THE RADAR RECEIVER 807 



will be defined as the ability of the AFC circuit to restore proper tuning con- 

 ditions with sudden application of the signal. The "hold in" characteristic 

 of the AFC system will be defined as the ability of the circuit to maintain 

 proper tuning conditions as slow changes occur in the frequency of the con- 

 trol signal. In the AN/APS-4 airborne radar equipment previously re- 

 ferred to, which employs an AFC circuit similar to the form here described, 

 the "pull in" range is approximately zt 5 mc from the 60-mc midband value 

 and the "hold in" range includes the entire tuning range of the reflex oscil- 

 lator employed which is of the order of ± 40 mc. This example will main- 

 tain the tuning within 0.5 mc of the desired tuning point over the range of 

 conditions encountered in wartime aircraft apphcations. 



In some applications use has been made of a frequency scanning process 

 whereby the AFC output voltage, in the absence of a suitable controlling 

 signal within the IF band, is caused to vary periodically in a saw-tooth 

 fashion thus causing the local beat oscillator frequency to vary, sweeping 

 across the complete tuning range of the receiver. When the desired signal 

 frequency is produced the AFC then functions in the normal manner. 

 This form of circuit was employed in certain radar equipments developed 

 during the early part of the war and a somewhat similar oscillatory AFC 

 circuit has been employed in connection with later developed thermally 

 tuned reflex oscillators and reported elsewhere. ^^ 



An automatic frequency control unit designed in connection with the 

 AN/APQ-7 radar bombing equipment which operates at 10,000 mc is illus- 

 trated in Fig. 70 and Fig. 71. The basic operation of this equipment ex- 

 ample is similar to the d-c amphfier type previously described but includes 

 certain modifications important for this particular application. In this cir- 

 cuit the plate potential of the first IF stage of the AFC unit is obtained as a 

 positive pulse from the transmitting modulator thus enabling the AFC cir- 

 cuit only during the short interval of time encompassing the outgoing radar 

 pulse. This arrangement assures that no detuning of the receiver will result 

 from spurious or nearby signals after the radar pulse has been transmitted. 

 The rectifier elements here consist of triodes operating near plate current 

 cut-off which results in improved hnearity of detection. The d-c amplifier 

 portion of this AFC circuit is arranged somewhat differently from theexample 

 previously discussed, including in this case balancing controls to account for 

 tube and circuit variations. At the condition of resonance, in this case 60 

 mc, the voltages applied to each grid of the amplifier are equal and the net 

 repeller potential is determined entirely by the manual control value. 

 The overall output range of voltage for this circuit is ± 20 volts, which in 

 this application represents a ± 40-mc frequency change for the associated 



" "Considerations in the Design of Centimeter-Wave Radar Receivers", Stewart E. 

 Miller, Proc. I. R. R., Vol. 35, No. 4, April, 1947. 



