THE RADAR RECEIVER 753 



ment which has in the past been required of the radar indicator and in this 

 respect has contributed some measure of improved performance under 

 miUtary operating conditions. 



2.33 D-c Restoration Methods 



It is pertinent to examine the exact form of the video signal encountered 

 at the output of the video amplifier as it exists available for indicator use. 

 The presence of series coupling condensers in the video amplifier has re- 

 moved the d-c component from the signal as detected and, therefore, the 

 average value of the signal is zero. In this form the amplitude of the posi- 

 tive and negative signal excursions are dependent on the form of the signal 

 itself. If such a signal is impressed upon an indicator of the intensity 

 modulated type the average brightness of the scene will remain constant, 

 and the presence of several large amplitude signals will tend to drive any 

 accompanying weak signals below the useful reproduction threshold and 

 effectively fail to reproduce them. In the case of an A-type display where 

 the video signal deflection modulates the beam, the no-signal base line will 

 assume a position on the screen dependent on the video signal form. For 

 these applications it is required that the d-c component be restored to the 

 signal before display. In many other parts of the radar system d-c restora- 

 tion is required to enable utilizing to the fullest extent the load capabilities 

 of vacuum tubes under the conditions of varying duty cycles of the im- 

 pressed wave forms. In sweep circuit design a considerable economy of 

 power is achieved by operating the amplifier tubes at or near plate current 

 cut-off for no-signal conditions. Through the medium of d-c restoration, 

 the signal excursions are confined to positive regions only and then regard- 

 less of the duty cycle the signal range of amplitude impressed upon the tube 

 is maintained within desired limits. 



Figure 34 illustrates three circuit forms which are employed to "re- 

 insert" the d-c component of an a-c video signal. The diode restorer com- 

 monly employed in radar systems is shown in Fig. 34a. The impressed 

 input wave, assumed to have an average value of zero as shown, will cause 

 the diode to conduct whenever the signal polarity is negative. During this 

 diode conducting period the condenser C will be charged rapidly, the full 

 effective negative peak signal voltage appearing across its terminals. Dur- 

 ing the following positive excursion of the signal this voltage difference 

 will be applied effectively in series with the signal. The time constant 

 RC is chosen large with respect to the period of the signal repetition rate 

 and thus maintains this additive bias for the remainder of the signal cycle. 

 Since the effective time constant during the diode conducting period is 

 extremely small valued, limited only by the conductive internal resistance 

 of the diode itself, an extremely small negative excursion time will suffice to 



