Laboratory Design Validation Testing (DVT) of Electronic Counter Measure (ECM) Systems Using Stimulus-Response Techniques
By Mike McKinley, PhD, Aeroflex

The best way to understand any Electronic Counter Measure (ECM) system’s behavior is to stimulate it with a known radar signal and to observe the system’s response. The ability to stimulate an ECM system with a known signal and view, analyze, and compare the stimulus and response signals side-by-side gives designers the opportunity to visualize, understand, and verify how their device operates. The Aeroflex AWARES (Advanced Wideband Analyzer Recorder and Emitter System) system, using its stimulus/response operating mode, supports this direct testing approach, thereby providing a means to fully understand the behavior of an ECM system.

As shown in Figure 1, the AWARES stimulus/response mode allows a stimulus radar signal to be played into an ECM system under test, while at the same time the ECM system’s response signal is recorded. These stimulus and response signals are time synchronized. Analysis software on the AWARES system allows their side-by-side comparison, so that the detailed behavior of the ECM system can be clearly observed and quantified. This comparison can include pulse measurements (e.g., ToA [Time of Arrival], frequency, pulse width, PRI [Pulse Repetition Interval], etc.), or plots versus time of the signal’s instantaneous envelope, frequency, and phase.

The AWARES System
An Aeroflex AWARES system, shown in Figure 2, is composed of the following functional components:

• Signal recorder (frequency tuning up to 18 GHz, 400/70 MHz instantaneous BW)
• Signal playback (frequency tuning up to 18 GHz, 400/70 MHz instantaneous BW)
• Signal storage (32 GB RAM per channel, additional 8 TB RAID)
• Server machine (CPU)
• Signal analysis software (Broadband Signal Analyzer [BSA] GUI application)
• Signal creation software (Vector Signal Simulator [VSS] GUI application)

Physically, a rack-mounted AWARES system is composed of two 3U chassis (RF and analog/digital conversion), and one 2U server (controller). For portability, these three chassis, plus a 1U power conditioner and a 1U combined monitor/keyboard, are combined into a single transit case, creating a self-contained platform.

Stimulus Signal Options
The stimulus signal provided by the AWARES system can come from a variety of sources. It can be recorded from an actual radar emitter; it can be generated from scratch using the VSS GUI application on the AWARES system; or it can be a combination of the two. The VSS software allows simple individual test signals to be generated (e.g., tones), as well as arbitrarily complex full signal environments to be generated by combining any of the following: recorded signals, user-generated signals (e.g., from Matlab), and signals taken from the VSS signal library. This library includes:

• General test signals
• Tone comb
• AWGN (Additive White Gaussian Noise)
• Communications signals
• PSK (Phase Shift Keyed)
• QAM (Quadrature Amplitude Modulation)
• Pulsed signals, with user controlled:
• Pulse rise/fall times
• Width
• PRI
• Center frequency
• Frequency hopping
• Chirp
• MoP (Modulation on Pulse)

In addition, the VSS pulsed signal allows individual control over each pulse in a generated signal via a spreadsheet-like PDW (Pulse Descriptor Word) file.

Alternatively, when the VSS software is used to generate the stimulus signal, an ideal version of the radar signal can be created, possibly including user-selected controlled impairments (e.g., a frequency error). This signal can optionally be included within a controlled signal environment. For example, the generated stimulus signal might include the ideal radar and also AWGN, interference, other ECM, etc., so that the behavior of the ECM system in the presence of these real-world effects can be investigated.

Yet another alternative is the combination case, where the signal from an actual radar is recorded, and used as the basis for a signal environment. The total signal environment, as described, can be generated using the VSS software, which might include AWGN, or interference, other ECM, etc. In this case, both the actual radar signal and real-world effects can be combined, creating a complex, realistic testing scenario.

Analysis of a Stimulus/Response Measurement
In an AWARES system, the Broadband Signal Analzyer (BSA) GUI application is used to control the recording hardware as well as analyze live or archived signals. The BSA software is composed of a number of “analysis functions” (e.g., spectrum, modulation domain, pulse analysis, PSK/QAM demodulation, etc.), and ties together their operation and the display of their results.

The input to a stimulus/response analysis is two time-synchronized signals—the stimulus and the response. The goal is to be able to view and summarize these two together, so as to better understand how the ECM system responded to the stimulus. There are three levels to this analysis: spectrum, modulation domain, and full pulse analysis. The first two levels are primarily single-channel (i.e., either the stimulus or the response). The last level, full pulse analysis, shows side-by-side analysis of the stimulus and response signals, including delta measurements between the two signals.

The BSA spectrum display (bottom) and spectrogram display (top) of a generic frequency-hopping radar are shown in Figure 3. In the spectrogram, the horizontal axis is frequency, the vertical axis is time, and the signal power at each time-frequency point is encoded in the color of the plotted point. This spectrogram shows the time-versus-frequency behavior of the signal. At the bottom of this figure, the spectrum display, with the max-hold enabled, shows the hop frequencies. These measurements only require knowledge of the frequency band of the signal, and provide a high-level view of the signal, as well as the specific hop frequencies.

Now that the frequency content of the signal is known, the next level of analysis uses the modulation domain. The BSA modulation domain analysis function shows detailed time-domain behavior of a signal in a specified band. This band could either be the band containing all of the hop frequencies, or, for very detailed measurements, might be only the band around a single hop frequency. Figure 4 shows the instantaneous envelope, frequency, and phase (all versus time) for a pulsed signal containing a linear frequency-chirp. The center plot in the figure (instantaneous frequency versus time) shows the frequency-chirp. This modulation domain level in the analysis provides a view of the signal in three domains simultaneously, and can be used to make coarse estimates of the pulse parameters, as well as to see effects like pulse envelope over/under-shoot, frequency-to-amplitude leakage, etc.

By zooming in on a single pulse, the frequency-chirp is seen to be linear and extends over 3 MHz (see Figure 5).

The final and most comprehensive level of analysis uses the BSA pulse analysis function. The use of this analysis function requires some knowledge of the signal’s pulse characteristics (e.g., pulse width, PRI, etc.).

Figure 6 shows plots of the instantaneous envelope, frequency, and phase versus time for the paired stimulus and response pulses. For this example, both the stimulus and response pulses contain a phase-encoded 13-bit Barker code. The top two plots in this Figure show the stimulus and response pulse envelopes versus time, the middle two show plots of their frequency versus time, and the last two, which are the most interesting here, show their phase versus time. The last two plots in this Figure show the phase-encoded Barker code in stimulus and response pulses.

This display can be used to view the detailed characteristics of both signals, and their relation in time. Figure 7 shows the per-pulse measurements and delta measurements made for the paired stimulus and response pulses.

The sequence of these paired, per-pulse measurements can be logged to a spreadsheet-like file, or plotted on a “strip chart” as shown in Figure 8. Logging these measurements to a spreadsheet compatible file produces a permanent record of a test, and also provides the raw data for further test summarization and analysis.

Figure 8 shows plots of four measurements from the paired pulses on channels 1 and 2 (stimulus and response). Any of the measurements shown in Figure 7 can be used in a plot like this, but these four are relevant for this example. For this example, the response pulse train contains both velocity-gate and range-gate pull-offs. The first two plots in this Figure show the Doppler frequency offsets for the stimulus and response pulses (frequency hopping has been removed). The first plot shows that the stimulus pulse train has no Doppler offset. The second plot shows the velocity-gate pull-off in the response. The third plot in this figure shows the frequency difference (delta-frequency) between the stimulus and response pulses, and also shows the velocity-gate pull-off. In the last plot, the time-difference between the stimulus and response pulse start times (delta-ToA) is shown. This plot shows the range-gate pull-off. The vertical red lines in these plots indicate times when pulses were missing from either the stimulus or response. These are times at which the ECM system either did not generate a pulse corresponding to a stimulus pulse, or generated an extra pulse not corresponding to a stimulus pulse.

Other Uses for Aeroflex AWARES System
In addition to its stimulus/response operating mode, an AWARES system provides support for the general recording and playback of signals. A signal must be recorded to be used as the stimulus in a stimulus/response measurement, but it can also be useful to record signals and save them to RAID for later use. For example, signals can be recorded in the field during a live exercise or in the presence of an interesting device. Afterward, the data can be taken back to the laboratory for detailed analysis or used for the stimulus of an ECM system.

Operating as a signal recorder, an AWARES system can frequency-tune to between 10 MHz and 18 GHz, and supports either a 400 MHz instantaneous bandwidth (using 8-bit ADC samples), or a 70 MHz instantaneous bandwidth (using 12-bit ADC samples). At the 400 MHz bandwidth, over 28 seconds of signal may be recorded. At the 70 MHz bandwidth, with no digital tuning, over 80 seconds of signal may be recorded. At the 60 MHz bandwidth, digital tuning may be used to trade reduced signal bandwidth for longer maximum recording durations and quicker analysis times. In this case, a sub-band within the selected 70 MHz band is chosen, and only this sub-band is recorded. This results in a possibly dramatic decrease in the size of the recorded signal, which translates into longer maximum recording durations or quicker analysis times. The factor of the savings is the decimation order, which can vary between four and 4096, in powers of two. For example, if a 10 MHz instantaneous bandwidth is sufficient for the signal, then a decimation order of sixteen may be used, and over 10 minutes of signal may be recorded. Longer record durations can be supported, for smaller signal bandwidths, using higher decimation orders.

Recorded signals may be analyzed using the BSA GUI application on the AWARES system. In addition to the general spectrum, modulation domain, and pulse analysis modules mentioned previously, other analysis modules are available which support digital demodulation (e.g., PSK, QAM, FSK, ASK), analog demodulation (e.g., AM, FM, PM, SSB), the measurement of power in arbitrary frequency bands, and the Environment Signal Parameter (ESP), a module which finds, tracks, and makes measurements on spectral “bumps.”

Operating as a signal generator, an AWARES system can frequency-tune to between 10 MHz and 18 GHz, and supports either a 400 MHz instantaneous bandwidth (using 8-bit DAC samples), or a 70 MHz instantaneous bandwidth (using 12-bit DAC samples). At the 400 MHz bandwidth, over 28 seconds of signal may be played. At the 70 MHz bandwidth, with no digital up-conversion, over 80 seconds of signal may be played. At the 70 MHz bandwidth, digital up-conversion may be used to trade reduced signal bandwidth for longer maximum playback durations. This processing operates analogously to the digital tuning in a signal recorder, with similar benefits. The signal generator can play back all AWARES recorded signals.

Roadmap for Aeroflex AWARES System
• Future plans include:
• Digital tuning and digital up-conversion (decimation and interpolation) for the 400 MHz bandwidth signals
• FPGA-based acceleration of the analysis processing
• Two new pulse recording modes, explained below

In the first of these new pulse recording modes, the pulse descriptor word (PDW) characteristics (e.g., pulse start time [ToA], width, power) of each pulse in the input signal are estimated in real-time and only these characteristics are stored. This results in the ability to store the pulse characteristics of an input signal continuously for many hours.

In the second new recording mode, the PDWs are estimated for each pulse as in the first mode. Also, the ADC samples from just before the pulse begins to just after it ends are stored. This case is less compact than the first new recording mode, but is much more compact than the full storage case. (The gain is approximately the ratio of the PRI to the pulse width.) This second mode allows for more complete post-processing of the pulses (e.g., chirp, MOP, rise/falls times, etc.), and for plots of the instantaneous envelope, frequency, and phase for cases in which having only the PDW is insufficient.

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