Principles of Modern Radar - Volume 2 1891121537

570 CHAPTER 12 Electronic Protectionand low-Q. An alternative approach is to have a dedicated IF stage in which the LO is tunedto place the narrowband interference within a fixed notch filter at the IF. The same LOcould be used to translate the remaining signal back to the previous frequency to resumenormal signal processing if desired. Several such stages can potentially be cascaded formultiple notch generation if necessary. The narrowband interference power must be belowthe input third-order intercept point of the mixer, in order to prevent saturation effects.Ideally, pulse compression is performed entirely through digital signal processing. Inthis case, optimized notch filters or band-pass filters can be implemented digitally beforethe pulse compression process occurs, assuming the jamming has not previously saturatedthe receiver. Notch filtering has negative side effects such as signal loss, elevated rangesidelobe levels, and decreased range resolution.12.8.5 Wideband LimitingWideband limiting, also known as a Dicke-Fix, has been used, primarily in older systems,to reduce the effects of intermittent in-band interference in receivers [2,3,10]. A sweptnoise jammer or a blinking jammer may be in the radar IF band for only a fraction ofthe time, but the interference may still appear to be continuous if the in-band revisit rateis fast enough. This is because the band-limiting IF filter rings, effectively stretching outthe input signal by the impulse response time of the filter, nominally the inverse of thebandwidth. For example, suppose that the final IF filter is 2 MHz and the jammer is in-bandfor only 100 ns. The output of the filter would persist for a time of approximately 600 ns:100 ns for the jammer time in band, plus 500 ns for the approximate impulse responsetime of the filter (the inverse of 2 MHz). If the jammer returns in band before the ringingdies out, it will effectively be continuously present at the detector. The jammer achievesa desired average power level through a combination of high peak power and a low dutyfactor. The objective of wideband limiting is to limit the peak power of the jammer duringits intermittent in-band intervals, thereby also reducing the average power. The limitingaction definitely affects the signal quality, but the result may still be preferable to thejamming without limiting.The limiting cannot be applied after the final IF filter, otherwise the signal will beclipped 100% of the time due to the filter’s stretching effect. For a swept noise jammerthat is continually within the RF pass band of the radar receiver, limiting also cannotbe done immediately after the RF filter because the jammer is always in-band, and theclipping would again occur 100% of the time. Therefore, in the case of swept noise, thelimiting is performed at some intermediate stage, following an IF filter whose bandwidthis somewhere between the above extremes.As an example of how to select such a filter, suppose the jammer has an instantaneousspot noise bandwidth of B j and sweeps at a rate of ρ (frequency extent divided by sweeptime), and the radar IF filter preceding the limiter has a bandwidth of B LIM . The time thatthe jammer is in-band as it sweeps through the filter is t j,in = ( B j + B LIM) /ρ. To a firstapproximation, the jammer output of the filter persists for a time equal to the input timeplus the filter impulse response time (nominally 1/B LIM ):t j,out = ( B j + B LIM) /ρ + B−1LIM(12.24)A possible choice for the filter bandwidth, B LIM , is one that minimizes the amount of timeof the jammer duration after the filter, thereby minimizing the amount of clipping damage