MICROWAVE RADAR TESTING 443 



with good accuracy the over-all performance. Known, or so-called stand- 

 ard, targets have often been used in the field for checking performance. 

 Because of wide variations in transmission due to many factors, results 

 obtained from such targets are frequently misleading. 



SiGNTAL Generators 



Signal generators for radar work deliver one or more different types of 

 test signals, which may serve a variety of functions. More important among 

 these functions are tuning or alignment of the radar components (TR and 

 RT boxes, converter, beat oscillator, AFC, etc.), measurement of receiver 

 sensitivity, checking TR and receiver recovery, measurement of loss, detec- 

 tion of frequency pulling, check of AFC following, measurement of IF band- 

 width, check of automatic range tracking, measurement of standing wave 

 ratio and check of video "gating" circuits. Many signal generators include 

 means for measuring the power and frequency of the test signal, and also of 

 an incoming signal. 



Types of Signals 



The test signals delivered by a signal generator may be CW, pulsed, or 

 frequency-modulated. Occasionally square wave or sine wave modulation 

 is provided. 



Pulsed signal generators deliver a succession of single RF pulses or pulse 

 trains, either of these generally synchronous with the pulses of the radar 

 under test. Multivibrator or trigger techniques^ are used to generate the 

 pulses for modulating the microwave generator. The trigger pulse for 

 synchronizing the pulsing circuits is commonly produced by rectifying RF 

 pulses from the radar transmitter, thus avoiding a separate video connection 

 to the radar. To avoid possible difficulties in video response, the RF test 

 pulses should be of comparable width to those of the radar under test. For 

 observing the test signals, either the radar indicator or an auxiliary oscillo- 

 scope may be used. With the single pulse method, provision is usually 

 made for var}dng the delay of the test pulse with respect to the radar pulse. 

 The width of the test pulse may also be adjustable or variable. 



If the frequency of the signal generator is swept over a sufficiently wide 

 frequency band, the IF output of the radar traces the curve of IF selectivity, 

 thus producing a kind of pulse. With a suitable rate of frequency sweep, 

 this pulse becomes comparable in width to the transmitter pulse, and when 

 synchronized with the radar it can be used for test purposes. Since the 

 pulse is produced in the radar, comparison of the shapes of receiver input 

 and output pulses is not possible. The nominal duration of the pulse in 

 the IF output is 



T = B/y (2) 



