Signal intelligence or SIGINT plays an important role in modern warfare. SIGINT is a general term that includes radio band (communications intelligence or COMINT), radar band (electronic intelligence or ELINT), and measurement and signature intelligence (MASINT) systems. COMINT engages with technical information and intelligence derived from interception of foreign communications.

This means that a SIGINT receiver assigned to COMINT duty is required to gather information about all transmissions in the radio band. The specific tasks of a receiver include the ability to detect, analyze the received transmission, estimate frequency and type of modulation, extract intelligence (information) and locate the source. The receiver must cover a wide electro-magnetic (EM) spectrum to detect possible emissions. These emissions can originate from an independent source and are likely to be in burst, hop or continuous type of transmissions.

This article addresses the challenges involved in generating a wide range of real-world signals required to test an effective SIGINT receiver.

A SIGINT receiver has functional elements that receive and process radio frequency (RF): from low frequency (LF-30 kHz to 300 kHz), through extra-high-frequency (EHF -30 GHz to 300 GHz), received by the platform antenna/antenna arrays. The RF signals emitted by independent sources are likely to have an assortment of modulation schemes and transmission modes.

These RF antenna/antenna array types may be omnidirectional, directional, mechanically or electrically steered. In addition to the common front-end functions, the other functional elements include the RF distribution, low and high-band tuners, receivers, IF distributing IF digitizer and sub-band tuners, digitizers and down-converters. This is followed by baseband signal processing for detection and identification of modulation schemes and estimate direction of arrival.

The challenge for the designer is to simulate all types of the above scenarios during the design and testing of SIGINT receivers. This requires the following:

• multichannel wideband signal generator;
• variety of analog and digital modulation schemes;
• create spectrally clean frequency-hopping signals;
• ability to control signal parameters like power levels, carrier frequency, symbol rate, etc.; and
• ability to replicate subtle impairments (or signatures) that can identify specific radio attributes.

A typical scenario for testing a SIGINT receiver is depicted in Table 1. This scenario covers the broad range of frequencies, power levels and transmission types the receiver will be expected to accurately identify. Such a scenario can be implemented using a variety of signal generators and other equipment, or more simply with greater repeatability using software and an arbitrary waveform generator (AWG).

An AWG delivers a combination of strong signal stimulus, greater sample rates, bandwidth and signal fidelity and improved usability. The latest AWGs, such as the Tektronix AWG7000 series, typically exceed the capabilities of current test equipment to generate the wide band-width and fast-changing signals that are increasingly seen in many wireless applications such as SIGINT, radar, UWB and others.

For this application, the AWG provides an effective tool for SIGINT test and measurement through direct generation (direct synthesis) of RF signals using a digital-to-analog converter (DAC) for signals with a carrier center frequency of up to 5 GHz and a signal bandwidth of up to 5.8 GHz. The direct generation of IF or RF signals avoids I/Q degradations and lengthy adjustments associated with traditional generation using I/Q modulators. A simplified block diagram is shown in Figure 1.

The term direct synthesis describes a means of producing waveforms. These can be used as stimulus signals for a variety of test and measurement applications. But the direct synthesis concept is not limited to measurement uses, nor is it new. Direct synthesis is similar to the process used to reproduce audio signals in a CD player (though CDs use encoded data). In a CD system, stored samples are read out of a memory (the CD itself) to reconstruct analog waveforms — the music you hear.

Direct synthesis is a sampling-based technology. Whereas an oscilloscope acquires sample points from analog waveforms, a direct synthesis signal source (AWG) creates analog waveforms through the DAC from sample points. The sample points in an AWG's memory can define essentially any waveform, including SIGINT waveforms. Of course, the normal limitations of physics and bandwidth still apply, but within its specified range.

With a simple to use software interface, direct synthesis allows the user to create the various scenarios (for example, RF-Xpress from Tektronix) and then transfer it to an AWG for those signals to be reproduced. Since the scenarios including impairments are created in software, the user has full control over the scenes being generated and can adjust the parameters to meet the needs of thoroughly testing a SIGINT receiver.

For this article, we will use RFXpress as an example of how direct synthesis can be used to generate the mutli-signal scenario outlined in Table 1. RFXpress is a software package that synthesizes digitally modulated base-band, IF and RF signals. It is designed to take IF and RF signal generation to the next level and fully uses the wideband signal- generation capabilities of AWGs. Supporting a wide range of modulations, as well as the symbol map functions, the software also allows the designer to define custom modulations. In addition, real-world impairments from either interference, linear and non-linear behavior of transmission equipment, and channel effects can be convolved with the desired scenario to test the margins of the receiver performance.

In order to create the signals in Table 1, various parameters for each transmission are selected. The waveform length and oversampling parameters have to be selected in order to generate frequency hopping and burst transmissions and the RFXpress “wrap-around” feature ensures that these waveforms have minimal spectral re-growth or transient emissions (which would result from phase jumps at the point where waveforms sequences join). The steps involved in setting up the RFXpress tool for generating the required transmission are explained below.

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