THE RADAR RECEIVER 



713 



quency. One or more of the networks involved may, of course, be arranged 

 alternatively in a parallel contiguration if desired. The nonlinear element 

 may be a vacuum tube with properly chosen operating conditions or a point- 

 contact silicon crystal rectifier. The beating oscillator in the modern 

 military radar converter employs special types of vacuum tubes such as the 

 GL-2C49 type previously mentioned or the single-cavity velocity modulated 

 types'^ at the higher radar frequencies. 



\ typical voltage-current characteristic of a point-contact silicon rectifier 

 is shown in Fig. 9. This nonlinear characteristic may be expressed mathe- 

 matically as a power series with coefficients whose amplitude decreases 

 quite rapidly with the order of the associated term. If two sinusoidal 

 voltages represented by frequency /i and f-2 are simultaneously impressed 



NON- LINEAR 

 IMPEDANCE 



SIGNAL 

 INPUT 



INPUT 

 NETWORK 



BEATING 



OSCILLATOR 



NETWORK 



OUTPUT 

 NETWORK 



I-F 

 OUTPUT 



BEATING 

 OSCILLATOR 



Fig. 8. — Basic configuration of a radar converter. 



upon such a nonlinear element, the resultant current flowing in the output 

 impedance will produce output signals of the form nfi ± mf-i where // and m 

 are integers including zero. The effective amplitude of each of these modu- 

 lation products is related to the magnitude of the power series coefficients. 

 In the radar converter under consideration j\ may represent the received 

 signal frequency and fi represents the beat oscillator frequency. The 

 difference terms of the above e.xpression are of the greatest importance here 

 because the desired output signal frequency has been chosen at a low value 

 as compared with the input received radar signal. The selection of the 

 wanted tirst-order difference term is accomplished by the frequency selec- 

 tivity characteristics of the converter output coupling network and the 

 following IF amplitier. 



A major design problem encountered in the practical development of 

 microwave radar converters is one concerning the design of the coupling 

 networks. The previously referred to limitations of microwave network 



^"Reflex Oscillators," J. R. Pierce and \V. G. Shepherd, Bell Svstem Technical Journal, 

 Vol. XXVT, July 1947. 



