Patent GB-517486-A (GB)
Title: Improvements in or relating to instruments for measuring the peak value of electrical impulses
IPC: G01R19/04
Filed: 1938-07-29
Abstract: 517,486. Valve relaying circuits. STANDARD TELEPHONES & CABLES, Ltd., BLACK, D. H., GRANT, A. S., and MACNEE, D. H. July 29, 1938, No. 22637. [Class 40 (v)] [Also in Group XXXVI] In a meter for measuring the peak-value of a train of electrical impulses, the signal voltage is rectified and caused to charge successively a plurality of condensers of progressively increasing capacity interconnected in cascade and so arranged that the charging time of the first condenser is sufficiently short to reach full charge during the signal peak period, and the leakage of the condenser in the output stage is sufficiently small for the charge to remain substantially constant over the measuring period, the output stage condenser being arranged to bias back an amplifying valve in accordance with the peak value of the signal voltage to operate the voltage recording device. The time during which a signal voltage is above a certain strength may also be recorded. In applying the invention to the measurement of valve microphonicity, the valve under test forms part of a test amplifier with means for impressing a calibrating voltage on the grid of the test valve, the amplifier output being applied to the input terminals AA of the instrument shown. The input wave at AA is rectified by the diode V8 and charges up a condenser C5 which is so small that its charging time is shorter than that during which the peak value of the signal impulse persists. The voltage on C5 therefore builds up to the peak value of the signal impulse and negatively biases the grid of a triode V3 causing a drop in the voltage across the load resistance R3 thereof. A potentiometer R7 is adjusted so that under " no signal " conditions it counterbalances the voltage across resistance R3 so that the grid of a triode V4 energized through the rectifier V9 is at its cathode potential. The drop in voltage across R3 due to the signal therefore results in a potential being built up across the condenser C9 which is of larger capacity than, and will hold its charge for a much longer time than, C5 and allow a still larger condenser C10 to be charged, in a similar manner, to a potential corresponding to the signal peak value. The condenser C10 negatively biases a valve V5 and a recording meter M1 in the cathode circuit thereof is thus deflected in accordance with the peak signal voltage. The zero position of the meter Ml, at its maximum deflection, is obtained by momentarily closing a switch P1 to bring the grid of V5 to cathode potential. The time during which the signal impulses exceed a certain strength is recorded in the following manner. Under " no signal " conditions, the similar valves V5, V6 both have their grids at cathode i.e. earth potential, and are balanced so that no current flows through a relay R connected between the anodes of V5, V6. When a signal is present the anode current of V5 is reduced as hereinbefore described but the anode current of V6 is reduced still further since the signal input at AA is applied to a pentode amplifier V2, the output of which is rectified at W and then applied to bias back the valve V6. The relay R is thus operated and short circuits a condenser C11 through a resistance R22. A bridge circuit, the arms of which comprise a triode V7 and resistances R12, R13, R19 is balanced when the grid of the triode V7 is at cathode potential. No current then flows through a recording meter M2. By momentarily closing a switch P2, the grid is biased negatively by the voltage drop across a resistance R14, and the meter M2 is then arranged to read a full scale deflection, this being the zero position of the meter. When the relay R is subsequently operated as above described the condenser C11 will commence to discharge through the resistance R22 thereby reducing the negative potential on the grid of V7. The meter M2 therefore deflects from its zero position until the relay R releases. This is arranged to take place when the signal at AA has fallen, for example, 30 decibels below its peak value (depending on the amplification of V2) so that the deflection of meter M2 is a measure of the time during which the signal strength exceeds a value 30 decibels below its peak.