7-14] PROBLEMS AT HIGH-INPUT POWER LEVELS 381 



order of magnitude of the receiver thermal noise. In the other case the 

 receiver is required to furnish an output from a single signal which has been 

 time-selected. 



In the first case, cross modulation caused by the strong signals can 

 deteriorate the weak-signal performance; the extent to which this occurs is 

 a function of the detailed receiver design. If the receiver is linear, the 

 dynamic range for any particular gain setting will usually be between 10 

 and 20 db. Signals more than 20 db greater than the thermal noise level can 

 be expected to cause saturation in the receiver. The result of the saturation 

 is a paralysis of the receiver for a certain time following the removal of the 

 large signal. To minimize this effect it is necessary that attention be given 

 to the circuits mentioned in Paragraph 7-12, so that a suitable transient 

 characteristic is obtained from the IF amplifier. The transient should 

 exhibit small overshoot and short delay time. Loss of weak signals occurs 

 only when they are time-coincident with the strong signals if adequate IF 

 filtering is provided. In cases where signal information is required and 

 when the interference and signal occur at the same time (range), saturation 

 must be prevented and the two signals separated on the basis of their 

 difference in frequency spectra caused by the doppler shift. In the non- 

 coherent pulse radar this is accomplished by heterodyning the weak signal 

 against the strong signal at the IF second detector. 



The second case occurs when a signal is being tracked. The desired 

 signal is gated and may provide range and direction signals from sidebands 

 associated with each of the pulse signal sidebands. The effect of strong 

 signals is to add additional sidebands at the receiver output and thereby 

 cause errors in the range and direction signal. In a well-designed receiver, 

 negligible intermodulation occurs when a strong signal is present which is 

 not time coincident with the desired signal. 



In some instances the desired signal power level may approach the order 

 of magnitude of the local oscillator signal power. Fig. 7-11 shows the 

 transfer characteristic of a typical microwave mixer at large-signal levels. 

 The nonlinearities of this characteristic will cause signal distortion. Inter- 

 modulation components appear incident to the beating of the various signal 

 components. These components are not highly significant except with some 

 propeller-driven targets in which terms of the order 2wi + £02 may introduce 

 more fluctuation in the final bandwidth of the system. The reduction in 

 modulation percentage of the pulse signal at the fundamental modulating 

 frequency results in deterioration of tracking performance, since it corre- 

 sponds to a change in tracking loop gain. In many cases the signal at the 

 antenna terminals is greatly distorted before it reaches the signal mixer 

 because of the time varying attenuation of a gas discharge TR tube. A 

 controlled TR characteristic is therefore sometimes used to advantage to 

 minimize the deterioration in tracking loop performance. 



