390 THE RADAR RECEIVER 



and the difference signal heterodyned against a filtered carrier signal. This 

 latter operation, however, is accomplished only with sacrifice of the infor- 

 mation rate. 



In a conical scanning radar the desired target is selected by range gating 

 so that the other targets, which are also PAM signals, will not be demodu- 

 lated. The signal is then envelope-rectified and lengthened. Lengthening 

 is employed to minimize additional modulation resulting from PRF 

 variation. The signal at the output of the pulse lengthener still represents 

 the composite signal, i.e. the sum and the difference signal. The low- 

 frequency modulation of the composite signal is caused by the scintillation 

 noise of the target and is independent of the lobing frequency. Both the d-c 

 component of the signal and the low-frequency modulation are fed back 

 to the IF amplifier as a gain-control signal. Modulation at the lobing 

 frequency, however, is not allowed to effect a gain control of the receiver. 

 The signal at the output of the pulse lengthener thus contains primarily 

 the sidebands about the lobing frequency which are caused by the variation 

 in direction of arrival of the signal. To demodulate this signal, and provide 

 control signals for the antenna servo, the signal is multiplied by a noise-free 

 carrier at the lobing frequency. The carrier signal is phase-locked with the 

 antenna lobing. This is usually accomplished by means of an a-c generator 

 mechanically linked to the rotating antenna. 



Fig. 7-17 shows three typical demodulator circuits. In all three of these 

 circuits neither the signal nor the carrier frequency appears in the output. 

 The output contains only the beats between the signal and the carrier and 

 certain of their harmonics. Of the three demodulator circuits shown the 

 "ring modulator" is the most desirable because the modulation products 

 are effectively separated in various parts of the circuit. The carrier signal 

 should be as monochromatic as possible for maximum output signal-to- 

 noise ratio. 



The process of pulse lengthening merely concentrates all of the noise 

 appearing in the IF in a region less than the PRF. In order that the noise 

 reduction provided by the antenna servo be approximately Bi/PRF, where 

 Bi is the noise bandwidth of the antenna tracking loop, it is necessary that 

 the demodulator provide a true product demodulation. To approach this 

 performance the ring modulator is employed in conjunction with a bandpass 

 filter which filters the signal applied to the demodulator. 



7-20 SOME PROBLEMS IN THE MEASUREMENT OF 

 RECEIVER CHARACTERISTICS 



Noise Figure. The most practical method of making noise figure 

 measurements involves the use of a dispersed signal source. An argon-filled 

 gaseous discharge tube will produce a standard noise power output equiva- 



