THE RADAR RECEIVER 



791 



ranged to be unaffected by any subsequent chatter or false switch closures. 

 The pedestal generator which includes vacuum tubes Fi and V2 which are 

 normally operated below plate current cutoff produces upon closure of the 

 bearing marker switch contact a rectangular negative pulse having a dura- 

 tion of 10,000 microseconds. This pulse operation is independent of addi- 

 tional chatter effects following the initial contact closure. The following 

 single-cycle bearing mark multivibrator is normally held inoperative by the 

 bias voltage developed on the cathode of F3 . The grid of Vz is continu- 

 ously excited with the range sweep start pulses of an amplitude insufficient 

 to actuate this multivibrator circuit. The presence of the 10,00()-micro- 

 second pedestal is sufficient, however, to allow the following range sweep 



OUTPUT, 

 BEARING-MARK 

 SIGNAL (TO GRID 

 OF INDICATOR) 



O 1 



BEARING-'^/ 

 MARKER / 

 SWITCH A 



+300 VOLTS 



Fig. 58. — Electronic azimuth bearing marker circuit — simplified schematic. 



Start pulse lo operate the multivibrator. The output of this circuit is 

 then a 55()-microsecond pulse which represents a time shghtly longer than 

 the maximum range to be displayed (60,C0) yards) but shorter than the 

 period of the sweep repetition rate. This positive 550-microsecond pulse 

 is applied to the modulating grid of the PPI indicator tube through an 

 adjustable trace brightness control element. 



Another convenient azimuth display method which has been extensively 

 employed in naval and airborne radar systems involves the use of "true 

 North" presentations. Here the PPI azimuth indication is presented in 

 terms of a compass reference, the actual instantaneous position of the range 

 trace on the screen representing the compass direction of the antenna beam. 

 In the indicator previously illustrated in Fig. 44 the compass information 

 is introduced by means of a dilTerential synchro inserted in the antenna- 

 indicator synchronizing connections whose angular displacement is pro- 



