THE RADAR RECEIVER 



795 



range, is zb 200 yards at the maximum range with an accuracy of ± 100 

 yards for targets within 5 nautical miles. 



For more precise determination of range than is afforded in the circuit 

 just described, two methods have been extensively employed. The iirst 

 method involves the production of a known time delay by actual measure- 

 ment of the time of propagation of an acoustical wave through a liquid me- 

 dium. Here the physical length of path is varied to produce the variable 

 delay. The second method involves the phase shifting of a known precise 

 sinusoidal frequency standard which bears a fixed phase relationship to the 

 time of the outgoing radar pulse. 



The "liquid delay tank" variable range unit over-all operation may be 

 observed by reference to Fig. 62. The zero time range reference is obtained 

 in the form of a pulse coincident with the outgoing radar pulse. This pulse 

 actuates the one-cycle multivibrator shown to produce a sharp high-ampli- 



DELAY TANK 



CONTROLn5:g3gz tL [ jZ\/\/\A/V--~-r[:^ 



SYNC 

 PULSE 

 INPUT 



MULTI- 

 VIBRATOR 

 CIRCUIT 



AUTOMATIC 



GAIN 



CONTROL 



TRIMMER 

 CIRCUIT 



MULTI- 

 VIBRATOR 

 CIRCUIT 



RANGE- 

 PULSE 

 OUTPUT 



to tp 



tntp 



Fig. 62. — Liquid delay tank type of precision variable range unit — block diagram of 

 operation. 



tude output pulse, here relatively independent of amplitude and form char- 

 acteristics of the synchronizing pulse and which is applied directly to the 

 delay tank. This delay tank consists of a suitable container filled with a 

 mixture of iron-free ethylene glycol and water so composed as to produce a 

 zero temperature-velocity coefficient at 135°F, at which temperature the 

 liquid is maintained by thermostatically controlled electrical heaters. In 

 this temperature region the temperature-velocity characteristic is such as 

 to produce a decrease of velocity of 0.1% for a temperature variation of 

 14°F. Located at one end of this tank is a quartz crystal, approximately |" 

 square and 0.040" in thickness, mounted securely on a brass plate which 

 serves as one electrode and which is immersed in the liquid. A similar 

 crystal element is attached to a lead-screw carriage and located so that the 

 face of this crystal is parallel to the fixed crystal. The distance between the 

 crystal faces can be varied by rotation of the lead screw. The sharp voltage 

 wave ai)plied to the transmitting crystal causes it to oscillate in a damped 

 vibration at its natural frequency for longitudinal waves which in this case 



