Principles of Modern Radar - Volume 2 1891121537

12.2 Electronic Attack 537figure include gain control and a second T/R switch, which may be an integrated part ofthe transmit amplifier.The purpose of the T/R function in the jammer is to allow the receiver to sense theenvironment without being overpowered by the leakage from the transmitter back intothe receiver. The leakage may occur internally within the system or externally throughthe antennas. The ratio of the peak HPA transmit power to the leakage input power at thereceive antenna port is referred to as the system T/R isolation. If the leakage approaches orexceeds the detection threshold of the detector, the system will lose sensitivity. Generally,this type of jammer employs fairly high power, and T/R isolation is often not sufficientto maintain desired sensitivity while simultaneously transmitting and receiving. Undersuch conditions, the T/R switches are used to occasionally interrupt transmission to allowthe receiver an uncontaminated sample of the RF environment. This process is referredto as look-through. During transmission, the receiver is blanked to prevent saturation orfalse detection; during reception, the transmitter is blanked to prevent high-power leakageinto the receive path. The look-through periods are typically kept short relative to thetransmitter-on times to maintain a high jamming duty cycle and, therefore, average power.Some noise jammers exclude the receive function entirely from the design and simplygenerate noise in a manner that is independent of the current radar emissions. Such jammersforego the added cost of the receiver and the complexities of the transponder operation,but at the expense of being potentially less efficient in jamming the radar, as discussed inSection 12.2.3.1.12.2.2.2 Coherent JammersFigure 12-3 is a conceptual diagram of a coherent jammer. The coherent jammer generatesthe EA signal by receiving, modifying, amplifying, and retransmitting the radar’s ownsignal. A device that simply receives, amplifies, and retransmits the radar signal withoutany other change is providing a beacon to the radar—essentially an enhanced radar crosssection (RCS) target—that provides little or no value in a support jamming role and mayhave disastrous consequences in a self-protection role. The modifications to the radarsignal that produce the EA benefit are the coherent delay and the modulation functionsshown in the figure.The radar signal is received through the receive antenna, passes through an RF frontend, and is then split, with one portion of the RF being routed to the detector/analyzer andthe other continuing through the coherent delay, modulation, and transmitter chain andfinally radiated through the transmit antenna. The RF front end and the transmitter chainare functionally the same as those described previously for the noncoherent transponder.The detector function typically employs a diode detector and band-pass filter thatrectifies and filters the received RF, essentially stripping off the carrier and leaving amagnitude-only pulse envelope. The output is sometimes referred to as detected videobecause the bandwidth needed to preserve the pulse envelope shape is comparable to thatused to carry video information. The detected pulse envelope is often fed into a thresholddetector, which generates a saturated (constant amplitude) video pulse, sometimes referredto as pulse-present indicator (PPIN), if the signal exceeds the threshold and zero voltage ifit does not. The leading edge of the PPIN triggers a sample of a high-speed clock, whichrecords the pulse time of arrival (TOA). The difference in time between the leading edgeand trailing edge yields a pulse width (PW) measurement.Some systems measure the frequency of each pulse with a device such as a frequencydiscriminator or instantaneous frequency measurement (IFM) receiver. An IFM receiver