Principles of Modern Radar - Volume 2 1891121537

568 CHAPTER 12 Electronic Protectionthan the subsequent IF filtering stages, it serves the valuable function of rejecting strongsignal interference that might otherwise cause intermodulation products or other spuriousresponses in nonlinear components of the receiver. This has benefit for mitigating EMI aswell as high-power EA. Another specific application is in image rejection, discussed below.12.8.2 Image RejectionRadar receivers generally perform multiple frequency conversions as the received signalsare translated from RF to IF and finally to baseband for detection. A jamming signal thatis nominally outside the radar waveform bandwidth may end up being translated to thesame IF as the target return if it is at an image frequency of one of the mixers. The imagefrequency for a mixer is located an equal distance from the LO as the desired signal, butin the opposite direction. For example, if the RF signal frequency is f RF = 8.3 GHz, andthe LO frequency is f LO = 8.2 GHz, then the image frequency is f IMAGE = 8.1 GHz. Themixer is a nonlinear device that produces many mixing products; among them are the RFinput minus the LO reference (the desired IF signal in this example) and the LO minusthe RF (the undesired image in this example).There are two consequences of poor image rejection against EA. The first and mostobvious is that the radar is allowing the jamming energy to interfere with the otherwisespectrally separated target signal. The second is that an erroneous angle track error mayresult if the radar is tracking an SSJ using an image jamming technique. Image jammingexploits the fact that the monopulse error signal derived from the image frequency is inthe opposite sense as that derived from the desired frequency. To gain insight into this,recall that the monopulse error signal is the complex ratio of the and channels thatincludes their relative phase, β, as shown in Equation 12.18. The IF signals produced fromthe mixing process for the RF and image frequencies areS RF = e j(2π f RFt+β) e− j(2π f LO t) = ej2π f IF t e jβ (12.22)S IMAGE = e − j(2π f IMAGEt+β) ej(2π f LO t) = ej2π f IF t e − jβ (12.23)where it is assumed that f RF = f LO + f IF and f IMAGE = f LO − f IF , as in the previousexample. One can see that the sign of the monopulse phase, β, is different for the imagefrequency than the desired signal frequency, thereby producing an erroneous error signal.One method of achieving good image rejection is through RF preselection, describedin the preceding section. A complication arises for RF preselection in the case of wideband,frequency agile radars in that the image frequency may be within the tunable bandwidth ofthe radar. In such a case, a fixed-frequency RF preselector cannot be used. Instead, eithera switchable bank of preselectors or a tunable RF filter might be considered; these mayprove costly and lossy, however. Another approach is to employ a high first IF. The first IFrefers to the difference between the RF and the LO in the first down-conversion stage. Ifthe IF is more than half the tunable RF bandwidth of the radar, the image frequency can berejected by a fixed, broadband, RF filter that covers the entire tunable RF band of the radar.Standard receiver design methods typically achieve excellent image rejection. Thejammer would need extremely high JSR in order to overcome the radar’s image rejectionand still compete with the target return. In addition, the jammer must either have preciseknowledge of the LO frequency or it must spread its energy over a bandwidth correspondingto the uncertainty in the LO frequency, resulting in power dilution. As a result, jammers areneither likely to attempt nor succeed with image jamming techniques against modern radar.