GB569279A - Improvements in or relating to the measurement of the transmission characteristics of electrical apparatus - Google Patents

Abstract

569,279. Valve circuits. AUTOMATIC TELEPHONE & ELECTRIC CO., Ltd., TERRONI, T. B. D., and CANNON, J. R. Nov. 11, 1943, No. 18722. [Class 40 (v)] [Also in Group XXXVI] In an arrangement for determining the transmission characteristics of electrical apparatus, currents from a source of predetermined frequency are applied to an indicating device alternately by way of two circuits including, respectively, the apparatus to be tested and apparatus of known characteristics, switching from one circuit to the other being effected automatically by an electronic switching device and indications corresponding to the currents in the two circuits being maintained visually displayed in juxtaposition for a time greater than the switching cycle time so as to permit direct comparison between the two indications. The circuit under test is assumed to be a filter network F, Fig. 1, to which is connected an oscillator OSC through a band-pass filter BPF and line simulating resistances YA, YB. The band-pass filter is also connected through line simulating resistances YC, YD with a calibrated variable attenuation network VAN. The detector comprises a cathode-ray oscillograph CRT, the pair of horizontal deflection plates of which are connected to a time base TB, while the pair of vertical deflection plates are connected through an automatic change-over device ACD alternately to the filter F under test and to the attenuator VAN, individual amplifiers A1, A2 and a common amplifier A3 being provided. The device ACD governs the triggering of the time base circuit in such a manner that the image on the screen consists of two parts of equal width derived from the amplifiers A1, A2 respectively. The attenuator VAN is adjusted until these parts are of equal amplitude, when the attenuation of the filter can be read off the scale of the attenuator VAN. The inputs IA, IB, Figs. 6 and 7, to the amplifiers A1, A2 are terminated on 600 ohm terminations YE, YF and extend via volume control potentiometers PF, PG to the input valves VA, VF of the amplifiers Al, A2. The common amplifier A3 comprises valves VG, VH and has a volume control PH which is ganged with the volume controls PF, PG. The output valves VB, VE of the individual amplifiers A1, A2 share the same anode load, since they operate alternately under control of the multi-vibrator valves VC, VD. The valves VB, VC share a common cathode resistance PA, YG, the bias produced across which is sufficient to carry the valve VB beyond cut-off when the valve VC is conducting, so that the output from this amplifier is suppressed. During the same interval, the other multi-vibrator valve VD is also driven beyond cut-off by the multivibrator action so that the common cathode resistance YH for the valves VD, VE carries only the normal current of the valve VE and the output from this amplifier is passed on to the common amplifier via the coupling condenser QA. When the multi-vibrator switches the output from the amplifier A1 is passed on and that from the amplifier A2 is suppressed. The horizontal sweep of the cathode-ray oscillograph CRT is effected during the charging of the time base condenser QB, the instant of fly-back being determined by the valve VD which, each time it is rendered non-conductive, applies a positive pulse via a condenser QC to the grid of a gas discharge tube VJ, thereby discharging the condenser QB. A non-linear resistance NLR and resistances YJ, YK serve to straighten the operative part of the exponential charging curve of the condenser QB. The resistance PA serves to match the anode current of the valve VB with that of the valve VE. A resistance PB is provided to match the gain of the amplifiers Al, A2. A resistance PE is employed for adjusting the frequency of the multi-vibrator, and hence of the sweep, over a range of a few cycles per second on either side of the nominal 50 cycles per second. For testing an amplifier, it is connected in series with a variable attenuator in one circuit, while the other circuit extends directly from the oscillator to the cathode-ray oscillograph.