Communications, Radar & Electronic Warfare (201.. - Index of

72Communications, Radar and Electronic Warfare'\ r'---In termediateRF Amplifier Mixer frequen cy (IF) Demodulatorfilter an dTamplifier---....,.Baseban doutputLocaloscillatorFigure 4.5Simple generic radio receiver block diagram.The output of the antenna is not always immediately filtered. This means that anysignal within the passband of the antenna is fed into the remaining electronics. If thesystem features an automatic gain control or a signal higher than can be dealt with bythe electronics then a high level interferer outside of the tuned frequency can stillprevent successful reception. In the case of automatic gain control, the interferingsignal can push the wanted signal down into the noise floor. This is illustrated inFigure 4.6. On the left hand side is the normal condition with no interferer. Theautomatic gain control amplifies the signal to the wanted level for further processing.The right hand side shows an interfering signal that adds RF energy into the AGe.Since the AGC cannot distinguish between the wanted signal and the interferer, all ofthe incoming energy is summed and when the signal is amplified to the desired level thewanted signal is at far lower signal strength than wanted. It can in fact be pushed downinto the noise, but even if not, the signal to noise ratio will be significantly less andtherefore the error rate will be substantially higher than wanted.Even if there is no AGC, the power from an out-of-band receiver can still add energyinto the receiver if it is not filtered out effectively. This can cause problems if the inputpower level is too high because it can push electronic components into non-linearity,which happens when the input power is too high. If this occurs, the non-linearityintroduces additional spectrum components that may cause components downstreamto fail to operate correctly.If the input power from interferers is very much higher than the receiver circuitry cancope with, then the system components can fail permanently. This is the aim fordirected energy weapons, which seek to destroy radio receivers by introducingunacceptable power into the electronics. In electronic warfare terms, this would beclassified as a hard-kill. Indeed, if the power is so large, the energy may not enter only