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BELL SYSTEM TECHNICAL JOURNAL 



is of the order of 1.4 megacycles. The mounting plate and surrounding 

 liquid serves to highly dampen this oscillation. A short vibrational wave 

 train is projected through the liquid toward the receiving crystal. The 

 amplitude of this disturbance is only slightly attenuated by viscous dissipa- 

 tion for the maximum path length here employed. The large area of the 

 crystal relative to the wave length results in a highly directive radiation and 

 is reflected in a parallelism requirement for the crystal faces of the order of 

 .01". The voltage developed across the receiving crystal upon application 

 of this delayed acoustical wave consists of a main response followed by minor 

 disturbances due to re-reflections between the crystals. 



Fig. 63. — Liquid delay tank t}-pe of precision varialjle range unit. 



The following amplilier shown in Fig. 62 is required to increase the .005- 

 volt received signal to appro.ximately 20 volts. This gain supplied is con- 

 trollable by means of an automatic gain control circuit so as to provide a 

 relatively constant amplitude of the first response signal. The following 

 trimmer circuit consists of a pentode operating below cutoff such that a 

 signal of at least 20 volts is required for plate current flow. Since the AGC 

 circuit operates to adjust the gain of the amplifier to this condition, only the 

 first and highest response peak is transmitted to the final range j^ulse multi- 

 vibrator circuit where a sharp narrow rectangular pulse is produced to be 

 employed in the following indicator circuit. 



Figure 63 is a photograph of the liquid-tank type of variable range unit as 

 developed and manufactured early in the past war and employed extensively 

 in naval fire-control radar systems. This unit includes provision for a 



