Electronic Military & Defense Annual Resource

2nd Edition

Electronic Military & Defense magazine was developed for engineers, program managers, project managers, and those involved in the design and development of electronic and electro-optic systems for military, defense, and aerospace applications.

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Techniques When a spectrum analyzer runs in continuous mode, each sweep is much slower than the rate of change in the PRF, and any measurement will produce what is essentially a random value. Zero-span mode can provide better results, but only if a sufficient span of data can be captured (e.g., 4 ms). However, measurements will lack the phase or Doppler information needed for a detailed analysis. Also, a typical spectrum analyzer cannot display time- and frequency-domain information simultaneously, further limiting the efficiency of the analysis process. Defining A Viable Solution One alternative is a signal analyzer that uses a combination of high-speed, high-resolution sampling and digital signal processing (DSP). The interface of such instruments typically enables user control of time- and frequency-domain parameters and content. This level of control makes it possible to measure jamming techniques as they change over time — even very short periods of time. A suitable signal analyzer is equipped with a high-resolution analog-to-digital converter (ADC), a high-speed DSP for digital filtering and fast Fourier transform (FFT) calculations, and deep capture memory. For electronic warfare (EW) applications, desirable front end specifications include dynamic range and bandwidth equal to or greater than those of the jamming signal. Capture memory should be deep enough to record an entire nonrepeating jammer sequence from beginning to end. Some level of headroom in these key specifications will help ensure future readiness as jammers continue moving to wider bandwidths and increasingly complex techniques. For maximum flexibility in measurement and analysis, the signal analyzer should be equipped with vector signal analysis (VSA) software. Advanced VSA applications configure the ADC and perform math operations and signal processing on the samples according to user-selected parameters. 32 Electronic Military & Defense ■ www.vertmarkets.com/electronics Figure 2: This simple test configuration is capable of capturing, measuring, and analyzing nonrepeating jammer sequences. This combination enables sampling and storage of an entire technique or a combination of technique and threat. It also has the flexibility to provide detailed measurements in the time, frequency, and modulation domains. As implied by the word "vector" in VSA, the analyzer can measure in-phase and quadrature (I/Q) data, which enables a wide range of signal-processing techniques that require phase information. Measurements can be triggered using external signals or, within the analyzer, an internal IF magnitude trigger. In either case, the goal is to capture an entire scenario — the first pulse and all subsequent events. An example test setup is shown in Figure 2 above. If the jammer signal has a power level of 1 W or more, then it is usually necessary to connect an external attenuator to the input of the signal analyzer. Although the figure shows the VSA software running on an external PC, it may also be run inside a Windows- based signal analyzer. If the jamming scenario under investigation involves frequency diversity, then a deep-memory oscilloscope with a wider ADC bandwidth can be used as the front end capture device. To support detailed signal analysis, the oscilloscope must also be compatible with the VSA software. Capturing The Jamming Sequence Configuring a successful capture depends on tradeoffs between three key parameters: measurement span, required bandwidth, and sample rate. The combined effect of these settings interacts with the available memory to determine the maximum possible capture length. As a starting point for capture configuration, the required bandwidth of a pulsed measurement is determined by the rise time of the pulse. As a rule of thumb, bandwidth equals 0.35 divided by the pulse rise time in seconds. In a typical signal analyzer, the sample rate equals 1.28 times the span. This is where another rule of

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