5-8] APPLICATION TO EVALUATION OF ANGLE TRACKING NOISE 265 



The angular error information is contained in amplitude modulation at the 

 scanning frequency. We shall refer to this modulation as the a-c error signal. 

 Its amplitude is proportional to the error amplitude, while its phase gives 

 the error direction. The RF carrier of the received signal is transformed 

 to an intermediate frequency in the mixer or first detector. The IF amplifier 

 then provides the necessary gain and maintains the average level of the 

 signal at a convenient constant value in response to the feedback signal 

 from the AGC (automatic gain control) filter. The envelope of the IF 

 signal is developed by the second detector, which is basically a rectifier. 

 For our purposes we shall assume the second detector to be a square-law 

 device whose characteristics have already been discussed to some extent in 

 the preceding paragraph. A range gate selects only pulses occurring at the 

 proper radar time for use in deriving the angle error. The range gate is 

 positioned by an auxiliary range tracking loop which is not shown in Fig. 

 5-13. The AGC loop maintains the d-c value of the video signal during a 

 pulse at a constant value so as to preserve a fixed relation between per cent 

 modulation at the scanning frequency and angular error, independently of 

 the received signal strength. 



A pulse stretcher generates a continuous signal suitable for use in 

 the low-frequency control circuits from the pulsed signal delivered by 

 the range gate. The output of the pulse stretcher is delivered to a product 

 demodulator or synchronous detector which develops a servo control signal 

 from the a-c error. 



Internally generated noise arises primarily within the mixer and the 

 first stages of the IF amplifier. The noise may be represented exactly 

 as in Paragraph 5-7 (Equation 5-67). That is, in-phase and quadrature 

 components at the carrier frequency are modulated by independent low- 

 frequency noise processes which we denote by x and y. The noise power 

 is denoted by cr^ so that the average signal-to-noise ratio will be 



Signal-to-noise ratio - SIN = a^/ld"^. (5-83) 



This is an average signal-to-noise ratio because, on a short term basis, the 

 signal power is modulated by the a-c error signal. 



The signal plus noise during a pulse will be of the following form: 



Signal plus noise = [a{\ + ke cos (cos/ + <p)) + x] cos Wct + y sin oij. 



(5-84) 



The video envelope from the square law detector during a pulse consists of 

 the sum of the squares of the in-phase and quadrature components: 



Video signal plus noise = r^ = a'^[\ -\- 2ke cos (cos/ + tp) 



+ kh"" C0S2 (oj,/ -f if)] 



-\- 2ax[l + ke cos (co^/ + cp)] 



+ ^2_^y. (5-85) 



