Patent GB-677788-A (GB)
Title: Improvements in or relating to transmission circuits
Inventor: PULSFORD EDGAR WILLIAM
IPC: G06G7/24
Filed: 1949-08-19
Abstract: 677,788. Frequency measurements. NATIONAL RESEARCH DEVELOPMENT CORPORATION. Aug. 16, 1950 [Aug. 19, 1949], No. 21605/49. Class 37. A transmission circuit makes use of a train of pulses spaced in time and delivers a voltage or current which is a measure of the repetition rate on a logarithmic scale. It is applicable as a counting rate meter for pulses of non- uniform occurrence rate such as arise in Geiger Muller tubes and other detectors of radioactivity. The basic circuit is shown in Fig. 1, a recurrent voltage step being applied to terminals 1. A condenser C is alternately charged through a diode D1 and discharged through a diode D2 into a smoothing condenser C5, on which the voltage increases until the discharge current through a resistance R is just balanced by the charging current through the diode D2. The circuit diagram of the counting rate meter is shown in Fig. 3. The pulse train or other waveform to be measured is converted into negative pulses of about 50 volts amplitude and one or two microseconds duration and applied to the cathode of a double diode, VI, which together with a double triode V2A, V2B forms a divide-by-two circuit, i.e. V2A, V2B has two symmetrical stable states and is triggered from one to the other by the incoming pulses. The outputs from the valves V2A, V2B are stepped voltages, one rising as the other falls. The grids of V2A, V2B are directly connected to those of a double triode V3A, V3B as a long-tailed pair so that, whichever half of V3A, V3B is conducting the current in the cathode circuit is approximately constant. This cathode current passes through one or other of the anode loads R9, R10 and so the voltage step produced at one anode when the valve is switched over is constant. The outputs from the anodes of V3A, V3B are stepped voltages of uniform amplitude similar to each other but of opposite sense. This stage feeds six paralleled stages, each of the form shown in Fig. 1, to yield an output dependent on pulse rate. Each stage comprises a condenser C3 ... C8, a double diode V4 ... V9, a larger condenser C9 ... C14, and a resistance R22 ... R17. Four of the stages V4 ... V7 are coupled by their condensers C3 ... C6 to the anode of the valve V3B. A positive-going voltage step at this anode charges the condenser C3 to the full amplitude of the step through the left-hand diode of V4 and this charge is maintained for the duration of the step. At the end of the step current flows through the valve V3B and the condenser C3 discharges through the right-hand diode of V4 into the condenser C9. Similarly the condensers C4, C5, C6 are charged by the positivegoing step and their charges delivered to the larger condensers C10, C11, C12 at the end of the step. The condensers C9 ... C12 are connected through equal resistances R22 ... R19 to the input grid of a negative feed-back amplifier comprising a pentode V10 and a triode V11 having a milliammeter M1 in its cathode output circuit. The leakage currents from the several condensers flow through this input circuit and their sum is measured by the meter M1. The feed condensers C3 ... C6 are logarithmically graded, suitable values being 0.1, 0.01, 0.001 and 0.0001 mfds. The feed condensers C7, C8 are connected to the anode of the valve V3A instead of V3B to reduce the effect of stray capacitances and are shunted to earth by resistances R14, R15, R16.