THE RADAR RECEIVER S05 



sample of the IF signal after normal conversion and some amplification is 

 applied to a frequency sensitive discriminator circuit and the resulting error 

 signal is employed to readjust the beat oscillator frequency. The outgoing 

 radar pulse is normally attenuated effectively by the TR circuit, and thus 

 the remaining signal available for AFC purposes is due to inherent leakage 

 of the TR elements. As previously indicated, the frequency spectrum of 

 the outi)ut ''spike" of the TR device extends over a wide frequency range, 

 due primarily to the small finite delay in the breakdown of the TR tube. 

 The energy frequency distribution characteristic of this spike is to a large 

 degree independent of the magnetron frequency and, therefore, must be 

 considered as an undesirable masking signal and accordingly reduced to a 

 noninterfering level. As previously indicated this is usually accomplished 

 by disabling one or more of the IF amplifier input stages for a short time 

 interval coincident with the outgoing radar pulse. 



With a signal available which is related to the frequency of the outgoing 

 pulse, the remainder of the AFC design is concerned with the utilization of 

 this information to accomplish the automatic tuning of the radar receiver. 

 To determine the frequency gain characteristic of the discriminator circuit 

 it is pertinent to examine the frequency repeller potential relationship of the 

 local beat oscillator. This relationship for a 2K25-type reflex oscillator 

 operating at 10,000 mc shown in Fig. 19 is approximately 2 mc/volt and is 

 representative of the tubes of this type. This quantity provides an indi- 

 cation of the "loop gain" required for a satisfactory AFC circuit. 



Typical AFC Circuit Designs 



Figure 68b illustrates the essential elem.ents of a radar AFC discriminator 

 and amplifier circuit. This consists of an input circuit which is required to 

 furnish the means for frequency measurement, rectifier elements to convert 

 this frequency deviation information to a proportional voltage error signal, 

 followed by an amplifier to increase the amplitude of this signal to the re- 

 quired level to adequately control the frequency of the local beat oscillator. 



The operation of the discriminator input circuit may be observed by refer- 

 ence to the vector diagram of Fig. 69a. The input circuit, essentially a 

 double-tuned transformer having a low value of mutual inductance, serves 

 to couple the AFC rectifier to the preceding IF amplifier tube. The 

 resonance frequency of both primary and secondary circuits is main- 

 tained at the desired midband IF tuning point, in this example, 60 mc. The 

 output of the balanced secondary winding of this input network is applied to 

 a balanced rectifier shown in the vector diagram as E^ and Ei . In addition 

 a portion of the IF signal voltage which appears across the primary winding 

 is also applied to each element of the balanced rectifier. At resonance, the 

 primary and secondary voltages assume a quadrature relationship as indi- 



