. The Bell System technical journal . of the at-tenuator is two-fold: it provides control of the over-all sensitivity of themeasurements (in steps of db), and it introduces more than 20 dbattenuation into the dc output signal path. This loss is compensated byan 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 stabilitysince the range of random drifts, such as usually generated within thephase-discriminator and its direct-coupled output stage,
. The Bell System technical journal . of the at-tenuator is two-fold: it provides control of the over-all sensitivity of themeasurements (in steps of db), and it introduces more than 20 dbattenuation into the dc output signal path. This loss is compensated byan 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 stabilitysince the range of random drifts, such as usually generated within thephase-discriminator and its direct-coupled output stage, are materiallyreduced. ..■! MeaHuring Probes As has been mentioned above, two arms of the bridge circuit consistof a pair of admittances between the grounded metal core of the cable(D corner) and the pioljes sliding on the surface of the i)lastic cablesheathing. These probes are connectetl io the A and C corners ofthe bridge, respectively, with two shielded flexible conductors (each 360 THE BELL SYSTEM TECHNICAL JOURNAL, MARCH 1954. m INCREMENTAL SHEATH THICKNESS MEASUREMENTS 361 about 10 feet long) and are maintained mechanically in the testing posi-tion h}^ the probe assembly (see Figs. 3 and 5(a)). In the design of the probes and their assembly, various difhcultieshad to !)( overcome. The probes operate on cables subjected to someluunoidable swings and vibrations while moving with speeds up to 80feet per minute. The capacitance from either of these probes to the metalcable core, in equivalent conditions, should match each other withinapproximately one-thousandth of a mmF. This capacitance should not beappreciably affected by limited displacements of the probes with respectto the cable plane of symmetry, such as may occur in actual operatingconditions. The first experiments with probes of a conventional design, havingflat, or nearly flat, contact surfaces, were quite discouraging. The probe-to-core capacitances fluctuated to an intolerable degree as a result ofeven minute c
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