198 PHILIPS TECHNICAL REVIEW Vol. 4, No. 7

A CATHODE RAY OSCILLOGRAPH

by J. D. VEEGENS.-

621.317.755

Description of the portable cathode ray oscillograph GM 3152. In this article if is chiefly, the improvements which have been made on the apparatus compared with the previouslydescribed 1) apparatus GM3150which are discussed. Special attention is paid to the focussingand deflecting system of the cathode ray tube, and to the frequency characteristic of theamplifier. In conclusion the possibility of observing or photographing single phenomenaof short duration is discussed.

stee«Ing. 1. The electrode system of the cathode ray oscillograph GM 3152. k cathode, g controlelectrode, al anode with voltage of 275 volts, a2 anode with voltage of 1 000 volts.

Introduetion

An oscillograph is a suitable instrument forrecording or making visible rapidly occurringphenomena: Jf no frequencies greater than' abont5000 cyclesjseé occur in the phenomenon, mecha-nical or mechanical-electrical instrume;nts may beused, such as the loop oscillograph, With higherfrequencies entirely or partially: mechanical os-cillographs are unsuitable because of thc inertiaof their moving parts, and a purely electrical in-strument is to be preferred. In the latter case thecathode ray oscillograph is indicated.

kg

::=:::;L.===.l LH-------l

The modern cathode ray tube makes it possible.to construct oscillographs which are cheap, portable

. ,and easy to operate. For these reasons these in-struments are being employed in cases' where theyare not strictly necessary from the point of viewof the frequencies to be reproduced, In connectionwith the very varied possibilities of application ofthe cathode ray oscillograph 2) efforts were madetó develop an apparatus suitable for, producingoscillograms of voltages of very varied magnitudesand with very varied frequencies.Several years ago such a universal cathode ray

oscillograph was described in this periodical.That apparatus has meanwhile been considerablyimproved; the new type, GM 3152, with built-in

1) Philips techno Rev. 1, 147, 1936.2) A series of applioations in electrical and radio engineering

was discussed earlier in this periodical, sec Philips techn.Rev. 3, pp. 50,148,248,339,1938; 4,90,217,1939.

amplifier, gives undistorted oscillograms with twiceas high frequencies, and is, moreover, more 'sen-sitive than the older type. This new type has beenin regular use in this laboratory and investigationsperformed with it have been repeatedly publishedin this review, A photograph of the apparatuscan be found on page 206, of this number andoscillograms were given on page 171 and 172 ofNo.6.

In the following we shall describe the newcathode ray oscillograph.. with sp~cial emphasis

on the parts which were discussed in .less detailin the description of the older type .

The cathode ray tube

The cathode ray tube DN 9-3 used in the oscil-lograph GM 3152 is of the high vacuum type.Cathode ray tubes filled with gas are not suitedfor following very rapid changes in voltage becauseof the inertia of th~ ions.

Fig. 1 shows diagrammatically the cathode raytube used. The electrons which leave the cathode kpass through an opening in the control electrode g,which has a negative potential with respect to thecathode. By changing this' potential the intensityof the electron current can be adjusted as desired.The electron beam transmitted is accelerated by'the positive potentialof the following electredesa'l and a2 and focussed to a narrow ray.

This focussing is the result mainly of the electricfield between. the cylindrical anodes al and a2,

>.

JULY 1939 À CATHODE RAY OSCILLOGRAPH 199

The anode al is at a potentialof about 275 volts,while a2 has a potentialof 1 000 volts. .

In jig. 2 the field lines between these anodesare drawn, The arrows point in the direction fromlower toward higher potential, i.e. in the directionof the force acting on the electrons. It may be seenfrom the figure that the force in the cylinder withlower potential acts toward the axis, and thereforeexerts a focussing action on the beam.

~- - _'_'--'-'_'-'-

..~

Fig. 2. Field lines between ~he electrodes al and a~·.This fieldacts to concentrate the electron beam.

The focussed electron beam passes through thespace between the deflection plates Dl and thenthrough that between the' deflection plates D2 andits direction can be changed by means of voltageson these plates. The set of plates Dl is connectedto the amplifier of the oscillograph and gives adeflection in the vertical direction which is propor-tional to the voltage to be investigated, the setof plates D2 is connected to the so-called timebase and gives -a deflection in the horizontal direc-tion .

. Distortions of ihe image

, It is very important that the deflection in thevertical direction should be proportional to thevoltage on the first set of deflection plates, and thatit should not depend upon the voltage on the secondpair of deflection plates which give the horizontaldeflection.If one plate of the first pair were earthed and

a sinusoidal, alternating ,voltage applied to theother plate (see jig. 3a), the deflection as a function

.,

iL____. ----IT -%a -.,.,. ~/600 .

Fig. 3. a) Asymmetrical set of deflection plates. With apositive voltage on the non-earthed plate the elec-.tron beam-is less deflected than with an equallylarge negative voltage.

b) The asymmetrical oscillogram of a sine curve.

of time would not be sinusoïdal. .In the positivehalf of the period the potential on the path of theray is higher than in the negative half. Thereforeduring the positive half period the ray moves morequickly through the pair of plates 'and it is less

. deflected than in the negative half period. The

. nature of the distortion producèd is shown in fig. 3b.This distortion can be removed by connecting

the two plates in balance, so that the one plate isalways as negative as the other is positive. Thepotenrial on the path of the ray then remainspractically constant.

When this measure has been taken and a voltageis then also applied to the second pair of deflectionplates, with here again one plate earthed, a newdistortion is found to occur which is ~hown injig. 4a-d. In this figure a ;represents the variationof the voltage between the. plates of the firstpair, b the .variation of the voltage of the non-earthed plate of the second pair, c the 'oscillogramwhich would be obtained with no distortion ànd dthe. oscillogram which is actually obtained. The dis-tortion may be described by saying that a rect-angular oscillogram becomes t.rapeaium-ehaped,and it is called "trapezium distortion".

c d 3'/601

Fig. 4. Trapezium distortion: a) variation of. the voltage onthe first set of deflection plates. b) variation of the, voltageon the second set of deflection plates. c) the oscillogramwhichwould be obtained in the absence of distortion. d) the oscillo-gram obtained with trapezium distortion.

Trapezium distortion indicates that a posrtrvevoltage on the non-earthed plate .of;the second pairdecreases the sensitivity of the first pair. This iseasily understood when it is kept in mind that thesecond system is not in' balanced connection, sothat the potential on the point where the rayenters the system fluctuates in the same rhythm .as the voltage on the non-earthed plate. When, forexample, the latter voltage is positive, the elec-.trons which leave the first set of deflection plates areattracted by the second set and are accelerated

200 PHILIPS TECHNICAL REVIEW Vol. 4, No. 7

thereby in an axial direction. The angle of verticaldeflection is hereby reduced (see fig. 5) and thismeans that the vertical deviation on the screenalso becomes smaller.

Df

S1602,

Fig. 5. Explanation of trapezium distortion. Between thesecond set of deflection plates the electron beam passesthrough a space with positive potential. It is therefore ac-celerated in an axial direction so that the angle of verticaldeflection becomes smaller.

Trapezium distortion can be compensated forby introducing an auxiliary electrode between thetwo deflection systems, which is connected tothe non-earthed plate of the second pair, and soconstructed that it increases the deflection in thevertical direction when it is at a positive poten-tial. Fig. 6 shows a very simple solution of thisproblem as applied in the tube DN 9-3. The auxi-liary electrode consists of two wires which arewelded to the nonearthed plate of the secondpair, and which run parallel to the plates of thefirst pair. When the potentialof this auxiliaryelectrode is positive, it will actually increase thedeflection in the vertical direction.· A ray whichhas an upward deflection is attracted by the up'perwire and thereby deflected more strongly in anupward direction; a ray which has a downwarddeflection is attracted by the lower wire and its ,deflection is thus also increased.

'U60S

Fig. 6. Compensation of trapezium distortion. An auxiliaryelectrode consisting of two wires is connected to the non-:earthed plate of the second set. When this plate is positivethe vertical deflection 'is increased by the auxiliary electrode,and the trapezium distortion is thereby compensated, Thefigure shows the deflection of the ray very much exaggerated.

Sensitivity

The sensitivity of the deflection system, i.e.the deflection per unit of voltage between thedeflection plates, can be calculated by the formula

, L lG_' 1/2 d Va'

where l is the length of the deflection plates, d thedistance between them, and L the distance betweenthe plates and the screen, while Va represents thepotential in the space between the deflection plates,calculated with respect to the' cathode. Greatersensitivity could therefore he attained by loweringthe anode voltage Va. However, the ,brightness. and sharpness of thè fluorescent spot would atthe same time be reduced. A compromise musttherefore he found; and an anode voltage of 1000volts was chosen. With this voltage the sensitivityof the first set of deflection plates is 0.4 mmjvolt,and that ofthe second set 0.3mm/volt. A sinusoïdaloscillogram with a total height of 1 cm thereforerequires a vertical deflection voltage of about 9Veff.

Ray modulationIn addition to a deflection in the horizontal and

vertical direction the cathode ray possesses anothermode of variation, namely a variation in intensity.In order to influence the intensity of the cathoderay, the cathode and control electrode of thecathoderay tube are connected via a switch' with connec-tion termináls mounted at the back of the oscillo-graph. If for example it is desired to entirelysuppress. the ray current, the control electrodemust be made about 40 volts negative with respectto the cathode.'

A possible application of ray modulation is theperiodic suppression of the ray during the recordingof an oscillogram with a knowri high frequency, sothat a series of dots appears on the screen insteadof a line. The number of dots between two pointson the oscillogram is an accurate measure of thetime interval.'The time intervals could be' determined of course

more simply from the distance between two pointson the oscillogram and the known frequency of thetime base. Such a determination is often howeverless accurate because the frequency of the time .base is not precisely known, especially when thetime base is synchronized with the' unknoWn, phenomenon.

The amplification

As stated above, a symmetrical voltage is appliedto the plates for vertical deflection of the cathoderay. In order to obtain this, the voltage ofthe signalto be recorded is amplified by means of a push-pullamplifier. Very high requirements must he madeof this amplifier with respect to the frequency rangeto be amplified and the freedom from distortion.As for the latter, it is not sufficient that a sinusoïdalinput signalof an arbitrary frequency should give

JULY 1939 A CATHODE RAY OSCILLOGRAPH

rise to a sinusoïdal putput signal, but the time lag obtain a lower limit of 10 cycles 4) by making Ckof.the output signal with respect to the input signal sufficiently large.'must also be small or at least independent of the If the circuit were built up exactly as shown infrequency. fig. 7, the amplification would be constant up toA constant amplification over a large frequency indefinitely high frequencies. Actually the ampli-

range can only be attai~ed by choosing the coupling fication decreases for high frequencies due to theunits such that the degree of amplification per stage harmful capacity Cs which is the' sum of thebecomes relatively low. On the other hand the am- capacities of anode, grid, connections and couplingplification per stage may not be too low, since other- units with respect to earth. It is scarcely possible

. . .wise a large n~ber of stages would be needed in to reduce Cs to less than 15 to 20 [L[LF;in practicalorder to reach the required amplification, which cases values two or three times as high mustis not only undesirable for economic reasons, but usually be counted on.to which there are also technical objections. In The equationthe first place the total degree of amplification would

. . 2 n 12 Rp Cs = 1then depend to a greater extent on the mains .voltage, and in the second place. the noise of, the determines the frequency f2' above' ~hieh the am-amplifier increases with the number of amplifier plification is lower than ~ of the maximum value.valves 3). The amplified range of frequencies thus extendsBy using amplifier valves with a steep slope, an f 0 ç to çr m Jl J2·

amplification of 1500 times can be obtained with It is possible to attempt to compensate theonly two stages of resistance amplification, and influence of the harmful capacity by introducingthe amplification is constant in the frequency range a combination of self-inductions, resistances and,between ten and one million cycles. capacities instead of the coupling resistance Rp,

which combination together with the capacity Csin parallel with it, forms an impedance which isindependent of the frequency,

3'1604

Fig. 7. Diagram of the circuit of a resistance amplifier. Inaddition to the rcsistances Rp' and Rg and the condenser Ck.inevitable harmful capacities Cp and Cg are present in thecircuit.

In order to attain this satisfactory result, specialattention must be given to the coupling units whichare introduced between. the two stages of ampli-fication, and between the second stage and thedeflection plates. In fig. 7 a diagram is given ofthe circuit of a resistance. amplifier, The variationsof the anode current of the first valve cause voltagevariations on the resistance Rp which are fed tothe grid of the following valve via the condenserCk. At low frequencies the amplification will de-crease because the impedance of the condenser Ckincreases and is finally no longer small compared'with Rg. When

2 nit Ck Rg = 1,

the amplification is exactly f!ï;. = 72 per cent, andthis may be considered as the lower limit of thefrequency band amplified. It is not possible to

3) See in this connection the article by M. Ziegler, Philipstechno Rev. 2, 136, 329, 1937.

Cs Cs

II 3'1605

Fig. 8. Composite impedances whose value is practicallyconstant as a function of the frequency up to a limitingfrequency !which is given by 2 n] ~ 2JRp Cs' The connection11gives a still better compensation than I.

o ' ~ •

Several examples of such combinations are' givenin fig. 8: Thè~e have approximatelyrthe desiredproperties within a definite frequency range. Thefrequency characteristic of the amplifier, comparedto that of an amplifier with resistance coupling, ishereby extended toward higher frequencies, asmay' be seen from the curves' of fig. 9, where theamplification of two stages in cascade connectionis plotted:a) for the case of pure resistance amplification,b) 'after introduetion of the compensation connec-

tions according to fig. 6.The anode resistance Rp was chosen the same in

both cases, while as amplifier valves pentodes

4) If it is necessary to amplify very much lower frequencies,the coupling condensers are best avoided, and complicatedconnections result for the supply arrangement.

201

202 PHILIPS TECHNICAL REVIEW Vol. 4" No. 7

I I 11V r-,

1\\ bé" \

1\ -" I

\

were used with a slope of about 5 mA/volt and aninternal resistance of more than 1.5 X 106 n, It'may he seen from the characteristics that the fre-

quency at which the amplification has fallen to Vl/2is increased from 400 kilocycles to 1050 kilocyclesby the appli<:ation of the compensation 'connections.

Circuit of the ~mplifier

In jig. 10 the complete circuit of 'the amplifier, is given. The last two of the three amplifier valvesBl' B2' B3 are in push-pull connection and eachis connected with one of the .defleotion plates ofthe pair Dl" The anode impedance of the first am-plifier valve is made up according the scheme Iof fig. 8, while the anode impedances of the valves'B2 and B3 a~e made up according to the scheme 11,

%110

100

90

80

70

60

50

40

sa20

10

ola 100 1000Hz

be put 'entirely out of action, if the intensity of thesignal to he recorded permits. In position I thesignal is fed directly to the grid of the first valve.In position IJ the signal current, besides flowingthrough the input resistan<:e of the first valve, alsoflows through a series resistance Rs; the input im-pedance is thereforè higher, but the sensitivity isless. In position III the signal voltage is applieddirectly to the deflection plates. In the table belowthe sensitivities and the input impedances are givenfor the three. different switch positions.

The time axis voltage

The signal voltage to be recorded gives the elec-tron ray a deflection in the vertical direction. Inorder to obtain an oscillogram of this voltage the

31606

10kHz 100 1000kHz

Fig. 9. Frequency characteristic ofthe amplifier for the cathode ray oscillograph GM 3152.a) with resistances as coupling units 'b) when the anode resistances of the first and second stages are replaced by circuits

according to fig. 8 I and 11, respectively.

which permits a compensation of still greater harm-ful capacities. , .' ,By means of the switches Sl' S2' .$3 and S'l ~hich

are coupled mechanically with each other, the am-plification can be diminished or the amplifier', may

Minimum *) voltagenecessary for an image 1 cm'

Position of high Imputswitch

Direct ,Effective impedance

voltage alternating. voltage

I 17 mV 6 mV 10 000 0-10G 0*11 280 mV 100 mV 1700000,III 25 mV 8.8 mV 12 (L(LF

'*) The sensitivity can in the first-two 'positions be 'regulatedby means of a potentiometer Pbetween zero and a. max-imum value. In the first position it is possible to disconnectthe potentiometer; the possibility of regulating the sizeof image on the screen is then lost, but a very high inputimpedance is obtained.

electron ray must also be given a horizontal de-flection depending upon the time.'If the .signal voltage is sinusoidal the frequency .

and phase of the signal can be investigated by ap-plying a sinusoidal voltage along the time axis also.In this way Lis saj ous figures 5) are obtained,which make it possible to deduce the ratio' of fre-quencies and the, mutual phase relation of signaland time axis voltage.

It is often desired' to obtain an image whichrepresents the voltage to he investigated as a func-tion of the time. In such a case a time axis voltageis best used 'which has a sawtooth form as a functionof time, namely a voltage which increases linearlywith the time over a certain interval and thensuddenly drops to its initial value.

The oscillograph GM 3152 is provided with' a

5) See Philips techno Rev. 3, 342, 1938.

JULY 1939 A CATHODE RAY OSCILLOGRAPH 203

time base apparatus which gives a sawtoothedvoltage (see jig. 11) whose frequency can be ad-justed between 2 cycles and 150 kilocycles. Abuilt-in adjustable synchronization arrangementprovides that in the recording of periodic phenomena

oscillograph it is unnecessary to record the os-cillogram in a permanent form, it is sufficient toobserve it visually on the screen of the cathoderay tube. The observation of single,' i.e. non-periodic, phenomena is facilitated by the fact that

+ L-~ ~ __ +- ~

Fig. 10. Complete circuit of the amplifier of the cathode ray oscillograph GM 3152. Maxi-mum amplification 1500 times for frequencies from 10 to 106 cycles. The amplifier valvesB2 and B3 are in push-pull connection in order to obtain a symmetrical deflection of theelectron beam. SI to S4 are switches for regulating the sensitivity in three stages; Ppotentiometer for continuous regulation, S5 switch by means of which the potentiometercan be disconnected in order to obtain not only maximum sensitivity but also a veryhigh input impedance.

the time base period always corresponds to a def-inite multiple of the period of the phenomenonrecorded, so that the image on the screen becomesstationary.

In addition to the voltage of the built-in saw-'tooth generator, an external voltage mayalso heapplied to the horizontally deflecting plates, forexample the voltage of the alternating currentmain. It is possible furthermore to let the synchro-nization of the sawtooth generator he carried out,not by the signal voltage, but by the alternatingcurrent main or by an external voltage. The dif-ferent possibilities of synchronization and sup-plying of time axis voltages can be combined witheach other at will simply by turning a switch intodifferent positions.The construction and the different switching

possibilities of the time base are not appreciablydifferent from those in the earlier model which isdiscussed in detail in the article cited in footnote 2),and we shall not therefore discuss them again atthis point.

The observation of the oscillogram

For most of the applications of the cathode ray

+

31607

a substance is used as fluorescent material on thescreen which possesses phosphorescent properties.The practical time of phosphorescence, i;e. the

time during which the curve can be seen on thescreen, depends upon the original intensity, andthus decreases with increasing writing speed. Injig. 12 the experimentally found relation betweenphosphorescence time and writing speed is given.Since it was found in these experiments that an

31787

Fig. 11. Oscillogram of the shape of the time base voltage.The discharging time is 1/20 to 1/5 of the charging time.

204 PHILIPS TECHNICAL REVIEW Vol. 4, No. 7

observation time of 1 sec is necessary to judge anoscillogram, a maximum writing speed of 1.5 km/secwas deduced. When the total length of the curveon the oscillogram amounts for instance to 2 cm,the total duration must be 1.3 X 10-5 sec.If it is desired to record a phenomenon of still

shorter duration, the image must be photographed.A special stand has been constructed for thispurpose which makes the adjustment of the cameravery much easier (see jig. 13). Panchromatic ma-terial which is still sensitive to the yellow-greenlight of the screen is best used.

The attainable writing speed depends not onlyon the camera and the photographic emulsion, but

secf2

S1623

4

\1\'\ ,,",0

<,-,['-.._

<,

---r--r--r--

la

8

6

2

oo 0,2 0,4 0,6 0,8 f,O 1,2 f,4 1,b f,S 2,0 krnfecc

Fig. 12. Time during which the phosphorescent image is ob-servable on the screen of the cathode ray tube as a functionof the writing speed. If it is assumed tbat the image must beseen for 1 sec in order to observe its most important properties,a maximum writing speed of 1.5 km/sec is arrived at.

also on the intensity of the ray current. It is per-missible to use a higher ray current than normalduring the exposure. For this purpose the negativevoltage of the control electrode may be changed

Fig. 13. The cathode ray oscillograph GM 3152 with the foldingstand GM 4192 for setting up a camera.

by means of a battery with switch connected ex-ternally at the rear of the apparatus. There is,however, the disadvantage that the bright linebecomes thicker with a high ray current, so thatdetails of the image are not brought out so well.

In practical cases with a lens aperture of 1 : 218°

and a film sensitivity of - DIN a writing speed10

of about 2.5 km/sec can be reached, and singlephenomena can thus be recorded with a durationof about 10-5 sec.