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BELL SYSTEM TECHNICAL JOURNAL 



As indicated in Fig. 13, a tuned echo box cavity is usually provided with 

 two couplings. One of these is to the radar pick-up; the other to an attenu- 

 ating device, crystal rectifier, and meter, which serve for tuning the cavity 

 and for other purposes. With such an instrument, not only can the radar 

 be tuned up and its over-all performance determined, but many other tests 

 can be made, to wit: (1) the setting of the plunger at resonance indicates 

 the transmitter frequency or wavelength; (2) calibration of the cr}^stal 

 affords a rough measure of output power; (3) since the Q required for ade- 

 quate ringtime is so high that the cavity selects only a narrow segment of 

 the transmitter spectrum, a spectrum analysis can be made by plotting fre- 

 quency versus crystal current reading; (4) slow recovery of TR box and 

 receiver after the transmitted pulse can be detected by noting the behavior 

 of the ringtime pattern at short ranges as the echo box is detuned; (5) 



I 



TO TUNING DIAL 

 MECHANISM 



VANE ATTENUATOR 



-X- 



Fig. 13 — Functional schematic of tuned echo box 



inability of the receiver to recover promptly after a strong signal (the result 

 of imperfect d.c. reinsertion in the video amplifier or of overloading of the I.F. 

 amplifier) is indicated by a blank following the end of the ring; (6) improper 

 pulsing (e.g. double moding or misfiring) can be determined with a class A 

 oscilloscope; (7) the frequency and power of the local oscillator can be 

 measured. In tuned echo boxes, requirements for extreme fineness of tuning 

 control and precise resettability have given rise to interesting problems in 

 the design of the mechanical drive and indicating mechanism. 



In another type of echo box, hand tuning is supplemented by motor-driven 

 tuning or so-called "wobbling" over a frequency range wide enough to 

 embrace expected variations in transmitter frequency. Operation is con- 

 trolled by a single push-button which energizes the motor and actuates the 

 cavity coupling. Such an instrument may be permanently installed in a 

 plane and used to check the radar during flight. 



