Patent GB-631203-A (GB)
Title: Improvements in and relating to cathode-ray oscillograph apparatus
IPC: G01R27/32
Filed: 1947-03-21
Abstract: 631,203. Cathode-ray oscillographs. COMMUNICATIONS PATENTS, Ltd., and HAMBURGER, G. L. March 21, 1947, No. 7799. [Class 37] A cathode-ray oscillograph is arranged to display simultaneously a trace representing the momentary co-relationship of two electrical quantities and calibrated scales corresponding to each of the co-ordinates. The arrangement shown may be used to ascertain the audiofrequency response of a transmission line which serves to distribute a broadcast relay programme to a number of subscribers. Test signals are obtained from an oscillator which, when triggered by an initiating signal, applies automatically to the line, as an alternative to the broadcast programme, a gliding tone which sweeps at constant amplitude from say, 16,000 c.p.s. down to 50 c.p.s. within a few seconds. This test signal is applied by way of terminals 14, 15, Fig. 1, to an input amplifier which includes a calibrated adjustable attenuation system whereby the mean level of the test signal can be brought to a mean value of 50 millivolts plus or minus 20 decibels. The output from the input amplifier is applied to a pre-channel amplifier where it is divided into two distinct channels, one pertaining to the frequency of the test signal and the X-deflection of the cathode-ray tube 16, and the other pertaining to the amplitude of the test signal and the Y deflection of the tube. For the frequency channel the test signal is passed from the pre-channel amplifier to a limiter consisting of a number of amplifying stages interspersed with metal rectifier clipping stages. The output from this is a rectangular waveform with a peak-to-peak value which is constant for all signal amplitudes into the pre-channel amplifier of from 5 millivolts upwards. This square-wave signal is applied to a frequency-toamplitude converter comprising an arrangement of gas-filled trigger tubes and other means for integrating the signal over unit time to obtain a unidirectional E.M.F. having a value which rises with the frequency of the input waves but is independent of their amplitude. This converter, also includes means for converting this E.M.F. into an E.M.F. which is a logarithmic function of the frequency of the input waves. The output from this converter is applied to a D.C. amplifier and thence to the X-deflecting plates of the cathode-ray oscillograph 16, which has a screen of the longpersistence type. For the amplitude or Y- channel the output from the pre-channel amplifier is applied to a phase-splitter and balanced rectifier to obtain an output which represents a linear measure of the amplitude of the test signal independent of frequency. This output is applied to a linear-to-logarithmic amplitude converter, the output from which is passed to a D.C. amplifier and thence to the Y-denection plates of the oscillograph. For the incoming gliding tone test signal a trace 17, Fig. 13, is obtained in which the vertical position of the spot indicates the amplitude of the test signal at the frequency corresponding to the horizontal position. The characteristic of the line under examination is thus indicated. A system of intersecting co-ordinates 18, 19 is set up upon the screen by means of a switchable oscillator and attenuator for obtaining two sets of standard E.M.F.'s representing frequency and amplitude for application to the X and Y channels. The oscillator produces, under the control of switching means, electric waves of nine predetermined different frequencies. Those suitable for logarithmic frequency scales are 16,000, 10,000, 5,000, 2,000, 1,000, 500, 200, 100, and 50 c.p.s. These standard frequency signals are applied successively to the pre. channel amplifier by way of a change-over switch S1. A separate scanning signal is simultaneously applied to the Y-plates so as to obtain nine vertical and parallel lines 18 on the screen. The oscillator then supplies a constant frequency signal of 1000 c.p.s. to an attenuator, which applies in succession fifteen different standard amplitude levels, at this frequency, to the pre-channel amplifiers, the output of which is passed through the amplitude channel to produce fifteen Y-deflecting E.M.F.'s corresponding to the amplitude levels, and are spaced 3 decibels apart. For the application of each amplitude level a separate scanning voltage is applied to the X-plates so that fifteen horizontal and parallel lines are traced on the screen and constitute the logarithmic amplitude calibration. Certain lines of one or both calibrated scales may be given a distinctive appearance, for example, a dot or dash line as distinct from a continuous line. During the time when the standard frequency and amplitude signals are being transmitted the Y and X plates respectively are disconnected from their channels by change-over switches S2, S3 so that they receive the separate scanning voltages from the sequence and scanning controller. This is controlled by a calibration starter which is triggered - on the arrival of the lowest frequency (50 c.p.s.) of the gliding tone. A loudspeaker 20 is connected to the output of a monitoring amplifier which receives an input from the pre-channel amplifier and enables the broadcast programme, the gliding tone, and the calibration signals to be heard.