US2438420A - Calibration of cathode-ray oscillographs - Google Patents

Description

Patented Mar. 23, 1948 ream CALIBRATION OF CATHODE-RAY OSCILLOGRAPHS Graham John. Scoles, East Sheen,

Charles Walter Miller, to Metropolitan-Woke Limited, London, Britain London, and

Sale, England, assignors rs Electrical Company England, a company of Great Application October 9, 1943, Serial No. 505,710 In Great Britain September 18, 1941 Section 1, Public Law 690, August 8, 1946 Patent expires September 18, 1961 1 Claim. 1 This invention relates to the calibration of time bases produced on the screens of cathode ray tubes for investigating phenomena, more particularly phenomena occurring at high frequency of some kilocycles or some megacycles, the time base itself being provided magnetically or electrostatically at a frequency not less than that for persistence of vision while the calibration is obtained by blocking the cathode ray beam at regular intervals (appropriate to the nature of the phenomena to be observedon the cathoder'ay screen) whereby the visual time base is interrupted by a series of dark spaces, and the invention has for its object toprovide improved means for obtaining more accurate and clearer calibration of time bases, giving at the same time a clear indication of the phenomena under investigation.

Heretofore such calibration has been effected to some extent satisfactorily by the application of a voltage of fixed periodicity which may conveniently be produced by What maybe called shock excitation, that is, the application of regularly spaced voltage pulses of steep wave front to a resonant circuit of high decrement, the resulting sinusoidal, decaying voltage wave of higher frequency being applied to the grid r cathode of the cathode ray tube. By steep wave-front it is meant that the rate of rise with time is greater than sinusoidal. Itis essential of course that the frequency of this applied calibrating voltage should be locked in any well known way in respect of the recurrence frequency of the time base, and one method that has been used involves the application of the steep wave front calibrating voltage at or shortly before the commencement of the time base sweep of the cathode ray beam, this steep wave front voltage having been fed to said oscillatory circuit causing the latter to ring, the damped substantially sine wave voltage thus produced being applied to the grid or cathode of the cathode ray tube. This method, however, has the disadvantagethat the brightness modulation along the time base is substantially sinusoidal so that accurate calibration of the time base is diflicult, if not impossible, whilst, furthermore, the depth of the brightness modulation and the rate of decay of the oscillation are mutually dependent and consequently if it is arranged to produce good definition at the commencement of the time base the definition deteriorates rapidly along the time base, Conversely if it is arranged that the deterioration of definition is small then the definition tends to be unsatisfactory throughout.

The present invention provides means whereby these disadvantages are avoided or minimised.

According to the present invention a resonant circuit is provided which is made to ring as heretofore, but this circuit is constructed to have a low decrement, and in practice this ingeneral will require a high capacity inductance ratio, or to include a piezo crystal, whereby to provide the train of effectively undamped sinusoidal calibrating waves for each period of the time base sweep which waves are fed to the control grid of an amplifier valve having an anode circuit of high reactance/resistance ratio (Q) Or equivalently a piezo crystal filter, each being per se of common knowledge. The output from this valve may be amplified or further amplified and also it may be shaped if desired such as by another valve whichprovides, from its sine wave input, pulses of short duration which are applied to the grid or cathode of the cathode ray tube producing, when applied to the grid thereof, a series of short dark spaces along the length of the time base, or when applied to the cathode of the oathode ray tube, a series of short bright spaces along its length, the visual efiect being, of course, much the same, but much superior to that obtained by the aforesaid known arrangement. The second valve may have associated with it means for adjusting the bias thereon whereby the ratio of dark and bright portions of the time base may be varied as desired.

The steep wave-front equally spaced exciting pulses are obtained from any per se known generator of such pulses, which generator may be the same one as that efiecting the time base sweeps.

The anode load of the aforesaid second valve may if desired have associated with it (conveniently to be shared as a common anode load) another valve, the grid of which is fed with equally spaced voltage waves or pulses such as to modulate out the return stroke of the cathode ray beam in a manner per so well known and sometimes re-- ferred to as return stroke beam trapping.

The arrangement according to the invention has been found to give in practice clear and accurate calibration of the time base, whilst the clearness of calibration is uniform throughout the length of the time base: the arrangement has exhibited the advantage that it can be used for quite high calibration frequencies as well as relatively low frequencies, for example 200 kilocycles per second and 2 megacycles per second, and even higher.

Whilst both grid and anode circuits of the first valve may each comprise a per so well known piezo crystal with a choke or resistor across it, it is preferred that according to one arrangement a crystal may be made a high Q and contains no crystal. In this case a "sub-standard calibration wave will be obcrystal filter (per se well known) used in place of the anode circuit of high Q, so that unless the grid circuit be tuned to ring at the pass frequency of the filter no calibration wave will be obtained, a wave of sub-standard accuracy being thus available.

Fig. 1 of the accompanying drawings is an electrical diagram illustrating the invention purely diagrammatically, and

Fig. 2 is a diagrammatic view of an addition which can be made to the circuit shown in Fig. 1 in accordance with a subsidiary feature of the invention.

Referring to Fig. 1, the valve VI, which is conveniently a pentode having a cathode c, grids g, g and g and anode a, as shown, has applied to its control grid g through the condenser Co regularly spaced steep-fronted voltage pulses derived from the output terminal of a generator of such pulses connected to the input terminal 1. The impulse generator may be in itself a well known arrangement. The impulses supplied to the terminal I may be those of the impulse generator which efiects the time base sweep of the cathode ray beam, or the impulses supplied to the terminal I may be derived from the time base sweep impulse generator in any convenient manner all as per se common knowledge.

The grid circuit of the valve V! comprises the inductance LI and condenser Cl, this circuit having-a low decrement and in general a high capacity/inductance ratio, or it may include a piezo crystal, as hereinbefore set forth, per se common knowledge. R is the usual cathode bias resistor bridged by the usual condenser Ca. Cl) is the usual condenser placed between accelerator grid 9 and cathode c. g is the usual suppressor grid anchored to the cathode. At 1' is shown a voltage-dropping resistance. The anode circuit L2, C2 connected to the anode a of the valve Vl has a high reactance/resistance ratio or Q, or, alternatively, it may be a piezo crystal filter. The output of the valve VI is applied to the control grid 9 of the valve V2, also conveniently a pentode and having a cathode c grids g and g and anodea by means of a coupling coil L3 of suitable size The valve V2 is normally biased beyond the anode current cut-off point by the resistance combination RI and R2 in series with the resistance R3, conveniently constituting a potential divider, so that the control grid of this valve requires a positive drive before current is passed in the form of the positive peaks of the sine wave drive.

The resistance RI is of high value, The resistance R2 is conveniently variableand in series with the switch S. A condenserC3 is connected across the resistances RI and R2 for smoothing the adjustable bias of the valve V2.

The valve V2 has the resistive anode load R4 across which will appear a series of short negative-going voltage pulses which are applied to to. ring in the grid cir-- cuit of the valve, the anode circuit of which has tained. Alternatively the crystal may be in a the grid G of the or alternatively to cathode ray tube as aforesaid, the cathode 0 thereof, this all being per se in accordance with common knowledge. 1" is another voltage-dropping resistance.

It will be appreciated that the variable resistance R2 enables the bias on the valve V2 to be varied and that by opening the switch S the valve V2 becomes biased so far beyond cut-ofi as to pass no current at all, thus removing the calibration marks from the time base.

The optional beam trapping circuit shown in Fig. 2 includes a third valve V3, also conveniently a pentode having a cathode c grids g", g and g and an anode a and having its anode directly connected to the output lead 2 of the valve V2 so that the anode load of valve V3 conveniently is the resistance R4 in the anode circuit of the valve V2. The voltage to produce this normal beam trap is applied to the input lead 3 of the grid 9'' of the valve V3.

We claim:

An electric circuit arrangement for calibrating the time base of a cathode ray tube having cathode and grid electrodes, and beam deflecting members to receive respectively pulses to obtain the time base sweep, and a voltage representative of the phenomena to be observed, comprising in combination, a first valve having an input grid circuit of low decrement, means for applying to said input grid circuit equally spaced steep wave-front voltage pulses in synchronism with said time base sweep pulses, and an anode circuit for said valve having a high reactance-resistance ratio and adapted to provide an output of effectively undamped sine Wave calibrating voltage of higher frequency than said time base sweep pulses, and means for connecting said anode circuit to one of said electrodes of the cathode ray tube for suppressing the beam thereof including a second valve having an input circuit connected to said anode circuit and an output circuit connected to said electrode of the cathode ray tube and providing, from its substantially sine wave input, voltage pulses of short duration which are applied to said electrode of the cathode ray tube, and a high cathode resistance and a variable resistance in parallel therewith for normally biasing said second valve beyond cut-off.

GRAHAM JOHN SCOLES. CHARLES WALTER MILLER.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 1,908,249 I-Iund May 9, 1933 2,103,090 Blebanski Dec. 21, 1937 2,121,359 Luck et a1 June 21, 1938 2,178,074 Jakel et a1 Oct. 31, 1939 2,232,044 Bedford Feb. 18, 1941 2,266,668 Tubbs Dec. 16, 1941 OTHER REFERENCES Sherman: Generation of synchronizing pulses by impulse excitation, proceedings of I. R. E., Sept. 1940-pp. 406 to 409. 250-363-132.

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