80 



USRL TEST STATIONS 



Figure 14. Polar recorder turntable assembly with re- 

 corder arm raised. 



Figure 15. Magnetic clutch and drive assembly of polar 

 recorder turntable. 



the use of a full-wave rectifier, operating on a square 

 law characteristic over a level range of some 9 db. 

 These points of difference permit the recorder to be 

 used for energy measurements of wide-band complex 

 wave signals such as noise. 



Polar Level Recorder 



ft is often desirable to know the response of an 

 instrument for various directions of projection or re- 

 ception, and auxiliary apparatus for this purpose is 

 provided for both systems. It involves a rotator on 

 which the instrument is mounted and a recorder in 

 polar coordinates. It is evident that these must rotate 

 in exact synchronism for the record to be correctly 

 interpreted. Figures 13 and 14 show the turntable as- 

 sembly of the polar recorder for system 2, and Figure 

 15, the driving mechanism and motor. The turntable 

 has suitable positioning and holding devices for 

 8t/,xl l-inch sheets of polar coordinate paper. The 

 gear train and several electromagnetic clutches allow 

 rotation of the turntable in either direction at an 

 angular rate of 1, y 3 , or % rpm, both direction and 

 rate being selected by switches. A 5F synchro is 

 mounted on the end of the drive shaft, which thus 

 couples it to the turntable through a 60: 1 worm gear. 

 This synchro, is the director of a 5CT synchro at- 

 tached to the rotator carrying the acoustic unit being 

 tested. Whenever the angular position of this synchro 

 does not correspond with that of the director, it gen- 

 erates error signals which are impressed on a thyra- 

 tron servo amplifier. The altered output of the ampli- 

 fier at once modifies the speed of the 14 hp d-c motor 

 driving the rotator and thus keeps the turntable and 

 rotator in nearly the same angular position. The er- 

 ror signal is about 1 volt for each degree of angular 

 difference and is thus a measure of the lack of syn- 

 chronization which, with proper adjustment, should 

 be less than 0.1 of a degree. A hand crank permits 

 angular positioning of the turntable independent of 

 the motor drive. As shown in Figure 15, the turntable 

 may be set readily with the hand wheel, if it is dis- 

 connected from the driving motor by the release of 

 the clutches. This allows it to be set to any desired 

 relation to the rotator, if the circuit between the syn- 

 chros is open. The strip attenuator, the sliding con- 

 tactor, penholder assembly, and a dual armature 

 motor are mounted on a tilt arm pivoted in such a 

 manner that it may be lowered into position over the 

 turntable (see Figure 14). The strip attenuator has 

 been wound at 10 db per inch to a total of 50 db, the 



