Principles of Modern Radar - Volume 2 1891121537

12.8 Receiver-Based EP 56912.8.3 Wide Instantaneous Dynamic RangeWide instantaneous dynamic range (IDR) helps prevent receiver saturation while reducingor eliminating the need for AGC. The IDR is the difference between the thermal noise andthe strongest signal that can be accurately processed at a given time. Receiver componentssuch as amplifiers, mixers, and limiters can potentially saturate at high input signal levels,although in many current radar systems, it is the ADC that limits IDR. When a componentis in saturation, spurious intermodulation products are generated, corrupting the inputspectrum. This can occur when strong signals such as clutter, EMI, or EA mix with eachother or with target returns. A small-signal suppression effect may also occur whereby thepower of a small (desired) signal is further reduced relative to that of a large (undesired)signal at the output of the saturated device.Automatic gain control is used to partially compensate for limited IDR by addingattenuation in the receiver path to keep the incoming signal within the linear region ofoperation. The use of AGC creates opportunities for the jammer, however. Two examplesare AGC capture and AGC deception. The AGC capture effect is used in conjunction withrange and velocity gate stealing techniques. The radar AGC is set based on the amplitudeof the deceptive jammer’s power before the jammer walks the range or velocity track gateoff the target skin return. The radar AGC adds attenuation because of the high JSR ofthe jammer thereby pushing the skin return down into the noise. As the jammer signalseparates from the target in the range or velocity dimension, the target signal is too fardown in the noise to be redetected. An instantaneous AGC (IAGC), which can vary theAGC from range bin to range bin, may help mitigate this effect against RGPO [8].The AGC deception technique introduces angle errors through very fast, high-power,on–off blinking. The AGC attenuation is set based on the average power of the blinkingjamming because the AGC time constant is long compared to the jammer blink period.As a result, the AGC attenuation is either too small when the jammer is on or too largewhen the jammer is off: the radar is alternately saturated and starved, never getting anaccurate signal. Against conical scan based angle tracking systems, this AM strips off thescan modulation of the return signal. Against monopulse tracking systems, the jammingdistorts the complex difference-to-sum voltage ratio by forcing the signal into nonlinearregions of the receiver where the phase and gain matching characteristics of the monopulsechannels are not valid (jammer on) or by forcing low signals levels to be contaminatedby thermal noise (jammer off). In all such examples of counter-AGC jamming, the mosteffective EP solution for the radar is wide IDR.12.8.4 Notch FilteringNotch filtering is potentially useful in rejecting narrowband interference when the radar isemploying wide or medium bandwidth pulse compression waveforms. Narrowband spotnoise or even a CW tone will be decorrelated during pulse compression processing causingthe interference to be spread in range. This results in a post-compression interference effectthat is comparable to a broadband noise waveform whose bandwidth is approximatelymatched to that of the radar.The notch filtering must be performed before any pulse compression processing isdone. Initially, this would suggest a series of tunable (or selectable) RF notch filters priorto the first IF conversion. It is often not practical to employ tunable RF notch filters or alarge bank of fixed-tuned RF notch filters, however, due to their cost, size, insertion loss,