Spectrum is an invaluable resource, and it is growing more crowded by the day. An RF/microwave spectrum overcrowded with too many signals can make it difficult for some sensors and systems to perform optimally. One solution is to locate a source of interference and either eliminate it or develop a filter to remove it from a system’s signal path. But many times, the cause of degraded performance may be more than a single interference signal.

Taking into account the constant, legal signal activity from such sources as mobile telephones, frequency-modulated (FM) radios, and automotive radar systems—and how these signals can mix and interact—here is no recourse but to design systems that minimize their susceptibility to such interference. Also, in wideband, multiple-channel systems, it may be signal distortion from the system itself that is the cause of that very system’s reduced performance. Spectrum simulation can provide an effective means of analyzing problems caused by too many signals and finding solutions.

Some systems, such as signal-intelligence (SIGINT), geolocation, and counter-improvised-explosive-device (counter-IED) systems, must operate in spectrally cluttered environments--yet still be capable of detecting, identifying, and (in some cases) suppressing signals near a broadband noise floor. Understanding the interaction of different signals within the operating spectrum can be a critical part of achieving optimum performance from these types of systems.

Of course, it is not always possible to perform spectrum measurements in some types of operating environments, and the availability of a wideband multiple-emitter signal environment that is controlled and repeatable could be a tremendous tool for system developers. Fortunately, methods exist for accurate and realistic spectrum simulation in the laboratory for signal environments from DC through and beyond Ka-band frequencies. One of the key tools for simulating these realistic signal environments is the arbitrary waveform generator (AWG).

Vector modulators are important instruments for creating a signal environment. They provide the means of imposing both amplitude and phase changes on a signal’s in-phase (I) and quadrature (Q) components, thus creating the I/Q-modulated signals widely used in modern communications systems. An I/Q modulator includes a 90-deg. phase shifter, two mixers, and an RF summing junction to generate the required arbitrary phase and amplitude of the modulated RF signal (Fig. 1). This makes it possible to modulate carriers to a bandwidth equal that of the I and Q signals combined. 

Spectrum Simulation Helps Verify System Performance, Fig. 1

Numerous alternatives are available for creating I/Q baseband waveform files. These include MATLAB from MathWorks, Visual Basic from Microsoft, and other general-purpose software offerings, although many more specialized software packages are also available. For example, many test-equipment manufacturers offer software products that provide the capability to create very detailed waveforms for specific communications, radar, and electronic-warfare (EW) applications. One example of these more specialized tools is Signal Studio vector signal creation software from Agilent Technologies. It is available for the creation and analysis of a wide range of signal formats, from digitally modulated cellular communications signals to advanced EW and pulsed radar signals.

Digital-to-analog-conversion (DAC) technology has evolved to the point where wide instantaneous bandwidths and high resolution are available with the latest commercial-off-the-shelf (COTS) AWGs. An AWG provides the capability to generate virtually any waveform loaded into its memory and within the performance limits of its DAC circuitry. The right AWG makes it easy to create radar and communications emitters scattered throughout a synthetic test range that simulates thousands of cubic miles of airspace and across several GHz of spectrum.

By programming multiple modulated signals into a single waveform file and loading that file into an AWG, it is possible to generate a composite signal and build a full electromagnetic (EM) signal environment across a given frequency band of interest (Fig. 2). Creating a waveform such as this can be done with an AWG and a general-purpose programming tool such as MATLAB, although other available methods can make the process much more straightforward.

Spectrum Simulation Helps Verify System Performance, Fig. 2