216I

PHILIPS TECHNICAL REVIEW Vol. 3, ,No. 7

AN APPARATUS FOR THE MEASUREMENT OF SCANNING SPEEDS OF CATHODERAY TUBES

general very greatly influenced by the potentialof the insulator which will change about equallywith the anode voltage. The secondary, electroncurrent which reaches the anode will therefore alsochange sharply with the anode voltage, and thismeans that the internal resistance, becomes con-siderably lower and the amplification of the high-frequency stage diminishes.

In order to avoid, these harmful effects hereagain several methods may be chosen. When, forexample, the disturbing phenomenum is -due toinsulating supports within the system, the mostpractical solution will be to cover them with asubstance whose secondary emission is less thanunity (for instance ~ layer of carbon). If the dan-ger spot is the inner side of the bulb, the jumpin potential can also be avoided byearthing thissurface through a capacity. In order ~o do this theoutside of the bulb may be covered with a layer

of metal which is kept.at a low potential (that ofthe cathode for instance). Finally lthe charging ofthe wall of the bulb may be avoided by entirely pre-venting electrons from leaving the electrode systemand reaching the glass wall. This ;may be achievedby means of shields. Fig. J2a shows a power am-plifier 'valve with a massive ail.od~ whose ends areclosed by caps which make it impossible for elec-trons to leave the anode. If an anode in the formof a gauze is used as is often necessary in order toimprove the heat radiation of the componentswithin the anode, this shielding must also. beextended behind. the anode. For this purpose awire gauze cage at, cathode potential is suitable.Electrons are unable to pass through this cage to

, the outside (fig. 12b). Shields with a low potentialmayalso be introduced in front of the muchused mica supports at the extremities of the systemin order to prevent their being struck by electrons.

by L. BLOK. 621.317.087: 621.317.755: 711.3

A simple apparatus is described for the determination of the greatest scanning speedof cathode ray tubes which can be photographed. 'The light spot on the screen is allowedto describe a logarithmic spiral, at every point of which the speed can easily he determined.The point ofleast intensity on the spiral which is still sufficiently visible in the photographdetermines the maximum scanning speed required. -

If, with the help of a cathode ray tube, phenom-ena are investigated which' are not periodic orstationary, but only occur once, it will in generalbe 'found practical to record the, phenomenonphotographically. In the most commonly occurringcases the details cannot be adequately studied byvisual observation.

As examples of phenomena which are investigatedin this way we may mention the switching on oftransformers, break-down in the testing of insu-lating materials, back-surge in rectifiers, atmosphericdischarges, etc.

In the photographic recording of such a phenom-cnon the blackening of the light-sensitive plate

I will depend on: 'a) the brightness B of the cathode ray light spot,h.) the diameter of the spot D,c) the velocity v at which the spot moves,d) the optical enlargement N,e) the light sensitivity 1] of the photographic

material.

A detailed treatment of the relation betweenblackening and the factors a) to e) has alreadybeen published in this periodical,1), together witha discussion of measurements relating to the max-imum scanning speed. As a control in the manufac-ture of cathode ray tubes it is desirable to be ableto determine this scanning speed' in a simple way.In the following a description is given of a meas-uring apparatus solved for this purpose.

Principle

In order to determine the scanning speed bymeans of a single photograph it is necessary toallow the spot to describe a path on which thevelocity changes continuously', while at every pointof the path the velocity must be known with suf-ficient accuracy. One may then determine on thephotograph the point where the path of the spot

1) J. F. H. Cu st er s, The recording of rapidly occurringphenomena with the aid of the cathode ray tube and thecamera, Philips techno Rev. 2, 148, 1937. .

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JULY 1938 SCANNING SPEEDS OF CATHODE RAY TUBES 217

Application of principle

ln order to find out how a spiral can he obtainedlc;}H::;\L:...jL-l---I----I----'-'rJlc_::=O on a cathode ray tube by an electrical method, we

pass over to rectangular coordinates.F?r this, purpose we take:

is still just sufficiently visible. The velocity at thatpoint is then 'the maximum scanning speed whichcan be photographed under the given conditions.

A very simple solution of the problem is obtained,when the light spot describes a logarithmic spiralwith a constant angular velocity.'This curve may he represented in polar coor-

dinates by the following equation:

(]= A e-arp

26902

Fig.:l. Logarithmic spiral: e = A e-a lp

The exponent of e is taken negative so that withincreasing argument cp the radius vector (J decreases(fig. 1). If ~is made linearly dependent on thetimeby setting

. cp = tot '.: . • . . • • • (2)

where w is a suitable angular veloeity we obtain:

(]= Ae-awt ••••••• (3)

when t = 0, (J = A, when t = 00, (J = O.The velocity at any point P of the spiral (v.

fig. 1) is give_n by:

v = Yv12 + V22,

where 'VI is the radial veloeity an~ V2 the velocityperpendicular to the radius vector. We now havethe relation:

d(JVI = - and

dt

dcpV2 = (J de = (J w •

From equation (3) we find that: .

d(JVi = - = - a w A «: a cot = - a w (J ,, dt

so that:

v = Ya2w2(]2 + (!2W2 , (! w ya2 + l' (4)

, If,a2 is made small with respect to unity, theradial velocity' VI .becomes small with respect tothe vèlocity V2' and the distance between successive'coils of the spiral 'becomes small with respect to,the radius (J. Then for the velocity V we obtainby approximation:

V=(!W , . . . " . . ,(5)

(1)

, .This expression gives 'the velocity of a point

which describes a eircle with an angular velocity to

and a radius (J. The approximation employedtherefore comes down to this, that the numherof windings of the spiral must he, so great -thatail element of the curve may he considered as anelement of a circle.

x (! cos cpy - (!sincp

Ae- a rpCOS cp ,- Aer:» rpsin cp •

With cp =Qt this becomes.

x A e-bt = wt' = A e-bt sin (wt ~ 'i)y A e-bt s~ tot ,

where aio is set equal to b. .x and y as functions of the time t have the form

of two damped oscillations which differ in phasehy 90°. Since the deflection of the spot on the screenis proportional to the potential difference betweenthe plates, it is sufficient to apply a dampedsinusoidal oscillation ~o each set of plates differingin phase by 90°.

Such a voltage is obt~ined in a simple mannerhy giving an LCR circuit an electrical impulse,and allowing it to die out in oscillations of its ö-i'vnfrequency according to the well-known equation:

R-2L'

Vt = Vo e cos on, .where Vo is the voltage, on the circuit due to theimpulse at t~e moment t . 0, Vi the voltage at thetime t, R the series resistance, L the self-inductanceof the circuit, Rj2L the damping factor and w =2:n; f, in which f is the frequency of the circuit it-self.,By applying this-voltage to the primary winding

P of a transformer which has' two separate equalsecondary'windings SI and S2 (fig. 2), ,two similarsecondary voltages are obtained of the same formas the primary voltage.

21'8 PHILIPS TECHNICAL REVIEW Vol. 3, No. 7

In order to obtain the desired phase difference,SI is loaded with the resistance Rl and the con-

p

26!J85

Fig. 2. Transformer with two similar symmetrically loadedsecondary windings. The voltages on Rl and Ca differ 90°in phase and are symmetrical with respect to earth.

densers Cl and C2, and S2 with R2' Ra and 'Ca,which are chosen so large that:

III-- + -- = Rl en R2 + Ra = -C 'cv Cl cv C2 cv a,

difference may he compensated and the spiralbecomes "circular".

,Description of the circnit

The diagram of the circuit may be seen in fig. 3.•The positive pole of the high voltage apparatuswhich supplies the anode voltage is earthed. Whenthe switch SI is in position 1, with the help ofpotenti?meter R4 the voltage of the grid g of thecathode ray tube can, be ,regulated, and therebythe strength of the electron current. When SI is inposition 2, g becomes about 200 volts mort' negativethan the cathode k and the current of electrons isthereby suppressed.

With switch S2 in position 1, on changing SIfrom 2 to 1, the adjusted electron current beginsto flow and a voltage impulse occurs on the gridof valve Ll' The oscillating circuit LRC therebyreceives an impulse so that on the primarywinding of the transformer in the' anode circuitof L2' the desired damped sinusoidal oscillation

26!JB4

Fig. 3. Circuit,diagram for the determinatioiI of the scanning speed. Ll is a, pentode AF 7, L2 a pentode AL 5.

while in additi,on it is provided that:

RI =' 2 R2 • == 2 Ra and

Cl = C2 = 2 Ca'

In. this way the voltage VRI on RI is made equalto the voltage VCaoh Ca' while both are symmetrîcalwitli\espect to earth and, moreover, the phasedifference between VR{ and VCa is exactly 90°:With a cathode ray tube the ~roportionality

factor between voltage and deflection is not thesame for both pairs' of deflection plates. The spiralobtained will therefore be ."oval". By applying thevoltage VRI which can be regulated with thepotentiometer Ro to the more sensit~ve plates, this

occurs. There is a potentialof more than 2 000 voltson condenser Co which must be constructedaccordingly. .

For good adjustment of the' camera it is desir-able that it be focussed upon the' spiral itself.For this purpose it is also necessary to be able toobtain a permanent image of the spiral on the screenof the cathode ray tube. To do this, a sawtoothvoltage generator e(t) is" connected over the con-denser C' by means of switch S2 in position '2.When R' and C' are correctly chosen, this sawtoothvoltage e(t) gives periodic impulses to Ll witha frequency depending on the frequency of thesawtooth voltage. For the circuit in wliich thisvoltage works th~ following holds:

e(t) = VC' + VR',

in which VC' is the voltage on the condenser C'and VR' that on R'.

Upon substituting VC' = f..!:__ dt wherè i is th'eC' ,"

;~ ,

JULY 1938 SCANNING SPEEDS OF CATHODE RAY TUBES 219

prevailing current and i = VR'jR', we may write:

j. VR'e(t) = R'C' dt + VR',

d e(t) VR' ddt R'C' + dt VR'·

or, differentiating,

If C'R' is sufficiently small, the first term of theright hand member of the equation dominates,and we may safely write the voltage derivationIII approximation:

de(t) VR'dt R'C'

The voltage on R' and therefore supplied to thegrid of Lt is proportional to the derivative of thevoltage e(t) with respect to time.

e(t)

al~d e (t)_ .YE'_dt - R'C'

bI j \1 \r-

~.~26:365

Fig. 4,. Form of the sawtooth voltage e(t) and of its derivationdel dt as a function of the time.

The form of the voltage e(t) as a function of thetime is indicated in jig. 4a, while in fig. 4b the va-riation of VR' as a function of t is drawn for suf-ficiently small values of C'R'. The latter figureshows plainly the occurrence of periodic impulses.Each of these impulses causes the spiral to bedescribed once. In this way a permanent imageappears on thc screen of the tube. If the timebetween two impulses is malie shorter than thatnecessary for the almost complete fading of theoscillations of the circuit, the light spot can nolonger reach its point of rest. It then does notform a point in the center, but ends at an arbitrarypoint on one of the windings. An interestingeffect is obtained when the frequency of the im··pulses is continously increased. The spiral is thenseen to be erased from the centre outwards.In recording a spiral the method is as follows.

SI is set in position 1, S2in position 2 and the camerais focussed on the spiral on the screen. After thelens has been closed the plate or film is introduced.SI is then set on 2 and S2 on 1, thc shatter isopened and SI switched back to 1, which causes the

spiral to be described once on the screen. Then SIis returned to position 2. Fig. 5 shows a photographtaken in the way described with a Philips cathoderay tube DG 16 with a green-fluorescing screen.The scanning velocity reached amounts to (for-mula (5)): v = e . 2 n f = 3.5 X 2 n X 1000 =22000 cmjs = 220 mjs. If the photograph is takensmaller than the true size of the image on the screen,for instance six times reduced, then, ceteris paribus,the maximum scanning speed which may bephotographed is about three times as great as ina photograph in natural size.In measurements carried out on this principle

the maximum scanning speed can be determinedas a function of the anode voltage and the electroncurrent with all types of tubes. Moreover, thismethod presents a means on the one hand ofchoosing the most sensitive photographic materialin connection with a given fluorescent substanceon the screen of the cathode ray tube, and on theother hand of comparing the quality of differentfluorescing substances.

26986.

Fig. 5. Photograph for the determination of the scanningspeed of a cathode ray tube DG 16. Electron current 50 [LA,anode voltage 2 000 volts, ratio of size of photograph to object1 : 1, aperture of lens 1 : 6.3, Agfa Isochrom plate.w = 2 n. f = 2:n 1 000. The spiral is still just visible for(! = 3.5 cm. Maximum scanning speed 220 m/so The enlarge-ment of the reproduction is 1 : 3.