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BELL SYSTEM TECHNICAL JOURNAL 



portional to the instantaneous heading of the aircraft. In the SL radar 

 indicator shown in Fig. 61 the compass information is introduced by means 

 of a mechanical differential rotation of the indicator deflection coil propor- 

 tional to the angular position of the compass repeater mechanism. 



2.62 Range Marker Circuits — Fixed Range Markers 



A simplified schematic diagram of a convenient fixed range mark gener- 

 ator circuit which has been extensively employed on airborne radar search 

 systems is given in Fig. 59. Here the radar system requires 1- and 5- 

 statute mile fixed range markers. The sweep start multivibrator pulse is 

 applied to the grid of Fi as shown. In the absence of a signal, this tube 



-1-300 VOLTS 



INPUT, 

 SWEEP-RANGE 

 START PULSE 



RANGE 

 MARKER 

 OUTPUT 

 O 



(-375 VOLTS 



RANGE-MARKER 

 SWITCH 



L, (80.9KC) c, 



Fig. 59. — Fixed range marker circuit — simplified schematic. 



operates at effectively zero bias, and because of the large plate current flow 

 reduces the effective plate potential of F2 and the grid potential of F3 to a 

 low value. Since the cathode of F3 is subject to a large positive potential, 

 this tube is cut off and the oscillatory circuit shown is inoperative. Upon 

 application of the negative start pulse, Fi is cut off for the duration of this 

 pulse and the attendant rise in the F3 grid potential permits the oscillator 

 circuit to function. The series resonant elements Li Ci and L2 Ci determine 

 the frequency of oscillation by providing a high value of positive feedback at 

 the series resonant frequency. The output of F3 which consists of approxi- 

 mate sinusoidal pulses is differentiated by means of the air-core transformer 

 shown. The differentiated pulses are then applied to a cathode follower 

 amplifier stage biased to cut off where the output is limited to the desired 

 positive fixed range mark pulses for indicator display. By careful choice of 

 circuit elements and equipment arrangement, this simple circuit form has 



