IXCREMEXTAI, SIIKATII TnirK\i:sS MKASTHKMEXTS 350 



is actually higher than the sensitivity of the usually available laboiatoiy 

 equipment. 



'I'he bridge network is supplied by a 10-kc ac power source. 



2.2 Phase Sensitive Detector 



For eccentricity measurements and control of the sheathing process 

 it is essential to register the direction of incremental deviations from an 

 arbitrary level. For this purpose a phase sensitive detector ' has been 

 provided. Its simplified version is shown on Fig. 4. 



By proper adjustment of the phase-shifter, the reactive component of 

 the bridge unbalance signal can be oriented to be in-phase with the 

 reference potential (b-a). In this condition, the capacitance unbalance 

 sensitivity of the discriminator is at its maximum, and for a certain 

 range of capacitance unbalances, linearity of- the indicator may be as- 

 sured. Also, when the above phase condition is fulfilled, the circuit is 

 not sensitive to limited conductance unbalances (this fact also renders 

 the cii-ciiit remarkably more stable than a similar circuit using a con- 

 ventional null detector). 



The dc output from the discriminator is fed through a balanced output 

 stage (V2a and V2b), and an attenuator to a Leeds & Northrup zero- 

 centered recorder. At the operating sensitivity level, each of the 100 

 divisions of the recorder scale corresponds to 0.005 mmF, or approximatel}^ 

 to 0.001 inch of the incremental sheath thickness. The role of the at- 

 tenuator is two-fold : it provides control of the over-all sensitivity of the 

 measurements (in steps of 0.2 db), and it introduces more than 20 db 

 attenuation into the dc output signal path. This loss is compensated by 

 an added gain within the feedback-controlled ac amplifier (AC-A) pre- 

 ceding the phase discriminator. The net result of this "ac for dc gain- 

 trading" is a considerable improvement of the over-all circuit stability 

 since the range of random drifts, such as usually generated within the 

 phase-discriminator and its direct-coupled output stage, are materially 

 reduced. 



.'.■! MeaHuring Probes 



As has been mentioned above, two arms of the bridge circuit consist 

 of a pair of admittances between the grounded metal core of the cable 

 (D corner) and the pi'oljes sliding on the surface of the i)lastic cable 

 sheathing. These probes are connectetl io the "A" and "C" corners of 

 the bridge, respectively, with two shielded flexible conductors (each 



