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Different ways of simulating wideband signals

Posted: 26 Oct 2011 ?? ?Print Version ?Bookmark and Share

Keywords:baseband signals? arbitrary waveform generators? wideband?

Taking baseband signals and modulating them onto higher frequency carriers are fundamental for both the design and evaluation of communication and radar systems. The bandwidths of these signals have become much wider to accommodate advanced system capabilities such as higher data rates, resistance to interference or jamming, and greater range and velocity resolution.

Of course, one of the cardinal rules of test engineering is that the stimulus and measuring equipment must have performance significantly better than the device or system under test. This is particularly true of RF measurements due to the rigorous requirements imposed for the systems to work optimally and to accurately simulate the operational environment.

Currently available arbitrary waveform generators (AWGs) can achieve a very large bandwidth but how do we go about modulating and then upconverting these wideband signals to their operational carrier frequencies? There are three alternatives we will consider. The first is the more classical analog IQ (in-phase and quadrature) modulation where the modulator is a hardware component of the test equipment (figure 1). The second and third involve digital IQ modulation where the modulation is realized mathematically.

Figure 1: Analog IQ modulation after the AWG within the RF signal generator.

There are a number of alternatives for creating the IQ baseband waveform files. MATLAB, Visual Basic and other programming tools are the most general purpose but there are many specialized tools available. For instance, Agilent Signal Studio products provide a means of creating very detailed waveforms for specific communications and radar technologies.

For analog IQ modulation, an AWG generates I and Q signals on separate channels that are fed into an IQ modulator, which is often part of a vector signal generator. An advantage of the analog approach is the fact that the bandwidth of the baseband signal required at the output of the AWG is only half of the bandwidth that can be achieved for the RF modulation bandwidth of the output signal. For example, with a 500MHz AWG, you can generate a signal with 1GHz modulation bandwidth. On the negative side, the analog IQ modulator creates a number of undesired distortions, such as images and carrier feed-through, which can only be reduced to a certain extent.

Figure 2: Analog IQ modulation with the relative skew, amplitude and phase of the I and Q waveforms optimized. The amplitude response has also been equalized.

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