THE ILiDAR RECEIVER 787 



the deflection time function desired. If a linear deflection function is 

 assumed, as shown in Fig. 55, producing a linear sweep, the necessary form 

 of the appHed voltage wave will vary depending on the inductance, the 

 resistance and the parasitic capacitance of the coil circuit. These condi- 

 tions are illustrated in Fig. 55. It is entirely possible to generate sweep 

 voltage functions of the forms indicated here for application to a linear 

 amplifier and deflection coil circuits and in fact such an approach was em- 

 ployed in early military radar designs of World War II. It has, however, 

 proven more convenient to employ negative feedback amplifiers whereby 

 the deflection coil current output is maintained proportional to the applied 

 voltage at the input of the sweep amplifier. In this manner, a sweep 

 generator voltage wave form can be employed which has the characteristics 

 desired of the final deflection. 



A simplified schematic of a feedback sweep amplifier to be employed in 

 connection with a magnetic deflection radar indicator is shown in Fig. 56. 

 In this example the impressed sweep wave form voltage having the essential 

 characteristics of the desired deflection time function is impressed upon 

 the grid of T'l . This sweep form is amplified and the deflection coil current 

 of the output stage which flows through the 80-ohm cathode resistance 

 common to the first and third stages produces a voltage drop proportional 

 to this current which is effectively applied between the cathode and grid 

 of the first stage thus completing the negative feedback loop. If sufficient 

 forward gain and adequate feedback is provided, the deflection coil current 

 can be made to assume the essential characteristics of the original im- 

 pressed voltage sweep wave form. It should be observed that the grid of 

 the third stage is biased negatively beyond plate current cut-off to insure 

 that the deflection coil current has an initial value of zero before the start 

 of the sweep. If this condition is not observed, the zero range point on the 

 indicator will be a function of the d-c current of the output stage and in the 

 case of a PPI form of display, the zero range region will assume the charac- 

 teristics of an open circle and map distortion at the short ranges will result. 

 In this amplifier circuit, application of the sweep signal to the grid of F3 

 will not result in deflection current flow until the tube is driven above cut- 

 off. During this time the feedback is not effective and the over-all gain of 

 the amplifier is at its maximum value. Due to the inductive characteristics 

 of the amplifier load impedance, the initial rise in current will be delayed 

 slightly with respect to the applied voltage and accordingly a further delay 

 of the feedback voltage is introduced by the use of a time constant in the 

 common feedback network. The result is a delaying of the applied feedback 

 voltage with a corresponding period of maximum gain of the amplifier which 

 tends to produce a sharp increase in deflection coil current at the time of the 



