TABLE OF CONTENTS The simplest ELINT system is the crystal video type consisting of an antenna, a detector, and a video amplifier, as shown in Figure 3.12(a). Adding a low-noise RF preamplifier, as in Figure 3.12(b), provides additional sensitivity. In practical receivers, there is always some bandpass filtering action ahead of the detector to restrict the receiver output to some band of interest. The band-limiting function may, in some simple receivers, be that provided by the antenna alone. In the low sensitivity receiver, the sensitivity is determined by the detector characteristics and the noise generated in the video amplifier. In the high sensitivity receiver, it is determined by the noise figure of the RF preamplifier. In both cases, the RF and video bandwidths place fundamental limits on the receiver s sensitivity. In some applications, high sensitivity may be undesirable as for a radar warning application in which an aircraft is to be warned of being tracked at relatively short range by radar associated with a surface-to-air missile system.
The sensitivity of a crystal video receiver has been studied extensively [16]. The sensitivity can be characterized as either gain-limited (when it can be improved by increasing the gain of the RF preamplifier) or noise-limited (when improvement is possible only by reducing the noise figure of the RF preamplifier or the receiver bandwidths). The discussion of sensitivity above was based on a noise-limited receiver in which...
