Patent GB-567571-A (GB)
Title: Improvements in electrical frequency meters
IPC: G01R23/02
Filed: 1941-12-26
Abstract: 567,571. Frequency measurements. GENERAL ELECTRIC CO., Ltd., and PHILLIPS, F. C. F. Dec. 26, 1941, No. 16694. [Class 37] Input oscillations are-applied. to a pair of discharge devices in such a manner that during one part of the cycle one of the pair is caused to pass current which charges a condenser and during another part of the cycle the other discharge device is caused to pass current which discharges the condenser, a unidirectional current being thereby produced whose mean value is proportional to the applied frequency. In order to render this value independent of the amplitude of the oscillations, a voltage is applied to each of the pair of discharge device. during a cycle, to cause it to pass current, which voltage consists of a pulse whose duration. is less than one-half of any period to be measured and is substantially independent both of the frequency and the amplitude of the input oscillations. In the apparatus shown in Fig. 1, a pair of triodes V2, V3 in series are arranged with a galvanometer G in the anode lead of the valve V3 and a condenser C6 between the anode and cathode of the valve V2. A discharge device N and resistor R8 keep the voltage feeding the two valves constant. Grid bias is applied to the valve V3 through a rectifying and smoothing device W2, C5 and to the valve V2 through a similar device W1, C4, the bias being adjustable by potentiometers P1, P2. The frequency to be measured is applied between the cathode and grid of a pentode V1 through a resistor R2, a blocking condenser C1 and grid leak R1 being arranged as shown. The screen grid is fed by a resistor R4 and condenser C2. The anode is fed through a resistor R3 and the primary of a transformer T1, which primary is in parallel with a condenser C3 and resistor R5. Two secondaries of the transformer T1 feed in opposite phases the grid-cathode circuits of the valves V2, V3, these circuits containing resistors R6, R7. Normally, the current flowing in the resistor R3 has a square wave form, the duration of which depends on the frequency. By adjusting the condenser C3 and resistor R5 relatively to the inductance of the primary so that the period of oscillation of the primary circuit is less than one-half of the period of any oscillation to be measured and so that this oscillation is critically damped, the pulses applied to the valves V2, V3 are caused to have a peaked form such that the period during which the pulse lies above the value at which current begins to pass in the valve is independent of the. frequency. The condenser C6, which is charged and discharged at each alternation by the valves V3, V2, forms one arm of a Wheatstone bridge of which the other arms are fixed resistors Q, S, Fig. 5, and a part of a resistor T selected by tapping arms 30, 31 operated by a dial 32. The galvanometer G is associated with a network R10, R11, R12 such that the sensitivity is increased 10 times by throwing the switch 33 while the input resistance remains unchanged. A lamp SL operated through a switch 34 ganged with the switch 33 indicates that the reading is to be multiplied by 0.1. Operation of a relay D changes over contacts D3, D4 and thus reverses the galvanometer connections. The contact D1 selects one of two lamps NL, IL which illuminate particular figures on the dial 32, while the contact D2 connects one or other of the arms 30, 31 to the resistance S. The frequency to be measured is derived from a previous stage, which may be of the kind described in Specification 567,572, in which the. units digit has been determined, so that the measurement to be made is to determine the digits following the decimal point. The first of these digits is indicated on the dial 32 and the others on the galvanometer scale. Under certain conditions in the previous stages the relay D is operated and its contacts D1, D2, D3, D4 thrown over. The reading then obtained is the difference between the actual value and unity.