The need to take baseband signals and modulate them onto higher frequency carriers is 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 (see 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 500 MHz AWG, you can generate a signal with 1 GHz 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.
In the case where a digital technique is used, the IQ modulation is carried out as a mathematical operation, either in real-time by a digital signal processor (figure 2) or ahead of time in the software used to create the AWG waveform file stored in AWG memory (figure 3). The result of this calculation is fed into a digital-to-analog converter and up-converted using a mixer or multiplier together with an RF or microwave signal generator. With this approach, the images and LO feed-through can be filtered out more easily as long as the IF is high enough. However, this method requires a higher AWG bandwidth.
Figure 3 - Real time digital IQ modulation within the AWG
A disadvantage of the digital techniques compared to analog IQ modulation is that in general the conversion loss of an external mixer is greater than using analog wideband IQ modulation within a signal generator (note the difference in figures 2 and 5) to up convert the signal. An external amplifier may need to be employed when using a mixer. When using the Agilent E8267D microwave vector signal generator, for example, the modulated signal is multiplied up to the carrier frequency and the output power level is amplified, controlled and calibrated all internal to the signal generator.
Figure 4 - Digital IQ modulation at time of waveform file creation
One issue with both analog and digital modulation methods is the non-flat frequency response of the hardware including the DAC, IQ modulator, signal generator, and the mixer or multiplier stages. Corrections can be made to the signal by measuring the frequency response of the amplitude and phase across the bandwidth, then pre-distorting the created waveform file and reloading it into AWG memory.
Figure 5 - Digital IQ modulation with amplitude equalization applied
A precision AWG allows you to generate realistic scenarios for radar, communications and other application areas. In order to utilize these scenarios there exist different viable alternatives for converting the signal to the operational frequency of your wideband application.
About the Author: John Hansen is currently a senior application engineer for Agilent Technologiesí Electronic Measurements Group. He has more than 20 years of experience in system engineering and new product development within the wireless, microelectronics and defense industries. At Agilent, he has been responsible for the launch of new high frequency microwave signal generator products and is currently involved in market analysis and generation of technical content for the aerospace & defense markets. Prior to joining Agilent, Hansen worked at Hughes Network Systems, where he participated in the development of terrestrial cellular and satellite communication products as an engineering test manager.
Hansen received his BS in Engineering from UCLA, an MSEE (communications systems) from USC, and his MBA from San Diego State University. He is also a registered Professional Engineer in the State of California.