Patent GB-628579-A (GB)
Title: Improvements in or relating to cathode ray tube oscillograph apparatus
IPC: H03K12/00
Filed: 1946-09-12
Abstract: 628,579. Cathode-ray oscillographs. AIRMEC LABORATORIES, Ltd., (formerly P.R.T. LABORATORIES, Ltd.), and HULME, V. B. Sept. 12, 1946, No. 27475. [Class 37] [Also in Group XL (c)] In a cathode-ray oscillograph for producing scanning in synchronism with the instants of occurrence of a recurrent phenomenon to be displayed, a synchronizing signal having a suitable tilming relationship with the repetition frequency of the recurrent phenomenon is applied by way of phase-adjusting means to effect triggering of a pulsegenerating arrangement and a time-base generating device adapted to provide a current or voltage scanning wave form for producing linear forward and return movements of the cathode-ray beam during the positive and negative going portions of the pulses developed in the pulse generator, the scanning wave form being applied to the beam deflecting means of the oscillograph and the output of the pulse generator being applied to a beam modulating electrode of the cathode-ray tube to cause illumination of the screen during one only of the forward and return movements of the beam. The synchronizing voltage is applied to terminals t1, t2, Fig. 1, and may be, as shown at a, Fig. 2, a version of the 50 cycle A.C. employed for operating a synchronous motor driving the modulating condenser of a F.M. oscillator. The capacity variation of such condenser is shown at b, Fig. 2, and comprises four capacity sweeps in each cycle, two in' each direction. The grid voltage of a double diode triode valve V1 is normally limited, in a positive-going direction, to nearly cathode potential by virtue of the voltage drop, due to grid current, in a resistance R2 which connects the grid to the junction of two potential dividing resistances R1, R4 and the voltage drop across a resistance VR1. In a negative-going direction, however, the grid voltage can drop below the anode current cut-off value on account of the charge accumulating on a condenser C1 for a period during each input cycle dependent on the setting of the resistance VR1. If this resistance is at minimum setting the input voltage at the grid of the valve V1 is as shown at c, Fig. 2, where o indicates the anode current cut-off level of the valve V1. The anode voltage wave form comprises a series of steep-sided pulses d, negative-going and coinciding with the periods when the grid voltage wave form c is above cut-off level. These pulses are differentiated by a network C2, R5, Fig. 1, and the positive going pulses are suppressed by the diode sections of the valve V1. The resultant waveform, comprising a sharp negative-going pulse e, Fig. 2, is applied as a triggering voltage to the suppressor grid of a pentode V2, where each pulse initiates an anode cut-off period as in a transitron oscillator. The duration of the cutoff periods is determined by the time constant of a network C4, VR2, whereby a square-wave output f, Fig. 2, is available at the screen grid of the valve V2. The corresponding anode voltage is shown at g, Fig. 2, the linear falling portion s being employed as the time-base scanning potential. This is fed at desired amplitude from a resistance VR3 to the deflection plates X1, X2 of the cathode-ray tube CRT. The screen-grid wave form f is applied to the control grid c, g of the tube CRT in order to suppress the beam during the fly-back period. Under these conditions the scanning wave form period s (g, Fig. 2), coincides with the falling capacity sweep u, (b, Fig. 2), of the modulating condenser. If the resistance VR1 is at its maximum value the voltage drop of the input synchronizing voltage is such that the input wave form to the grid of the valve V1 is as shown at c1, Fig. 2, where the input voltage is below cut-off level o for only a short period per cycle. The corresponding anode output wave form is shown at d1 and comprises extended negative-going pulses whose leading edges have been phase-advanced with relation to those shown at d. The differentiated negative-going triggering pulses e1 and the square-wave pulses f1 of the screen-grid wave form are correspondingly advanced and result in the time-base wave form g1 being similarly phase-advanced so that the scanning period s is now coincident with the period v of the modulating condenser capacity variation b. Intermediate values of the resistance VR1 produce intermediate values of phase advancement so that this resistance serves as a phasing control. The ratio of the fly-back period may be varied by the resistance VR2. The display of all but one sweep per cycle of the synchronizing wave form may be suppressed by selecting either an increasing or decreasing capacity sweep and centring this by appropriate adjustment of the resistance VR1. A portion of a recurrent wave form of any character may similarly be chosen for display.