E W101 Independent Samples Figure 2: An I & Q digitizer digitizes a signal at two points one quarter wavelength apart to capture the frequency and phase of the signal. The Journal of Electronic Defense | October 2013 64 Wideband DRFMs are highly desirable, because they can handle signals with wide frequency modulations and frequency-agile threats. We will be discussing the implications of frequency-agile threats in detail later in this series. Simply put, as the state of the art in digitizers improves, wideband DRFMs can be expected to be wider and more plentiful. There is an inverse relationship between the digitizing speed and the number of bits that can be provided; the driving requirement for future DRFMs is more samples per second with more bits per sample. As we will discuss in later columns in this series, there are several other performance specifications which bear on the direction that DRFM specifications will take. We will also be covering a number of approaches to the generation of faster sampling with more bits than can be produced by a single ADC. NARROWBAND DRFM A narrowband DRFM need only be wide enough to capture the widest signal the jammer must handle. This means that a narrowband DRFM can operate with an ADC that is reasonably within the state of the art. As shown in Figure 4, the jammer system converts a frequency range of interest into the frequency range covered by multiple narrowband DRFMs. The DRFM input signal is power divided to the individual DRFMs. Each of the DRFMs is tuned to an individual signal and performs its function in support of the jamming operation. Then, the analog RF outputs from the DRMS are combined and converted (coherently) back to the original frequency range. It should be noted that spurious responses are less a problem in narrowband DRFMs, because each contains only one signal. DRFM FUNCTIONS WIDEBAND DRFM IF Band Input FILTER SYSTEM L.O. IF Band Output DRFM L.O. WIDEBAND ADC MEMORY FILTER DAC Figure 3: A wideband DRFM handles a frequency range containing multiple signals. COMPUTER DRFMs are particularly valuable in dealing with pulse-compressed radars. The August 2010 “EW 101” column describes radars which improve their range resolution through pulse compression. This column has some illustrations that will help your understanding of pulse compression, if you are not familiar with it. The two techniques discussed are “chirp” and “barker code.” Chirp involves adding a linear frequency modulation across each transmitted pulse. In the radar’s re-