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A TWELVE-INCH MAGNEHC DEFLECTION
CATHODE-‘RAY OSCILLOSCOPE
FOR LECTURE DEMO?~J$TRAT!ON

‘ffiesis fer fiw Deana of M. S.
MICHEGAN STATE CQLLEGE

Kmnei‘h ‘Wiifiiam Saunders

“$948

*- __,~.— “.4.--

MICHIGAN

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. x
; This a to certify that the
4 I
,' thesis entitled
t
A MINI-INCH MAGT'ETIC DEFLECTION CATHOL‘E-RAY OSCILLOSOCPE
FOB LECTURE DEMONSTRATION
' '3 presented by
-‘ Kenneth William Saunder-
; has been accepted towards fulfillment
‘ of the requirement: for
' 11.8. degree in Physic-
1
Major prof
\\ ; Date 216! 1‘8
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PLACE IN RETURN BOX to remove this checkout from your record.
TO AVOID FINE retum on or before date due.
MAY BE RECALLED with earlier due date if requested.

DATE DUE DATE DUE DATE DUE

 

A TWELVE-INCH MAGNETIC DEFLECTION CATHODl‘p-PAY OSCILLOSCOPE

FOR LECTURE DEMONSTRATION

Kenneth William Seundere

A Thesis

Submitted. to the School of Graduate Studies of Michigan
State College of Agriculture and Applied Science
in partial fulfillment of the requiremente
for the degree of

MASTER 01 SCIENCE

Department of Phyeicl
19h:

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‘3

Acknowledgement

I went to expreee my appreciation to Dr. Ealph Bowereox
for hie instruction very patiently given during the couree of
this problem. Dr. Beryl Dickinson also willingly gave many

helpful euggestione during the early part of the project for

Zééauxzz/%%V£Z:;-~a&;4/’

which I am grateful.

1 989,55)

CONTENTS

I. Problems involved

II. Theoretical coneideratione

A. Magnetic focueing

3. Magnetic deflection

III. Detail: of construction

A.

I.

High voltage power eupply

local and deflection ooile

Sweep oscillator and deflection circuit
12 D P 7 tube in parallel with the 208-3
Dumont Oecilloecope

Low voltagejpover Inpply

IV. Performance of the 12" oecilloecope

1.

1. Problems Involved

Building a cathode-ray tube into an oscilloscope involves
the planning and construction of several auxillary circuits. These
circuits depend considerably on the characteristics of the tube to
be need. A.twe1ve inch tube needs six to seven thousand volts of
potential applied between the electron source and the high voltage
electrode of the tube-according to the manufacturerh ratings. A
high voltage power pack of low current output is required.

The tube may be made for electrostatic focusing or
magnetic focusing. If it is focused electrostatically. about twelve
to fourteen.hundred volts must be available from the high voltage
power pack to be applied to the focusing grid. If the tube is
focused magnetically a suitable coil for a magnetic lens must be
secured. The tube used in this problem is magnetically focused and
depends on magnetic deflection of the stream of electrons. Characteristics
of magnetic deflection coils available influence the type of deflection
circuit planned. If the coils are of lov'inductance a much larger current
is needed and. therefore, a transformer may be needed in both the horisontal
sweep amplifier and vertical amplifier plate circuits to supply the right
current to the defledtion coils. The coils used in this problem were of
high inductance and consequently relatively small currents were required.
Inductance of the deflection coils influences the form of the sweep circuit
wave. the external connections to the coil windings also must be considered.

If the two coils for any deflection, say horizontal. are in series and

are not center-tapped as W they cannot very well

2.
be connected in a.push§pull circuit which.wcu1d need a center
tap as (quTWFU‘hT-/WTFFTU]

A.horisontal sweep circuit must be constructed of such
wave form as to give constant current increase in the deflection
coil instead of constant voltage increase as is necessary in electro-
static deflection. Ihe sweep frequency for demonstration.purposes ought
to be variable from a few cycles per second to two or three thousand cps.
The seeep oscillator should be provided with some means of feeding into
its grid circuit some of the vertical signal so that the sweep frequency
may be stabalised ia.reference to the vertical frequency.

Two amplifiers. one for the vertical signal and one for the
horizontal sweep must be planned. They should have a fairly constant
gain over the band of frequencies for which the oscilloscope is intended.

Along with the fundamental problems already mentioned there
will be other miscellaneous problems encountered during experimentation
such as stray oscillations occuring. induced disturbances picked up in
leads. flybback oscillations occuring at the return of the sweep. and
blanking out of return sweep trace. All of these problems will be
discussed in the report of the actual construction of the scope. Before
this is done, a few theoretical considerations will be discussed concern-

ing magnetic focusing lens. and magnetic deflection.

II. IHEORETIGAL CONSIDERATIONS
1. Magnetic focusing lens
For an electron traveling in both an electrostatic field (-grad V)
and an electromagnetic field (H3cur1.d) in.which.a is a vector potential

and the electron.having a velocity. v. the following field and force

U
0

components can be expressed:

Components of F in cylindrical coordinates are:

Hr = Curer ”[31,: _. %_____(er0)]
Hz = CurIzA -[A(""t—Aa)’ 9542*?
H9=C0k19A—;(%—A%L" j’é‘z)
Because of s axial symmetry in H
Ar = Ax = ”jfia‘: 0—
Therefore, the components of H are
Hr -_-, -I/ £14.91
Hz= / :JCZAJ.
Hg = 0

Expression of force on the electron in the two fields:

“

F: Md =-a¢[;6rad v +(7xfi)
z=_fi_[-Gr¢=/V+(\7Xn)
F; (V9 Hz.- Vz Ho)
(4) (W9 == aw. H."V.H7.I
ECVr He "V9 Hr)
l'or acceleration:
4r=F-V'éz
qe=2Fé+r°é
411:2.

3.!
Jr

0) CnJVr- ‘J—‘L— 0

j: Ksince V is constant and

d

J 2 symmetrical with respect to

the a axis. Using these three sets of components. the component

i4.

expressions for acceleration are:

(Go) 4r=rarez=m ——-—- [—37. 3.! 4.. ‘e 3(__I:________.Ae)]
(53) 493% cyzzé) =——.—-—-[— ”a JCS 56.)] (consider i: o from

(6c) 42:.- i ~—-‘-‘-—[—:V +9 5193:1191] atobinfigure II)

92
a b
\ 3v 0:)
sq .4
flue—.1 —>-_—_T_ ===-h-
H
Flaunt H

integrating (6b)
r2 9 g: %(VAQ)+C

At r -_-, O, A“ 3 0 for an axially symmetric field. Therefore
(7) r b ° 34;} A9

 

substitutin (7) in (6a) ,1 ‘l
r_yW= .. .41. 3 '77 _. 3 V J
z )r 7" Q)- t-

D
I J(“L’ ”2 on.
(a) r=-/z a— —-;m,,—2 + a—g—i¢=a—§-[v-tetj
Substituting (7) in (Se)
.. z.
(9) 1' {7:};- [V-‘A‘i‘n" a]
From considerations of axial distribution of potential‘ and equations

(8) and (9) the differential equation for the trajectory of a paraxial

electron in a magnetic field is:

d‘ V +V__’__ J____r; __...— ” r=o
(I'm—— ale—T 2v Jz. ++V (V +T Hi)

If V is considered constant from a to b over the short path of
influence of B then equation (10) becomes
(11) i— " +——-—- ‘H: r: o

integrating eq. W<lll once between a and b on the s axis
J'- _ = ___/__, 1a nf‘ a
(12) J: )4 (Jz)b 9V ”2 J2.

*Ilectron Optics in Television by Maloff and Epstein. pgs. 76, 77.
and 1550

5.
r0 is substituted for r because for the short distance on s

for a thin lens r can be considered constant. Then 415;

J; (4) 1.
the slope of the incident electron and 33-:- (b) is the slope of the

refracted electron. These rays are labelled in this manner in
figure II. Equation (12) may be rewritten in terms of focal length of
the magnetic lens. 5
(13)J’=--——l-- " Vo/ sz‘lz
F ?V 7" 4
He depends upon the geometry of the coil. but can be quite easily

 

computed as a function of s. coil dimensions. number of turns. and
current. if there is no iron present in the core. From the two
distances. cathode to focus coil. and focus coil to screen. the
required focal length of the coil can be computed. Then. by the

use of equation (13). with the appropriate substitution for H? . the
coil dimensions and current may be varied until the correct focal
length is obtained. A.plot of focal length compared to current for
a short coil is given at

the right.

{(rocat leagth)

 

 

CUII'QD‘It through COlk

From equation (13) it is apparent that the lens will be
converging regardless of the direction of current through the focusing
coil. Reversal of current results only in reversal of direction of
rotation of the electron image. Since. in a scope. only a spot is

desired. direction and angle of rotation of the image are of no concern.

b.
For focusing. then. H is parallel to the general direction of the motion
of the electron whereas for deflection H is perpendicular to the electron
path. Since velocity (v.1 does not appear in eq. 12. the varying speed
of the proJected electrons from the cathode has nothing to do with where

they will come to focus.

3. Xagnetic Deflection
Considerations for deflection are much simpler than those
for a magnetic focusing lens. A stream of electrons passing through
a cross magnetic field will be deflected in the same manner as a

conductor is in a magnetic field

 

 

:r----~-,
/ Hy.
Ha

 

FIGURE IV

The force. 1' : Eh ev. which is applied to forcing the electron out

of its path will be resisted by centrifugal force. I a mvz .

Then Eh ev ; ave or f

f
(13}: H. = 3‘7?

e/mo is known to be about 1.758 x 107 emu/g and v can be computed

from the relation

(114) V8 :3 [/2 ”70 Va (The kinetic energy of the
electron is equal to the work done on the charge by the potential of
the prOpelling electric field) Relativity considerations of mass

change with velocity are not necessary for velocities encountered in

7.
cathode ray Oscilloscopes.* In these equations the rest mass (no)

is used. Substituting in (13)“ from (11+)

(15) Hit: ’4‘ ”zfiv

In a tube using 6000 volts on the second anode. V'would be
approximately 3000 volts at the point of deflection. From the geometry
of the tube f can be computed and. then also. the value of H from
equation (15) for full width deflection. The width of deflection. then.
according to equation (15). is proportional to the field. lb. of the
deflection coil and this in turn is directly'proportional to the current

in the coil. This current equation is given on page 15.

III. Details of Construction
‘A. High voltage Power Supply

The only requirements of the high voltage power supply are
that it will furnish the required high potential at low current. Most
cathode ray tubes require currents not over 2 or 3 milliamperes. The
transformer selected was rated at 3700 volts and n ma current. The
bsecondary also had two 2.5 volt windings. one of which was connected
to the high voltage winding at one end. The high.peak voltage ('/§VE)
is about 5200 volts. .After being filtered by a condenser it still
measured slightly over 5000 volts .

In the tube characteristics given for the 12' cathode ray
tube it was stated that the operating high voltage should be between
six and seven thousand volts. Operation at a lower voltage gives more
deflection sensitivity but less brilliance on the fluorescent screen.

At 5000 volts. control of brilliance was less sensitive and the trace

could easily be made bright enough.
' Television by Zworykin and Mortonp- pg. 77

8.

The rectifier selected was a Ratheon CEP-3B2h tube. which
is rated at 20 kv. peak inverse voltage. 'Half of the heater can be
operated at 2.5 volts or all of it at 5 volts. When the direction of
M reverses in the secondary of the transformer after the condenser
has been charged through the rectifier tube this tube has both the
voltage of the condenser and the transformer added in series across
it. Therefore. the tube must be rated to stand twice the peak voltage
of the transformer. The circuit for this power pack is shown in figure
V on the next page.

B. Focus and Deflection Coils

At the present time not much television equipment is available
and for that reason suitable focus and deflection coils may be scarce.
The housing. focus coil. and deflection coils for a 5-IP-7 radar tube
were salvaged from the no. 1d-l9/APS-3 viewer of the AN/APS-3 aircraft
radar equipment. In the detailed drawing. figure VI. page 10, are given
the circuit with.pin connection letters and circuit constants for the
convenience of anyone wishing to utilize similar equipment. Also. cor-
responding numbers of’pins on the socket of the cable which was spliced
onto these leads is given. This thirteen element cable socket plugs into

the back of the chasis which mounts the electronic elements of control.

 

 

LOW VOLTAGE POWER SUPPLY

 

 

5v. center iop

 

 

 

 

   
   
 

 
 

 

 

r’
i2 h. 8+
LW—L 4 350'.
b 3011f. __30uf. 250v.
504° focusing coil
iOK
»/
:‘P focus
control
f 2.5K
iiii n
say. ioIZDPT heater r<-—--b' cgn‘ifoi
» io coihods
Sumo. fuse i 2.5 K

 

 

 

 

 

 

[Epics voltage swiich
)Somp. fuse .

L high voltage switch

\Piug oi heck oi chisis _
_..,i-iiGi-i VOLTAGE POWERflGK
500014 high voltage
' grid of IZDP‘I

 

 

 

 

 

 

 

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10.

fl/lwrizoniol deflection coils

 

 

 

 

 

   
 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

7 Aj 400‘“ coihode ray tube
l3 8 aggoen. 1/
c::
O Pjfiqh
|| 0 = 720.“ volioge
l— 5‘: eriicoi
\deflecilon coils
anoA
9 G = —Lf ff
2 c: . lOOOO-n-
J
= ‘ e gfocus coil
3 K 1-*-' ‘r_i
6 N =
'2 P: Winn terminous of cable "A-B”
i L from pilots indicator of
AN/AP-3 aircraft radar set

numbers on pins of socket
at end of cable spliced to the
leads from this equipment

l'igure VI
The housing of this unit was used along with the focus

and deflection coils, cable and cathode ray tube socket. Some
alterations had to be made in the flare and of the aluminum housing
to allow the larger 12“ tube to fit into the coils far enough. But
making use of this housing and accessories saved quite a lot of time
in coil mounting and cable constructions. The 10000 ohm variable
carbon resistor used in controlling focus was removed from the (k)
lead between.points a and b and placed in series with the focusing

coil as shown in figure 7. Points a and b were then connected.

ll.
The relative positions of tube and coils is given in

figure VII below.

/ deflection coil

AMSCI’OO n

  
 

{1:1

 

 

 

iocus coil

Figure VII

0. Sweep Oscillator and Deflection Circuits
The frequency of a sweep oscillator must be variable in

order to be useful for sweeping vertical voltages of different
frequencies. Another and very important requisite is that it
will sweep the spot across the face of the tube at a constant
speed. .L sawtooth wave of the form
will give such linearity of sweep
for electrostatic deflection systems
but may not for electromagnetic deflection.
for magnetic flux to increase at a steady rate in a coil di/dt must
be a steady change. This means that the inductive reactance voltage
is constant. It then needs a square voltage wave. However. the £1
voltage required for the soil will be of sawtooth fora. ildding these

together the voltage applied to a coil should be 31 + L di/dt

12.

m /\/\/\/
L di/dt I I I I I l

m + L di/dt

This type of sawtooth wave will result across a condenser
and resistor in series. in an oscillator. between points a and b as

shown in figure VIII.

 

 

 

Figure 8.

However. if the inductance is not high in comparison to the resistance

of the coil. the resistance. Bi in figure VIII can be discarded. The
resistance and inductance of the horisontal deflection coil were read at
1000 cps. and found to be 650. ohms and 5.2 Ih.. respectively. So the
reactance of the coil is about 32 ohms as compared to a resistance of

650 ohms. It was found that a sawtooth wave would work satisfactorially
for a magnetic sweep pulse. Therefore. it is possible th use the sweep
from a Dumont 208.3 oscilloscope in parallel with the deflection coils

of the 12' tube.

13.
One defect in using a sawtooth sweep was that on
flyback of the spot. oscillations were set up in the coil circuit

by the sudden change involtege. This resulted in a scope configuration

such .. gvflv

mess harmful oscillations may be eliminated by “blanking out' the
spot during the interval of fly-back. This is accomplished as explained

in discussion of the blanking circuit on page 16.

D. 12 D P 7 Tube in Parallel with a Duont
206-3 Beeps

Probably the easiest‘ and most satisfactory way to build a
large demonstration scope is to parallel a large tube with the circuits
of some smaller scope commercially constructed. Added to the advantage
of eliminating a lot of circuit construction is the advantage that the
lecturer may observe what he is doing on the smaller scope and have the
audience get an unobstructed view of what is taking place on the large
tubs. Certain balancing adjustments need to be made to parallel the
behavior of the two scopes. A discussion of the work along with pertinent
circuits follows.

At the rear of a Dumont 208-3 scope is an outlet plate of

contacts with the four deflection plates arranged as follows viewed from

 

 

the rear: »
‘ 9:5 01» :5; ol%' fpk‘
I / f i l
W ground 6/
horisontal deflection vertical deflection plates

plates

1h.

 

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15.

Figure II on page 1h shows how these plate connections are utilised
to actuate signals for the large tube. The chases of both scopes are
connected by one lead from G to the chasis of the large scope. All
four plates of the 208-3 are grounded through high resistance which
act as potential dividers so that parts of the plate fluctuations of
the 20843 are fed directly to the grids of the pufihrpull amplifiers
of the horizontal and vertical deflection circuits. The high resistance
is used as potential dividers to prevent overloading the plates of the
Dumont Oscilloscope.

I Examine, now. the behavior of the horizontal deflection
system. As D2 becomes more positive than.D1 the spot of the 208-3
is deflected to the right. Simultaneously the grid of the right hand
5L6 tube becomes more positive than.that of the left hand 6L6. Since
both tubes have the same bias furnished by the 220 ohm cathode resistor the
right hand tube will be more conductive. Coil 13-h will be carrying
more current than 13-7 so the beam of the 12 D P 7 will be magnetically
deflected to the right. If Vi is the signal from.D2 which is fed to
the grid of the fight hand 6L6 tube; then the current equation for
coil.(l3-h) is: V

._,.,V,

Liz—4 ” [5, +rb 4-ij +(l+u)l’..J

18000 ohms wt 3 32 ohms r(cathode)

rb :
or rt 2 220 ohms
’1.

About 70 volts on D2 is required by the 208-3 scope for center to

650 ohms u.- 6

outside deflection. About 1/7 of this is applied to the grid of the
right-hand 6L6 tube. Then i (13-h) is about 3 me. when v; is 10 volts.

Selecting the right amount of potential from the plates.

16.

D1 and D2. to be fed to the grids of the two 5L6's. makes the two
scope patterns relatively the same size. To return the spot to the
left side D1 becomes more positive than D2 and the left 6L6 is more
conductive than the right 5L6. {me return of the spot takes place on
the downward stroke of the sawtooth wave from the oscillator. This
is a relatively short event as compared to the forward sweep. but still
the spot can be seen in its return trace although not as brightly as in
the forward direction. This return trace is objectionable but can be
blanked out if more bias voltage is applied between 01 and ‘ the
cathode of the 12 DP 7.

In order to accomplish this blanking some signal from D1
is fed through a 6837 tube as a cathode follower amplifier and on
to the cathode of the 12 D P 7 which is already operating about 50 volts
above 01 voltage. his positive pulse of the form [I II II ll -.
is enough to blank out the beam of the 12 D P 7 on return sweep. In
Order to prevent the 6337 from passing on the DO potential changes of
D1 . 0.005 uf. condenser is used in'the feed line from D1 and the grid
D0 potential of the 68K? is made more stable by the grid resistor of
2$00,000 ohms.

The characteristics of the two 6L6's may not be exactly the
same or for some other reason the spot of the 12 D P 7 may not be
centered when the spot of the 208-3 is at center position. A 50 ohm
potential divider is provided in the cathode circuit of the 6L6's so
that the relative grid biases of the two tubes may be varied to ac-

compli sh centering.

Variations of vertical plate voltages from D3 and pit are

l7.
likewise fed through another similar push-pull circuit to the

vertical deflection coils of the large scope.

B. Low Voltage Power Supply

The circuits of figure 11 show that there are various other
voltage requirements in the large scape in addition to the high voltage
of 5000 volts. The 616's operate at about 350 volts and the deflection
coils require about 85 me of current. each, for horizontal and vertical
full scale deflection. The focusing coil requires approximately 10 ma.
so the current requirements are roughly 200 ma. Voltages of 350, 250,
and 50 golts are needed for the elements of the 12 D P 7.

A transformer made by the Standard Transformer Corporation

was chosen which had the following specifications on secondary:

High voltage 700 volts ct at 200 ma.
Rectifier heater 5 volts at 3 amp.
Filament 5.3 volts at 5.5 amps.

The low voltage power circuit was constructed as shown in the upper
half of figure V. The 50‘“) gives full wave rectification. Two 30 uf.
condensers and a 12 henry inductance coil form a suitable Pi filter.
The two VB tubes stabalise the 12 D P 7 grid voltages from fluctuating
when focusing is being accomplished.

Iigure III shows the assembly plan of the three main elements.
12 D P 7. tube housing. high voltage power supply, low voltage supply
and push-pull amlifier circuits.

IV. Performance of the 12" Oscilloscope
The 12" oscilloscope works well in conjunction with the Dumont

208-3 oscilloscope from very low frequencies up to about 3500 cps. for

18.

 

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19.
vertical signals. Above this frequency the patterns of both scopes begin
to show distortion. The distortion an be due in part to overloading
the plates of the smaller scape at higher frequencies and to the failure
of the 61.6 amplifiers to give consistent gain at higher frequencies.
However. as the photographs of figure XIII show, both scope patterns
begin to show distortion at about the same frequencies. This would

indicate that the plates of the 208—3 were being overloaded.

   
   

Sweep 12D? 7 205-3 12DP7 208-3
freq. cps 3V”?
' freq. cps
500
2000
1000 3000

 

FIGURE XIII

Sweep frequencies up to about 800 cps. may be used. Above

this point the sweep velocity is no longer a constant. but is faster

at the ends than at the middle and the length of pattern begins to
decrease. These faults appear on the 12" tube only, indicating that
failure may be the horizontal amplifier's poor gain characteristics

.at higher frequencies. The system will work well. however, at frequencies

used for most demonstration purposes in elementary physics study.

Bibliography

Brainerd. Koehler. Reich. and Woodmff. “Ultra-High Frequency
Techniques”. Chapter V. (D. VanNoetrand 09.. Inc.. 19h2)

Kivers. Hilton 8.. “Television Simplified' (D. VanNostrsnd 00..
Inc.. 19%)

Cooking. W. T.. ”Television Receiving Equipment“. Chapters IV.
v, and n. (Iliffe and Sons LTD.. Great Britain)

Reich. Herbert J.. "Principles of Electron Tubes”. (IIcGraw-Hill
Book 00.. Inc.. 191a)

horykin and Morton. "Television“. (Wiley. 1910)

link. Donald 0.. "Principles of Television Engineering”.
(McGraw-Hill Book Co.. Inc.. 19%)

Maloff and Epstein. 'Electron Optics in Television“. (McGraw-Hill
Book 00.. Inc.. 1938)

 

HICHIGRN STQTE UNIV. LIBRQRIES

I||I||||||| IIHIII "NW |||| 1| III Illlll IIIHI IIINIIHIHII

 

3129301774560