Improvements in or relating to electric attenuation measuring apparatus

Abstract

Abstract 779,046. Attenuation measurement; potentiometric voltage measurement. STANDARD TELEPHONES & CABLES, Ltd. Nov. 1, 1955, No. 31182/55. Class 37. Apparatus comparing the levels of electric signals for attenuation measurements in transmission networks comprises a source of test frequency signal applied to the inputs of a test and a standard network, first time switch means for alternately connecting the outputs of the networks to a storage circuit for equal periods exceeding the time for completion of storage, and second time switch means synchronous with the first for sampling the stored energy from the test. and from the standard networks over equal periods having for duration a fixed fraction of the storage period and commencing after storage is complete, and for applying the sampled energy derived from each switching period to respective integrating circuits whose outputs are compared in a measuring device calibrated either to indicate equality or the difference in the attenuations of the two networks. In Fig. 2, oscillator 1 energizes test and standard networks 2, 3 over equal series resistances to develop corresponding outputs over equal loads which are alternately selected for e.g. 10 millisecond periods by change-over contacts 8 of relay 7 operated at 50 c.p.s. (Fig. 1) to energize amplifier 9 and square law triode or pentode detector 12 having a variable cathode resistance 14 and its anode circuit energized through a bridge network comprising anode resistance 17, variable resistance 18 and fixed resistances 20, -19 of which the latter is returned to the valve cathode while an indicating instrument is connected between junctions 17, 18 and 19, 20, and anode voltage is applied to the junctions 18, 20. The rectified output is stored in small capacitance 22, and contacts 25, 26 of sampling relays 23, 24 (Fig. 1) are operated on alternate half-cycles at 50 c.p.s. for e.g. 4 millisecond periods to connect the stored charge in capacitance 22 to respective larger integrating capacitances 27-28 energizing the grids of a comparison double triode 31 over equal series resistances 29, 30; a normally open switch 37 interconnecting correspondingly spaced taps therein. The cathodes are connected over equal resistances 32, 33 to a common cathode resistance 34, while the anodes are connected to a potentiometer 35 shunted by an indicating instrument 36, having its slider connected to the anode voltage source. The switching relay operating circuit (Fig. 1) is supplied with A.C. at e.g. 50 c.p.s. over transformer 41 having a secondary 42 driving the grid of limiter triode 45 biased by series grid resistance 44 alternately to cut off and saturation so that relay 7 in its cathode is operative and inoperative on alternate half-waves to operate contacts 8 in opposite directions for e.g. 10 millisecond periods. The opposite ends of secondary 43 centre tapped to earth are connected over variable phase-shifting resistance 48 and capacitance 49 to potential divider 50, 51 capacitance coupled to the input of phasesplitting triode 53 energizing in antiphase the respective elements of double triode 58 biased beyond cut-off by common cathode potentiometer 63, 65 so that anode relays 23, 24 are alternately energized on successive half-cycles to operate contacts 25, 26 for successive e.g. 4- millisecond intervals; determined by the bias of valve 58, the phasing of which is adjustable by resistance 48 to occur normally at the centre of the e.g. 10-millisecond operative period of contacts 8 in each respective direction. In operation, resistance 18 is first adjusted to balance out the residual current of meter 21, which is decibel calibrated in current values, and relay 7 is held inoperative while the attenuation of the standard network is varied in 1 db. steps, and resistance 14 is adjusted for corresponding 2 db. steps on meter 21, so that valve 12 has square law characteristics. Switch 37 is then closed and potentiometer 35 is adjusted for zero current in meter 36, with the relays operating normally after which switch 37 is opened for measurement. The stored charge on capacitance 22 is shared, on each operation of relay 23 or 24, with capacitance 27 or 28, so that the latter acquire integrated potentials respectively proportional to the outputs of the test and standard networks whose difference is measured by meter 36 which may be calibrated in decibels or restored to zero by insertion of attenuation in the standard network equivalent to that of the test network. Transients due to the successive charging of capacitance 22 have died out before capacitances 27 or 28 are charged so do not affect the accuracy of the measurements. In a modification (Fig. 3, not shown) the sampling relay contacts are fed from variable potentiometers energized from the anode circuit of the square law rectifier through a low-pass filter eliminating A.C. components from the rectified signal and automatic gain control (which may be disabled at will) is applied to the input amplifier from the cathode circuit of the double triode comparison valve 31 or through a D.C. amplifier from the potential acquired by capacitance 28 (Fig. 4, not shown). A potentiometer whose slider is returned to the junctions 32, 33, 34 may be connected between the cathodes of double triode 31 and shunted by a switch to increase sensitivity, the potentiometer enabling balance of the two sections to be obtained both for high and low sensitivity. The relays are replaceable by equivalent electronic switches.

Inventors

• 👤 Basil Bernard Foster

Bibliographic Details