350 THE RADAR RECEIVER 



It is desirable that the noise figure be minimized and the sensitivity be 

 maximized. This is not always feasible, as will be indicated in Paragraph 

 7-2. Selectivity is provided in both frequency and time. It is desirable to 

 provide the required selectivity at the low-level signal stages and prior 

 to envelope detection of the signal. 



Gain control characteristics are dictated by requirements to provide 

 error signals to range, speed, and angle tracking feedback mechanisms for a 

 specified range of input signal power. Automatic gain control (AGC) 

 systems are discussed in detail in Chapter 8. 



Dynamic range is the range of signal levels above the thermal noise 

 level for which a receiver will provide a normal usable output signal. To 

 reproduce faithfully the amplitude modulation on a received signal, the 

 incremental gain of a receiver whose output is controlled by the average 

 level of the received signal must be constant for a dynamic range on the 

 order of 12 db above the average signal level. The incremental gain is the 

 slope of the output /input voltage characteristic of the receiver. When an 

 undesired signal appears at the receiver input which is coincident in time 

 with the desired signal and nearly coincident in frequency, a much greater 

 linear dynamic range or range for which incremental gain remains constant 

 is required. In receivers which separate signals by frequency filtering, it is 

 not unusual to require a linear dynamic range on the order of 80 db up to 

 the point in the receiver at which the frequency separation of the desired 

 and undesired signals occurs. On the other hand, short-pulse, low-PRF 

 radar sets which separate signals by time filtering may require only a linear 

 dynamic range on the order of 15 db. 



Undesired signals which occur at a different time or frequency than the 

 desired signal may impart their modulation to the desired signal. This is 

 called cross modulation. Such a phenomenon arises from nonlinearities in 

 the receiver and is undesirable, since it degrades the output signal-to-noise 

 ratio. 



A proper radar system analysis defines the signal environment and allow- 

 able degradation of the receiver output signal; thus the principal factors 

 governing the selection of the dynamic-range and cross-modulation 

 characteristics are specified. 



Tuning characteristics are dictated by the radar transmitter. The 

 receiver is designed so that it can always be tuned to the transmitter fre- 

 quency. The design objective is to make the receiver tuning as accurate 

 as the state of the art permits. Both short-term and long-term frequency 

 stability is important. The effect of short-term frequency instability is 

 to introduce modulation on the signal in the receiver. Such modulation 

 degrades the output si.gnal-to-noise ratio. In a noncoherent pulse radar 

 set, the tuning accuracy that can be achieved is on the order of 1 part in 

 10^ Much greater stability is required in coherent radar sets. Automatic 



