switches optimized for agile signal generation

Comprehensive EW threat simulation requires generation of signals with a very wide range of accurate amplitudes, and amplitude agility that matches a source’s frequency agility. This agility requires coordinated switching from solid-state attenuators, and in the UXG this is implemented with new nanoFET MMIC devices invented and manufactured by Keysight.

These new switches are specifically designed for microwave and millimeter frequencies, with fast settling to avoid distorting the pulse shape. They provide an 80-dB agile amplitude range and their amplitude agility matches the frequency switching speed and latency of the synthesizer. The 82-dB agile amplitude range can be used between output levels of 0 to -120 dBm to handle the widest range of threat scenarios. To optimize cost and capability for different applications, the solid-state agile attenuator is optional and available as a retrofit or upgrade.

Pulse descriptor words: Speaking the language of the EW engineer

Most wireless signals are programmed in terms of I/Q parameters, but the signals gathered and organized for EW testing are defined in terms of PDWs. Each word describes all the parameters of an individual pulse: frequency, duration, amplitude, chirp rate, and so on. Signals are then created from lists of PDWs, as shown in Figure 4.

Marker outputs are available to coordinate execution of PDWs with other devices, and sequences of PDWs can be triggered and regulated by external triggers sent to the UXG. The UXG’s internal memory of 600,000 list elements is large enough to create a sequence 6 seconds long at 100,000 pulses/second, and fast enough to generate 5,000,000 pulses/second. The result is a signal generator that is easy to integrate in the EW simulation environment.

In addition to list-mode operation, the UXG can be operated in an “agile controller mode” via LVDS or BCD, where signals from an external control bus determine generator parameters on the fly (Figure 5).

Controller mode enables the generation of scenarios of unlimited length, controlled externally via signal parameters and triggers. Feedback to the external controller is used with routines in the controller to enable closed-loop testing (i.e., hardware-in-the-loop testing).

In development and mission-data reprogramming, better testing performed sooner enables deeper confidence in the performance of EW systems. The Keysight UXG agile signal generator lets EW engineers create complex scenarios when they need them.

Off the shelf, the UXG is a powerful building block, whether the need is for a dependable LO or a scalable threat simulator. Because the UXG is fluent in the language of EW systems, it accelerates the integration of newly acquired intelligence into up-to-date signal scenarios. With unmatched performance in areas such as switching speed and phase noise, the UXG makes it possible to generate increasingly complex simulations—and get closer to reality.

Keysight is a global electronic measurement technology and market leader helping to transform its customers’ measurement experience through innovation in wireless, modular and software solutions. Keysight provides electronic measurement instruments and systems and related software, software design tools and services used in the design, development, manufacture, installation, deployment and operation of electronic equipment. Information about Keysight is available at www.keysight.com.