a THMIS 1949 .leetriel
THE CONSTRUCTION .AND ANALYSI3 OF A LIONOSCO
a
by
FREDERICK GEORGE HERRING
B. 3., Uaivorsity of
A
t3ubm1tte
re
in
Ncla.L3shire,
1V48
THMIS
t1
m
ew
lfillnent of the
Its for the
deers°
AASTER OF SCIENCE
Depti.rtment of
.leetriel
.;.a6.ineering
KANSAS STATE COLLEGE
OF AGRICULTURE AUD A2.I.?LIED SCIENCE
1949
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74
INTRO
U
TION
The past decade has witnessed the evolution of television
from the research laboratory to a proved medium of visual com-
munication.
This evolution has been achieved through
bined efforts of scientists
and,
the
com-
engineers, whose objective has
been the realization of television's full possibilities.
To
accomplish this objective, and to facilitate the edvuseement
of the art,
the
development of test equipment for television
devices and circuits arose as a logical and aeoessary undertaking.
Accordingly television test equipment has been designed to satisfy the various needs of the industry.
One such piece of test equipment is the monoscope camera,
a static-image signal-generator.
It is the purpose of this
camera to produce a fixed picture and blanking signal output,
which may be combined with synchronizing pulses to provide a
composite video signal.
This composite signal conform
to
1
R. M.
.
staneards.
The monoscope camera is, therefore, a oom-
plete television camera which may be used interchanably, in a
standard television system, with the studio cameras.
The monoscope was designed primarily for the purpose of
aligning and checking television equipment such as receivers,
transmitters, monitors or other associated circuits.
The
picture signal from the camera makes it possible to observe
contrast, system resolution and to detect nonlinearily in the
deflections circuits.
It excels the looneecope for this
eadio eAnufacturers associations
this purpose since it is inexpensive to menufacture and requires
no auxiliary source of lighting.
strong, equal to 10 time
The signal is exceptionally
that obtained by a eonventional iocr-
os ape withel foot candle of plate illumination.
It is apperent
that the eeeision to conetruet a monoecope
camera was well justified.
The conetructien ef this camera has
been completed, ena it is now installed as an integral unit of
143.,'LeVis
television system,
preen
This paper
s,
therefore, the
construction and design considerations of the camera and a theoreticel analysis of the monoseope tube and the circuits fund-
emeneel to its operation.
THL 1ONO3COPZ
General Dee°
TUBi:
le
The monoscope is a special type of cathode ray tube resem-
bling the iconoacope in general principle, but differing in that
the video signal is derived from a printed pattern electrode
enclosed within the tube.
eceoraingly, it cannot be olaeuified
as a pier up tube since it is unable to develop a video signal
which repreeen,s action, but is necessarily restrieted to one
particular still picture or pattern.
The limitations of such
a tube are adearent, and if a comperisen of the monoscope and
the
iconoseope is to be made, then their eharacteristies ehould
be evaluated in the light of
cope
:as
intended.
the purpose for which the monos-
This, primarilye is the testing of tele-
vision system performance, and
the
ability of the moneeeope to
generate a strong video siz.na1, rich in
hear
content, that
is dependent of light conditions end free of spurious signals,
is identically oommensurable with its purpose as a piece of
test equipment.
There are in gendral two basic types of static-imae signal-
generating tubes.
The first of these employs a change ee primary
beam current to develop an output sienal.
Briefly this type of
tube has a signal plate of some designed configuration cut from
a conducting surface.
ehen the plate is scanned
signal our
rent is obtained each time the beam strikes the plate.
As a
result this video signal will produce a picture of the plate
when applied to the
grid, of a kinescope.
The second class of tube
able combinatione:
on
In general,
be made in a number of work-
two materials having Ceie:ferent
secoudary emission ratios are used to make the signal plate.
As the plate is scanned, one magnitude of ecoondary emission
ourrent is obtained from one material and another magnitude is
obtained from the second.
The difference in the magnitudes of
those secondary emission currents determine the amount of video
signal.
Therefore, if two materials hich have a numerical
difference in secondary eeieeion ratios greater than one are
used, it is possible to develop more video current than eould
be possible if only the primary current of the beam were util-
ized.
to
Beoauee of this advfintace, a number of combinations of
materials having different secondary emission ratios were
investigated by the tube manufactures.
The end result was a
technique of preparing sienal plates which peemitted the accurate reproduotion of all types of subject material (2).
hysical Characteristics
The statio-image signal-generating
monoscope eamera is an
4:C.ii
type arl,
tube employed in the
It hhs an overall length
of 13 inches, a maximum bulb diameter of b inches and a phys-
ical appearance characteristic of magnetic deflection type eathode ray tubes.
The base is a long-shell medium six-pin base
that fits a standard six contaut socket.
The electron gun con-
nectiens are made to the base, while the collector and pattern
leads are brought out two recessed small ball caps.
The pattern
eleatrode cap is loaoted at the center of the face of the tube
and the collector cap on the side of the bulb near the neck of
the tube.
The entire, tube, except for a small portion near the
baee, is coated externally with an insulating moisture-repellent
coating, which serves to prevent eaudensation of water vapor in
a conductive Tilm over
the glass eurface.
:i4.ratic
surface
sparking, which may be produced by such a film when a high volt-
age gradient is present, is thus eliminated,
Design L:onsiderat
lntorna1lr the
21721
conalute of an electron gun,
eleetrotW and a collector.
.Wlate I.
a
pattern
These are shown diaerammatically in
The electron gun, which produces
a oceleratos
and
focuses the electron beam, must be of high Quality if the best
video signal is to be obtained.
ifl
particular, the gun structure
must have been dehigned electrically and neehanically to be
measurable with two objectives.
The first of these would
com
b
be that of producing the maximum output signal.
since the out-
put signal is a function of the secondary emission current, the
potential of the pattern electrode
current are important factors.
talc'
the magnitude of the beam
The second objective is to secure
a beam whose diameter does not limit the resolution of the system.
It is of interest to investigate the particular choice of
bean current and final accelerating voltage employed. in the monoscope tube, sines it is essentially these values that will deter-
mine the resolution and strength of the video signal fed to the
video amplifiers.
The final accelerating voltage, which is the
potentiel of the target plate with respect to the cathode, is
given by the manufacturer to be 800 volts.
The choice of this
particular value may be justified by realizing that the accelerating voltege employed should be consistant with two primary
objectives.
Ihe first of these is that of obtaining the max-
imum coefficient of secondary emission for the white or alumin
um portion of the target, and
a
minimum secondary emlaion
coefficient for the black or carbon portion of the picture.
Here a good. seeroximation is that the secondary emission co-
efficient for a given surface is a function only of the primary
electron energy and thus of the accelerating potential.
The
actual secondary emission current will be to sane extent dependent upon space charge effects, the angle of incidence of
the
primary electrons, residual gas in the bombarded materiel,
and possibly temperature, but these effects are negligible
0).
Curves representing the desired relationship are plotted in
elate
I
for carbon and for a typical composite oxidized layer
LIP.L,NATIM
F 2I6TL
Fig. 1.
Simplified sehemtle of the monoseope tube
Pig, L.
Secondary emission ohareeteristiez of the materials
compri,'ing the
signal plate (1Z)
7
PLATE
I
GUN
ELECTRON
PATTERN
ELECTRODE
LOAD
COLLECTOR
RES151-0Fi
Fig. 1
a
7-
200
400
PRIMARY
J
600
IMPACT
Fig. 2
800
ENERGY,
1000
VOLTS
1200
1400
0
3ince the secondary emission coefficent for carbon is
(1:5).
prectioally independent of the primary electron energy, the
accelerating potential should be chosen so as to correspond
with that value ehich will give the
ximam secondary emission
coeeficient for the alumminum oxidized layer.
This maximum
occurs at potentials in the vicinity of 600 volts.
The potential of the pattern electrode also influences the
resolution of the camera in that it is instrumental
mining the diameter of the electron beam.
irk
deter-
Accordingly this po-
tential must be chosen to be consistant with a resolution ex-
ceeding b25 lines.
This in itself is analogous to the problem
of preeictine the change in beam diameter with a change in
accelerating potential.
The Kluwer to this seems to be provided
in some earlier papers end in the eell-kaown textbooks by
orykin ana
eprton (14).
The general opinion expressed seems
to be that the spot size ahould decrease in proportion to the
square root of the increase in the accelerating voltage of the
electron beam, all other conditions being held constant.
Thus
the formula
a
Where:
r is
:ea
= constant
the radius of the electron beam
is the accelerating potential
This theoretical prediction, however, does not agree at
all with the change of spot size actually observed;
the exper-
imentally found change of spot size with a change of acceler-
ating voltage is very much smaller than required by this theory.
A
rather thorough investigation of this subject
:as
undertaken
by G. Liebmann, and it is of interest to note the result of his
work since it provides an answer to the foregoing question.
Briefly, Liebmann proposed a new equivelent-optioal system
loon
slating of three lenses to represent the electron optical
system.
On this besis the conclusion is reaohed mathematically
that the spot si:4e is an image of the oathode and that its
diemeter
is
not a fuection of the accelerating voltage.
This
prediction was checked experimentally on a number of typical
oathode ray tubes employing both types of deflection.
The ex-
perimental results indicated that for accelerating voltages
above 1000 volts, the spot size remains unchanged within the
error of measurement.
Below a final accelerating voltage of
1000 volta, the obtainable resolution decreases, although the
decreaee is not as great as that predicted by the OTOSS over
theory.
The 2F21 uses accelerating voltages in the vicinity of
1000 volts,
i'rx1 the
above arguments. it is seen that a d
crease from this ficure, while resulting in a slightly in-
creased secondary omission coefficient for the aluminum oxidized, layer,
would also produce a eecrease in resolution.
SimiliarIy, higher accelerating voltages would be to no particular advantage.
The definition would not be increased, and
the secondary emission coeffivient would be adversely effected.
'Liebmann, G., "Image Formation in Oatho
the Relation of Spot Size and. Final Anode Voltage"
1.R.B.,
315::581,
Juni
1945.
Tubes
Proc.
am:,
IC
The magnitude of the beam current is another important
consideration and here again a compromise is made.
A high
beam current is desired since a percentage increase in beam
ourrent results in a like percentage increase in signal output.
However,
the beam current is primarily a function of
grid, voltage.
the control
This voltage ',Idle determining the magnitude of
the current, simultaneously determines the size of the emitting
area of the cathode.
ince the spot at the pattern electrode
is an image of this cathode area,
results in poorer resolution.
then an increased beam current
The size of the beam should not
be sacrificed to obtain high beam currents.
As a first appromaration it might well be argued that the
signal output from the mon.oscope tube could be increaed by
enlarging the area of the pattern electrode.
ith an increased
area, beam dLumeter could be correspondingly increased without
adversly effecting the b25 line resolution.
beam current,
:_hich is
he allowable
proportional to the beam diameter, is
greater, and the secondary emission current and thus output
voltage is also increased.
two objections manifest
rile the above argument is valid,
themselves.
The first is the well-
known defocusing of the beam as the defleution angle is increased.
The second is the variation of the secondary emission
coefficient for any surface as the angle of incidence of the
primary electrons in changed.
This effect becomes appreciable
at primary impact energies above 100 electron-volts
as such would be evident in the monoscope tube.
(1Z), and
The degree of
this effect is a function of the bombarded material and is
II
determiaed experimentally* eltnoegh an increase in the eucoedary
eetission coefficient for increasing angles of incidence is
characteristic of all eubstances.
For carbon the coefficient
of secondary eaiseion is practically independent of the angle
of incidenoe, while for aluminum it can be eoneidered constant
if the angle of incidence is not greater than Le degrees.
It 30 degrees the coefficient of secondary emiseion has in-
creased to about 16 per cent of its normal inci:lenee value,
and thereafter rises rapidly.
If the effective center of
deflection is considered to lie at the mid-point of the coil
jo7ee,
then the aaximum angle or incidence in the monoecope
tube is 17 degrees, so that the effect described could be
negleeted.
Haaever, this variation in the secondary emiseion
coefficient might well be a limiting factor in increasing
the signal strength by an enlargement of the pattern electrode
area.
he
'at
tern electrode
The heart of the monoseope tube is the signal plate or
pattern electrode,
tee
it is aumetimee called.
plate is made from aluminum foil and carbon.
-
The siaa.1
The eurfaee of
the aluminum has a natural coating of aluminum oxide whieh has
high secondary emission ratio while the carbon has a relatively
low ratio.
eyference to Plate
relative ratios obtainable.
I
confirms this and shows the
1:xpeldmentally the manufacturers
found that aluminunl foil developed for advertising and packing
purposes as well as special inks developed for printing on metal
foils were satisfactory materials for the signal plate.
was extremely fortunate
This
inee the advantages and flexibility
of coLlmercial printing could be utilized.
The desired picture
or pattern is priated on a piece of aluminum foil approximately
.034 inche thick ,A.th a blaokfoil ink.
The only other process
necessary before sealing the signal plate in the tube is to
fire it in hydrogen.
This process removes the volatile uaterial
from the ink and leaves it practically pure carbon (2).
Subject matter for reproduction on a signal plate can be
divided into two classes:
black and. white, and half tones.
Cartoons are a good example of the first, while snapshots,
which Contain
half tone group.
black and white, illustrate the
2hoto-engravings are made of the :;ubjeot
matter for printing the signal plate.
The black and white
material is treated as a line-cut, but the half tone mterial
must be broken into a number of dots of various sizes depending
on the half tone value.
This is done when the photo-engraving
is made by photographing the material through a suitable screen.
A screen is used which will break the picture into more elements
than are used in the television seanning system for which the
tube is designed.
As a result, this technique of obtaining half
tones does not limit the resolution of the television system,
and the half tone effect is reproduced just as in a newspaper
photograph.
The actual pattern in printed on the signal plate as a
13
'negative.
The pattern is printed as a negative in order to
add to the convenience of the tube's use in testing iconoscope
equipment.
polarity.
The iconoscope has a signal output of negative
That is, when n picture is transmitted by an loon-
oseope, a highlight in the picture is represented by a negative
velue of the iconeseope's signal output voltage.
In the menos-
cope, however, a white portion of the pEttorn is represented by
a positive value of
signal output voltage.
This is beceuee the
aluainum omide has a secondary emission ratio greater than earbon.
In order to melee the eignal output of the 2721 correspond
to an iconoscope,
2F1.
to
the pattern is printod as a negative in the
The result is that when the signal from a 21721 is applied
the ieput of a chain of amplifier stages betweea an ico.ioecope
and a kinescope, the pattern appears on the kinescope as a
positive.
The negative printing therefore is a convenience in
tests of iecnoecope equipment.
In tests of any television equip-
ment, a positive reproduction of the pattern cen be obtained
by using an oed number of video amelifier etegcs between the
monoscope and the kinescope.
aeam Defleetion
The
21e2,1
(eaploys electromagnetic deflection end the standard
rectilinear system of scanning.
Sawtoeth scanaing currents are
eupplied'to the respective coils by horizontal and vertical deflection circuits ehich sre
a.
ceeposite pert of the unit.
are described separately in detail.
These
The vertical,horizonal,
field and frame frequencies as well as the geometry of the inter-
14
laced pattern, eumber of scanning lines and the aspect ratio
all conform to the present R. a. A. standard.
Physically the deflection yoke is situated so as to cover
the neck of tube, extending fron the end of the electron gun
struoture to the beginning of the bulb.
a
de
the
to support the yoke on either
ft-0'1ns bolts were
llo, and two slots out in
tube shield so that the yoke might be positioned to cor-
rectly scan the pattern electrode.
The collo are formed within the yoke in the following
manner.
ach
coil is wound rectilinear in form, and is made
up of several concentric seotions each having the proper num-
ber of turns so that the magnetic field will be uniform in
the
finished yoke.
The two horizontal coils are placed on the
inside, 180 degrees apart, and the vertical coils are formed
over them and displaced 90 degrees from the horizontal coils.
Thus the horizontal coils provide a magnetio field at right
angles to that of the vertical coils, so that the resultant
horizontal deflection is at right angles to the vertical doflection (e).
An important design oriteria is that the de-
flecting coils provide a uniform flux distribution across the
neck of the tube,
If this requirement is not satisfied,
that portion of electron beam which passes
then
through the strong-
er region of the field suffers a greater deflection, and a
beam of circular cross section is distorted into an eclipse.
en electrostatic shield is provided between the horizontal
and vertical coils to minimize any cross coupling.
The cells
are entirely enclosed within a case of megnetio material,
to
Ib
insure ahielding and
leo to increase the inductance of
vertioal
In order to prevent utray electric and magnetic fields
from effecting the deflection
tnid
beam, a galvanized iron shield
16a,3
focusting of the electron
constructed to enclose the
entire tube and the deflecting yoke.
wall was built within
A
this case encircling the monoscope tube between the pattern
electrode and the yoke.
This shields the pattern electrode
from the electrostatic field of the coils vmd minimizes pickup by the pattern electrode and video amplifiers of high
frequency components of the voltage across the yoke.
Care
was taken in spacing this 'a 11 from the pattern electrode and
its terminal,
since too close spoing would result in an in-
crease in the output capacitance of the tube and thus the re-
duotion of output voltages at
GlICEILAL
Ugh
DE3CRI2TION OF
The fundamental arrang
frequencies.
CAML,74,
CIRCUITS
ho monosoope
nt of e
camera is shown in the bloat diagram
1ate Xl.
The vertical
deflection generator consists of four tubes and their associated circuits.
The first of these tubes amplifies the driv-
ing signal received from the synchronizing Generator and gen-
erates a sawtooth voltage wtich is amplified in the second
tube.
The fourth provides negative feedback to improve
laic
scanning linearity.
The horizontal deflection generator inclutis three tubes
and their associated circuits.
The first tube is
the driving
16
signal input aaplifier and sawtooth genc.ator;
third aaplify the output wave Aid f(ed it
deflecting coils of
th,
to
the second and
the horizontal
monoecope tube.
The blanking amplifier is used to provide the proper
level and polarity of the blanking pulses received from the
synchronizing generator before these pulses are fen into the
video anplifier for mixing with the video signal.
Video amplification is provided by compensated stages,
end the monoscope output sienea is fen to the first of these.
A clip)er stage is included to provide variation of the d-c
component of the camera signal.
This is acoomplished by con-
trolling the vias on the clipper which in turn determines the
height of the blanking pedestal.
One may be used to feed a monitor,
Two output stages are provided.
.11i10
the other is connected
to the system under test.
VID14
Li F
High- 2requency Compensation
The video amplifiers consists of a nine tube res
capncity coupled amplifier, compensated so as
.
frequency response.
to
ace
insure good
Actunlly only the first eight stages are
compensated amplifiers, no oompensation being required in the
output stage, sinoe it operates into a 7b ohm coaxiF1 line.
Theoretically, these amplifiers may be adjusted so as to obtain a response uniform to 8 megacycles.
difficulty
Aotually, however,
as experienced in extendning,the response beyond
3.
EXPLANATION Or ILATL II
Fie* Z.
Block diaeram of the monoscope camera Allowing
input end output voltnee wave forms
r-
VELI.THAAL.
/EAT CL
OUTPUT
H co-t
VO L-TA E
,OPE
MBE
SUPPLY
Hont-zoNTAL
DR.JNG
2a
HOR170
L
DEFLECTION
'ENERATOR
K1 NEti.coPt_
BLANKING
0
SIGNAL
BLANKING
AIVIPLIFIER
REGULATED;
POW E. Ci SUPPLY
VIDEO
E
NCLuDEs VIDED
,<:^1C2.
19
6.4'J
megacycles without peaking at the high end.
However, the
maximum video frequency generated by the monoscope, aseuming
equal horizontal and vertical resolution, is given by the
formula (6)
2
ra/a
2
= (625)
f =
Where:
(ZO)
6.5 megacycles
N is the number of scanning lines
r is the fraae ropition rate
a le the aspect ratio
f is the maximum video frequency.
The resolution of the system, therefore, will not be limited
by the video amplifiers.
It is important to realize
that
e
high frequency per-
formanoe of the Amplifier determines the quality of the picture
so far as the resolution and detail are concerned.
in enu relay characteristics are flat,
duced exactly.
If both
the picture is repro-
If the gain is constant in the video bend end
the time delay varies
ith frequency, all the high frequency
components are reproduced exactly in their proper relative
Naplitudes, but the location of the various picture elements
is not correct, because of the different amounts of time takea
for passage of th
different frequencies.
This results la in-
ferior reproduction of detail.
The signal plate of the .aonoscope tube is connected to
the grid of
the first picture amplifier tube
through a compen-
sating metwork composed of the transformer, 11, and the
20
resistors,
liZ
and Re, as shown in rlate V.
A similar network
is included in the plate circuit of each adeitional amplifier
tube.
The compensating circuit is a four terminal network
that presents a constant load impedance to the tube over the
required bandwidth.
A mathematical analysis of the network
appears to be rather complex and will not be attempted,
It
may be stated, however, that there exists essentially two
methods to reduce the effect of the
is
load,
capacitance which
the principle cause of loss of gain at the hiO4 frequen-
cies.
One method involves the use of a very small load resis-
tor, whose resistance is so low compared to the reactance of
the load capacitance at the highest video frequency, that the
reactance has no effect on the gain or phase characteristics.
This arrangement would possess no practical advantages, because
of the erent loss in gain per stage entailed, by the use of a
small plate resistor.
a circuit
The second method is that of employing
containing inductance and utilizing the resonance
effect to extend the gain at high frequencies.
This, basically,
is the principle employed by the coalpensating network.
In the actual construction of the video amplifiers all
possible precautions were taken as to the placement of component parts and the length of leads to insure a minimum of stray
capacitance.
Coupling condensers and load tires were held
well off the chassis.
It was found experimentally that
the
capacitance between a component having axlel symmetry, such
as
paper coupling condenser or a
1.rel,
and the chassis varies
xi e
approximately as the function Ce
;
here x is the distance
of separation, C is Lho capacitance at x equal to zero, and
is a constant proportional to the surface area of the coapon-
ent.
Thus the rate at which the capacity chaneeee with respect
to distance,
evaluated at any point, is proportional to the
capacity at that point.
This relationship is useful, inasmuch
as it indicates the advantage gained in increueing the distance
for small initial values of x.
Low-Frequency Compensation
In the frequency range below 200 oycles, the gain and phase
ehift of the vid.oc amellfiers arc both subject to a variation
from the aid-band values.
The phase shift is particularly ob-
jectionable at low frequencies
for this.
and. it
is necessary to compensate
The relationship between phase shift and time delay
for a perticular frequency is
=2-rrtf
Thus for a given phase shift the time delay is seen to increase
rapidly as the frequency is lowered.
et to
frequencies it is neeessary to compensate for phase
shift in the cathode circuit, the screen circuit end in the
coupling network.
Complete coepeneation may be obtained by
means of a network in the load circuit for any one, but only
one, of the-e.
In the video stages conetructed for the menoz-
cope no compensating network is employed; however, preceutions
are taken to effectively eliminate any phase shift that might
arise due to the above causes.
The 6AG7's use 18,000 oh
screen dropping resistors which
are bypassed by 20 microfarad condensers.
The 6eG7's use
,O00
ee
oha dropping resistors ehich are bypassed by 10 microfered
condensers.
This may be verified by reference to illete V.
The frequency response is
thus not effected by the screen grid
circuit for frequeecies exceedine approximately 5 or
t
cycles.
In the coupling network o.eb mierofarad condensers are emp7oyed
with grid resistors of approximately 500,000 ohms.
lere again
the phase shift is neeilible down to a few cycles per second.
In the cathode circuit no phase shift whatsoever is intro-
duced.
however, is accomplished at the expense of the
This
stge.
in of the
The method emplojed to accomplish this
is sieply to omit the cathode bypess cendenser.
of the stage at
loe;
frequencies is given by
3ince the gain
0)
1.
Ge
i
e
here:
G' is
+
the etin of the atage taking into
consideratlo.
the effect of the bias network
Is the load resistee,e
R
is
0,
e
the cathode resistor
is the cathode bypass condenser
and
u,
rp and ga are
then if
L;%
equals zero, the gain is real and independent of
frequency.
the tube parameters
This equation, with C.
to zero,
is equivalent
to
Where:
G is the gain of the emplifier without feedback
BG is the feedback factor
This, of csourse is the well-known equation of an wJlplifier
stage eaploying inverse feedback.
An approximation of the
gains realized, in the video atage will be made.
typical
J
value of load and cathode resistance are 100 ohaa respectively.
Considering the transconductunce of a
60
to be
approximately 10,000 reicrodlos, then the absolute value of
the
feedbae-faetor is
feedback is 10.
1
and the gain of the stage neglecting
Thus the gain is deoreased by a factor of
two and the overall gain of the stage is 5.
i.S
a first ap-
proximation it may be thought that this decrease in gain is
not commensurable with the result achieved.
ever, other advantages
resistor.
ance
There are, h
to be had by using an unbypassed cathode
The first of these is a decrease in input capacit-
since the gain of the stage has beca lowered.
This
effect is desirable since it improves the high frequency
posse of the preceding stage.
res...
The trunster chsraoteristic
curves of the tube also become MOT
linear and a sharp out off
is obtained.
It should be mentioned that the i'unction of the very low
video frequencies is
to
suvly
the background of
the pieture.
If the video stages fail to pass these frequencies without
distortion the result will appear as a gradual shadine of the
picture from the top to bottom (6)
Output St
The output of the video system is
fe'ti
to the
paralleled
grids of Ve and V10, the picture output and monitor output
L4
tubes.
The two output stages are identical, the additional out-
put stage being provided so that the picture signal may be view-
ed on a monitor while the signal is being fed to the equipment
No attempt is made to achieve high-frequency
being checked.
compensation, since both stages operate into a 75 ohm coaxial
line.
The necessity of feeding into a coaxial line reduces the
voltage gain of the output stages to a value less than unity.
Blanking
The blanking signal is also mixed in the video amplifiers.
The composite vertical and horizontal blanking pulses are de-
rived from the synchronizing generator and initially fed to V7,
the blanking amplifier.
The blanking pulses must be of nega.t-
ive polarity and should conform to tha R.4.A. standard.
The
composite pulses are aixed with the picture signal in the fourth
video stage .through the use of a common plate load, resistor.
It is of interest
to note here that during
:anoscope tube is blanked, the polarity of
ti..
the time the
output video
signal from the 2121 corresponds to that of a white section of
the picture.
Therefore, when the actual blanking pulses are
mixed with the video signal, the polarity of the monoscope
signal corresponding to the blankinking time is opposite
of the blanking pulse.
rather than additive.
The
that
two voltages are then subtractive
This necessitates
that the blanking
pulse be of much greater amplitude than the pulse generated by
the monosoope tube during the blanking time at the point where
they are mixed.
Lfter the mixing takes place,
the polarity of
the
signal during picture blanking is in the true black direction.
PICTURE; BRIGHTNLS3
Following five stages of video amplification the signal is
fed to the clipper tube, V4.
The clipper tube operates only on
the blanking pulses, v4.hich have previously been
mixed with the
video si nal, to establish a black level, and thus control the
light intensity, that will correspond to any particular amplitude
of the video signal.
lien
the blanking pulses are mixed with
the video signal the level of the signal during the blanking
period is raised far above the level of the actual signal eontali:Ate
picture information.
The clipper tube,
therefore, has
a wide range over whieh to operate on the blanking pulse, and
the average picture brightness can be adequately controlled.
The importance of the clipper tube in controlling the
black reference level cannot be overestimated, and an analysis
of the camera signal at this point nay be well justified.
Fundamentally, the information contained in the camera signal
consists of two components..
One is the a-c component, whose
relativainstanteous amplitude contains the picture element
brightness corresponding
being scanned.
to that point on the pattern electrode
The second is the d-e component
which is de-
fined as that value of voltage existing between a fixed
reference level and the average of the camera signal
Olek
(4)).
The
d-c component, therefore, corresponds to the average brightness
of the picture, since it is an average voltage which would
correspond
to an
average light intensity.
It is advantageous
2t.1
to be
able to vary these two components independently of each
other.
The relative amplitude of the a-c component is con-
trolled by the Lain of the video amplifiers.
This control is
incorporated by mfeking the screen grid voltege of the first
video amplifier variable with the potentiometer,
117.
While
this control is labled "Gain" it is in a sense also a contrast
control, since a variation in contrast is an inevitable result
of 'vryine the amplitude of the a-c component of the video
signal.
It nova remains
signal
cell
to be
be varied.
shown how the d-c component oT the
This is readily accomplished once the
fixed bleak reference level is defined.
This bleak reference
will be taken es the height of the blanking pulse.
Thus by
varying the difference between the picture average end the
blanking level, the d-c component corresponding to the average
picture brightness is controlled.
The average picture bright-
ness ef the test pattern in the monoscoee tube is obviously a
constant value.
Once the d-o component corresponding to this
average brightness is set for a satisfactory picture as vieeed
on a kinescope, further adjustment will not be necessary.
heference to Plate IiIwill help to clarify this discussion.
The brightness control, N55, is a potentiometer that controls the grid bias on the el/peer tube. -Glipping is accom-
plished by setting the grid bias of V4 to such a value that the
top of the
blanking pulse drives the tube into cut off.
even number of steges is had
the
An
between the blanking input end
elippe, so that the blanking pulses appear negative on
the
EXIMANATION OF 2LATZ III
Fig. 4.
Control of the average picture brightness by
varying the average of the caera signal with
respect to the black reference level. ikverage
light intensity of the scene being transitted
is high
Fig. b.
Average light intensity of the scene being
transmitted is low
28
PLATE III
BLACK
4
D-C
C
REFERENCE
OMPOtlEt IT
vv-"Ji-or-AvERAGE
0
LEVEL
OF
CAMERA
51:SNAL
1,VHI-TE
TitviE
Fig.
4
BLACK
REFERENCE
LEVEL
OF CAMERA
SIGNAL
D-C COMPONENT
AVERAGE
WhITE
Fig. 5
of V4.
Re
in varying the bias on V4,
determines the clipping
level and consequently the d-c component contained by the signal.
The action of the clipper is improved by using a 1000 ohm unby-
passed cathode resistor.
This gives a sharp out off character-
istic and thus a well defined blanking level.
01.414PER
TUBE ANAIr5I5
Fundamental Theory
clamp circuit using a
6116
third picture amplifier tube, V6.
operates on the grid of the
The
function of this eireuit
is to establish the peaks of the blanking pulse at a fixed
reference level in the video signal or at some potential fixed
with respect to this reference level
clu.rinL
the retrace tiete.
Coincidental with this action, spurious low frequency signals
such as microphenics, power supply surges or 60 cycle hum which
may have been introduced in the precedirw low
level.
picture
LLn)lifier stages are reduced to a negligible amplitude (7;,
The activation of any clamp circuit can be controlled
either by the signal to be claaped or a signal independent of
the cla.ipod signal (L2.).
ed
The particular clamp circuit employ-
in this equipment uses the latter method of clamping, de-
riving the keying signal from the horizontal driving circuits.
By using a keying signal it is possible to obtain greatly improved d-c restoration.
Keyed circuits can be made very fast
with low distortion and high immunity from noise.
They operate
satisfactorily with signal levels much lower than for the sim--
30
pie restorer circuits (7).
The operation of the clamp circuit may be understood :.ore
easily by reference to the idealized equivalent of a keyed d-c
restorer or claap circuit shoLn in Plate IV.
ated, or closed, for
They key ia oper-
n interval of time corresponding to the
width of the horizontal driving pulse.
The horizontal driving
pulse width is approximately 10 per cent of the horizontal
cycle or 6.35 microseconds.
When the key is closed the output
voltage goes to ground potential.
A charging or diacharging
current flows through C liaited only'by R.
so that before they key is opened,
C is small
enough
it becomes coapletely charged,
and the current through it has drcrdded to practically zero.
C
now possesses a charge representing the diffrence between
the signal voltage and ground.
After the key is opened the
charge oannot change since no path exists for the current to
flow.
The signal is transaitted through C as if it were in-
finite in size.
when the keying interval again returns, the
signal may be at an incorrect level, and the charge will be
changed to agree with the new difference between the input
voltage and the correct output voltage.
If the level, however,
needed no changing. no current could flow into or out of U.
This keyed circuit thus reatores the
si-c
component by holding
the eignal during the keying pulse at a fixed voltage which
may be considered the d-c axis.
The signal extends always in
one direction from this axis, end has a d-c coaponent exactly
as in all d-c restorers or elaapers (7).
In the light of the foregoing analysis the operation of
PLALIATION
01?
eLATL IV
Fig, 6.
Idealized equivalent of a keyed d-c res or
or clamp circuit
Fig. 7.
Simplified diagram of the doable keyed diode
clamp circuit use in the monoscope ca,716:0a.
Keying pulses are derived from the grid and
plrx te circuit of the horizontal pulse amplif
32
PLATE IV
Fig.
6
R,
Fig .
7
the monoscope clamp circuit may be discussed.
The key is closed
at the beginning of the horizontal retrace period which corres-
ponds to the leading
e
-a of the horizontal driving
pulses.
These driving pulses vhile employed as the keyin g signal are
simultaneously applied to the control grid of the
ing pulses.
Zn1
as
blaz-
The grid of ie, elate V, is now at zero signal
level or ground potential ana this, as previously stated, is
the clamping action.
blanking pulse,
ing.
Just prior to the end of the monoscope
the key is opened and the grid of 72 is float-
The remainder of the blanking pulse, therefore,
falls
on a predetermined point on the grid voltage-plate current
characteristic of V6.
This portion Of the blanking pulse there-
fore falls on the same point during successive cycles of hor-
izontal or line frequency.
In other words, undesirable low
frequenoy chances in preceding amplifier stages are removed by
resetting the level at the end of each line.
he desired low
frequency characteristics of the picture aignal are at the same
tie restored
by resetting the base of the blanking pedestal
at a fixed refereace level at the end of each line.
I]xcept for the difficulty in closing
at the proper time,
and opening the key
the keyed circuit fulfills
for a satisfactory clamp circuit.
the requirements
The advantages associated
with a keyed clamp are that d-c level at which the blanking is
held can be easily adjusted, and that the same circuit eill handle either positive or negative polarity signals.
it is of.interest to note
actual circuit is of for value.
that the capacitor, C84 in the
This keeps the time constant
LXPLUIATION OF PLATL
Actual
circuit
V
diagram of the muuoicope camera
i.
35
PLATE
V
VI
V'S
6AC7
PICNIC: ANIL.
6AC7
menace
1,49
Wt.
af
ve
GAC.7
0/C724211
cLAMP.E
R.14
/V \
MONOSCOPE
y ID
RCA -2FZI
a
R -7
11.
5
A.
0
R-47
C.41,
/es
A
r
I
't r
.-,ww,
4
C4113
T.3
6-24E.-
IA
C.37
I
or
--a
T__
I
utx(
C3
---11ruree
M
1.1
If
2....4
10000
(
..00n
,
OCN/
3
:;
:,..L40
...II'S°
J
F-
6.3 v
?AK D
C-
-4
-
V
'
AND DISCHARGE
4-2S
V 13
65N7-(-_,T VERT. S.T,
n
Ir
e
.
'lc.,
t. 4
l
tr,
,,.,20.,
APJ
CI'Y
4
N1 'Z.14-A
P
OWL
S933113
'e
6 ',..
MON
44107:?...,-E.A.:0
,cagan
HI*
VERT lue_SE AHR:_
5
5
Is
Vilr IWO
-*-la
f
'fcr
r
TT,
ALANKING
ASAC
28
nOn.,Va.
evrmar
A:5711
,9
V.13
*
F.1
z:
Ls AwsP.
nal
faxW%
a
h
I
7--11
P
t,
ea
11IVRLeCIA.
C2
DI
.12
ell,.
tow
)
s
011.
L tAriT
,
TE
0,
salTnhc.a
vows.
4
L
It
0
s-e
t
1 GO
FtE
Fig.
8
jr4o
MT%
No-,
yrlTf
'T
619213
1001 f
.,
wai...4_11 IN
:*P3 I1Anal ,ALucs
car
inaT
stt:12_,;:taty.aa,
in-,
04.101
CO%
T4 zeIttim*.deo..
c DWG% IS .4.Alf
idalLGarim&T
TAtf
0117704.0"
of the circuit to a suitable low value with respect to the
ation of the keying pulse.
.ur-
The low frequency response is not
deteriorated during the picture siEnal interval as the open cireuit resistance of the diode reaches a high value, end the new
RC value
1.3
of the proper order to pass these frequencies.
The Double Keyed. Diode Clamp
In the aotual circuit shown in the schueetic diagram, Plate
the switching action is accomplished by keying two dipdes by
means of the horizontal driving pulses.
These driering pulses are
180 degrees out of phase and are obtained from the input end. Output circuits of the horizontal pulse amplifier seotion of
tube, V18.
An analysis of the keying pulse:i, and tic
.t
te
particular
requirements eill be given later.
The simplified diagram of Pla e IV is coneistene with the
actual circuit.
The two diodes are driven through two condensers,
01 and C2, and are
connoted by two resistors RI and
R2,
the
common point of which is grounded through some voltage E for
supplying the bias to the amplifier.
end
R2c2, are long coapareti to the
For an-analysis this circuit
as shown in Plate'Vlo,
The time oonstants R101
duration of the horizontal
isbest redrawn
in bridge form
The circuit and letters deeignations are
the same as in the previous figure, except
that the two pulses
are indicated by PI and 22 with the center point of the two
pulses at ground.
opposite polarity.
The two pulses are equal in this case, and ef
In the actual circuit RI and. R2 are
2E,000
Cl
1m3,
9,11
C
are .01 mierofarads, and
C is
330 mio ofarads.
During the pulse time the diodes are driven into eondnotion
by the positive pulse on the plate of D1 and the negative pulse
A ourrent i1 flows through the diode and
on the cathode of DZ.
the capacitance Cl and C2.
A ourront 14 will flow into or out
of C until point 1 is brought to an equilibrum voltage which is
the clamping level.
Between pulses a current 12 flows through
Cl, C2, R1 and RZ slightly reducinc
the charge on G1 and C2.
The clamping level or output voltage to which point 1 is
brought during the pulse time is equal. to
IL
under the conditions
of equal pulse amplitudes, equal resiatanoes, and similar diodes.
The voltage
to ',hich
condenser
C
is brought during the pulse time,
is charged or to which point 1
depnd
upon the a-c an
voltage applied to the diodes at points 3 and 4.
are shown in Plate VI.
ay way of illustration,
d-c
These voltages
the pulses are
shown as 20 volts peak to peak, and of 5 percent width, whioh
would give they a peak value of 19 volts from the d-e values rumoured at points 3 and 4.
During conduction time points 3, 1, and
4 are all approximately the same instantaneous potential, which
is the clamping level.
The axis or d-o voltage at 4 is then min-
ua 19 volta and that at 3 is plus 19 volts from the clamping
level.
The clamping level is therefore midway between
voltage at
tv.een these
and. 4.
If R1 and R2 are equal,
1.0
is also midway be-
voltages, and equal to the clamping level..
pulses are unequal,
the d-c
the elamping level will equal
If the
whenever
12LANATIOS OF
Fig. 9,
'
T1
ouble keyed diode clamp eirouit redrawn in
bridge for4 to simplify aaalysis
Fig. 10. Clamp oirouit designed to supply grid bias for
the following amplifier stage
39
PLATE VI
,
Et
19 v
_
20v
E,+20v
1_
_L_
Fig.
4
RA
Fig. 10
Et+ !gv
-
-
40
R1
Clamp Level
=
Since
=
1
Then
C.L.
+
4
-
-f-
=.
2
÷
-
1.41
+
1
P1
1.
;-;
11 21
or
Via
C.L.=
Then
If the resistors are unbalanced
C,
ttnd G.
should be very
large, or unbalanced so that
he two keying signals are constant in amplitude,
may be obtained in effect from the diode oirouit itself and
tAle
voltage
eliminated, point 2 being grounded.
3uoh an
arrangement is shown in :elate VI, where the °enter portion of
the R1R2 combination has been replaced by a potentiometer.
There is a point 2 on this potentiometer vIlich has the same
41
potential aa the olamping level.
It is the junction of the now
hypothetical resistors 41 Lnd R2 which have the same ratio as
2.
and.
If soma other point on the potentiometer ,le ground-
voltage will exist between the
aria
and point 2, since a
In the example shown,
d-c voltage exists aorose 41 and 42.
negative voltage exists at point 2, since the positive portion.
of
R,
ground.
Any part of R1, R2 may be grounded, except
that as the grounding point leaves point 2, an inoreasing por-
tion of the pulse voltage also appears on the arm*
A resistor.
RA should therefore be ineerted between the arm and ground in
order to avoid loading the pulse. (7).
/eying
The
aocrate tLainz of
importance.
Requirements
the keying pulsee is of obvious
This requirement,
uits external
Sussed.
.Pulse
however,
to the monoscope camera
is fulfilled by cir-
nd will not be dis-
Incorrectly timed pulses to the keyed circuit are re-
vealed by a strong dark or light horizontal streaking effect on
the kinescope which varies erratically.
The pulses must, of course, be synchronous with the signal.
The front edge of the keying pulse shoald occur approximately at
the beginning of the portion or
the monoscope derives
which
is'
the signal to be clamped.
since
the keying signal and the blanking pulse
clamped from the same source, this requirement is
automatically satisfied.
The keying signal should end well be-
fore the end of the blanking pulse.
A tolerenoe is allowed
here, such that under no conditions vill the key last beyond
the signal re
-ranee level.
The keying pulses must elso bo large enough so that under
maximUm signal swings, neither diode will conduct between keying
pulses..
It is apparent
that even though the pulses place a
largo blocking voltage on the diodes
the signal also swings
the other element of each diode, and eill decrease at least on
one of them the bias provided by the keying pulse.
Greatly increased noise immunity is given by the keyed
restorer, since noise ooeurring between the keying pulses has
no effect on the circuit.
Since the keying pulses are apprcet
imately 10 percent of the horizontal cycle, the circuit is
effectively improved by a factor of 10.
TH
HORIZONTAL DEFleeCTION
CIA
CUITe
Horizontal .eulse Amplifier and Sawtooth Generator
The horizontal driving signal, derived from the aynchro-
nizing generator, is a square pulse of negative polarity, the
duration of which is 10 per cent of the horizontal cycle or
microseconds.
fe.;5
Its peak amplitude is from 3.5 to 5 volts
held within a tolerance of .5 volts.
The driving pulse is applied to the grid of V18, a 631,7,
uaed as the pulse amplifier.
The time constant of the grid
circuit, C30 end R95, is long (10,000 microseconds) compared
to
the
grid,
duration of the horizontal driving pulse.
As such,
leak bies is developed and the signal is effectively
Clamped at ground potential.
The clamping action of the air
ouit is a result of the low etatie grid reeistasee of the
vale?,
which can be approximated as 1000 microseconds, and the coupling
eandeneer changes quickly to a value equal to the aaplitude of
the input signal above its
ac
reference level.
The bias on the
tube is then zero and the signal is said to be clamped to Ground
potential.
At the time of the next negative pulse the time con
stant of the Grid circuit is, of course, its original value of
10,000 micro-seoonds, aad the waveshape of the negative pulse is
preserved.
If the amplitude of the input signal is above 4
volts the tube is probably driven into out off during the negate
ive pulse since
Ch
80
the of cut off)
(at
volts
The input horizontal driving pulse is also fed directly to
one section of a 61M, which is used in a keyed diode clamp cire
cult in the video amplifiers.
A voltage diVider in
the plate
circuit of the input section of V18 supplies a signal of opposite
polarity' but of
OV:
1
amplitude to the other diode section of
the eHee
The total output of the pulse amplifier sections of Vi8 is
fed to the second section of the
62147
which acts as the dis-
charge tube for the sawtooth generating circuit.
The peak to
peak amplitude of the driving pulse from the pulse amplifier is
20 volte, ehich drives the discharge tube into full conduction
44
duriag the driving pulse period.
ing
the interval between the
The tube remains out oft dur-
driving pulses.
Glamping is not had in the grid oircuit, a
case, due primarily to two reasons,
in the previous
The 2irst is that the time
durieg which the signal goes positive col-resads to 6.4 micro-
con de rather than 57.1 microseconds, since the waveform has
been inverted by the pulse amplifier.
To determine whether
°Lmping action is present
the
must be compared to this.
However, the resistance usei to cal-
time content of the grid circuit
culate the RC product is no longer merely the etetio grid resistance of the tube, but
or the drivingsource,
mist else include the internal impedances
iiich in
sistance of the pulse amplifiee.
this case is the plate load re-
This consideration could be
neglected in the grid circuiteof the pulse eaplifier since the
internal impedance of the driving source is only the characteristic impedance of the coaxial cable.
Thus the hC product, as
determined during the time which the grid is positive,
leis
in-
.
creesed greatly and the time to which the RC product must be
compared has deeecaeed.
the grid. circuit during
The result is a long time constant in
the period of the positive driving pulse
end no clamping action is obtained.
The savitooth generatin g. circuit actually produces a trap-
ezoidal voltage waveform.
This is necessary to produce
the saw-
tooth of current in the inductive-resistive circuit of the de-
flection coils,
The mathod employed to develop this voltage
waveform is conventional and is based on the gradual aoeumulation of charge on a condenser, C32, following by its rapid dis-
charge.
The output
volte
is
then taken across the condenser
and a .peaking resistor, R126, and applied directly to the grid
of the horizontal output tube, V17.
The horizontal width con-
trol is incorporated in this circuit by varying the potential
to
which C32 charges.
This potential is controlled by the set-
ting of the arm of the potentiometer, R92, since the potentio-
meter is part of a bleeder across the to
voltage power supply.
R92 will also control to a certain degree the horizontal linearity.
The Horizontal Damper Circuit
An analysis of the horizontal damper circuit will
undertaken*
The actual circuit is
simlified
to a
w be
fundamental
damper circuit: shown in Plate VII, Fig. II, and reference will
be made to this and to the associated waveforms in Plato VII,
Fig. 12.
The tubes, VI and V2, correspond respectively to
tubes, 717 and 118, of thc actual circuit diagram.
The initial assumed condition will be that V2 is not con-
ducting and that VI is eondueting but with an absence of signal
on its grid.
If a linearly increasing voltage is
on the grid of V1, the plate current,
nee with the grid signal.
i,
ow inpressed
will rise in accord-
This plate current flowing through
the horizontal deflection (soils* Lh, builds up an assumed pos-
itive magnetic field, the
respect to time.
to give
magnade
of which is increasing with
When the magnetic field strength is such as
the desired deflection,
the
plate current of V1 is cut
off by driving its grid into cut off.
This time is designated
1ATION OF PLAT Z VII
Fig. 11.
A fundamental damper circuit diagram employing an inverse power control tube as the
resistaaee. The direetion of eleotron
current flow is assumed
Fig. 12.
AssoeiaXed current and voltage waveforms of
the fundamental damped circuit
47
PLATE VII
8
eg
A
Fig. 11
0
eLRET-Lci1/4
1
T
0
d
= L
e=
Fig. 12
i$R
0
,4
48
by the letter B in Fig. 12.
iitiatc
e.
A period of energy reversal is now
tuned plate circuit,
in the
retrace period.
LhC, corresponding to the
Thus the magnetic field surrounding Lh collap-
By virtue of this induced
ses, inducing a voltage in the coil.
the polarity
voltage the coil now appears as a voltage source,
being such as to drive current in the same direction as it was
originally flowing.
This current is designated as
necessarily go to charge the capacity
in accordance with the
maximum when
large.
and must
since V1 is no longer
The voltage across the condenser is now increasing
conducting.
is also a
C
i
i
formula
equals zero.
maxim
at this
e,=
1/C
f idt,
and reaches a
The voltage appearing across Lh
time, since the expression di/dt is
The actual peak value of voltage across the deflection
coils is so large as to warrant its omission from the waveforms
of Fig. 12.
The potential energy, now stored in the condenser,
is released in the form of a condenser discharge current,
-i.
has increased to a value which is approximately 70 per cent of
the original value
of il at point B.
This decrease is a result
of the power loss that is inevitable in a tuned circuit with a
finite Q.
time is now
Since the rate of change of current with respect to
zero the voltage across Lh is zero and the waveform
representing ce is seen to cross the zero axis.
The system has
now cocepleted a complete cycle of energy reverse' or one half
cycle of free oscillation.
Again the magnetic field collapses and a voltage is induced
in the coils such as to substantiate the current
-1.
This ne-
cessitates that the coil, now appearing as a veltage source,
have a polarity such that point
1
is negative with respect to
Thus the plate of the inverse power control tube,. V2
point
becomes positive bhd the tube will caldnet.
sistanceof V2 now shunts
fie
and the circuit is highly damped.
a result of the shunting effect
is damped cut at point 4«
Lh
0
and. V2
The low plate re
of the tube, free oscillation
The current that now flows through
as a result of this action will be called 12.
This
current is forced to devrease in any desired manner by controlling the plate resistance of V2 with a proper signal voltage on
its grid*
At point A the tube Vi is simultaneously driven into con-
duetion by its control grid voltage*
The current that flows
through the deflection coils is therefore the summation of the
two currents, iz and.
i.
These combined currents result in
larger and linearly changing total scanning current,
Thin
i
summation current, ij, produoes the field which causes the for
ward trace of the horizontal sweep.
Its characteristics deter-
mine the amplitude and linearity of the horizontal deflection.
The results obtained with this type of damper circuit are
superior to that had 'when a conventional diode daapeT'is used.
Apparently better utilization of available power is secured and
certainly better control of the deflection linearity is provided
by being able to control the eifective damping resistance.
2late VIII
oomparison.
shos
the aetual horizontal damper circuit for
The modifications and refinements aade to WAO fund-
amental damper circuit will now be justified.
.
The insertion of the coupling transformer, T11
week
the
ZULANATION
Fig, 1Z,
O1'
2LATL
11111
.Atual circuit of the monoscope horizontal
damper,
ffC" is the lumped coil uld circuit
capacities in shunt with the deflection
coil. R126 &rid R102 are linearity adjustments
for the horizontal sweep, and control the
bias and Erid voltage ',aveforms respectively
of 716 the inverse power control tube
51
VIII
PLATE
A.C. L OAD
di*
C.
AP-
-r
.S!
V16
V1T
L
HcIZi
(11(ff-11C-1
V
N
_
4PAAAAr
COILS
V
L N
GRID
OF
PULSES
T3
MONoSC. Qp
TLj
si6
Fig. 13
e.
52
power tube,
117, and the damper tube,
T16 allows
the dumper tube
This in turn .laminates the necessity of opere
to be reinverted.
eting the cathode of V16 at a relatively high potential above
ground.
Thus it is u
eoessu.ry to use a special heater voltage
supply trunsformer.
The design requirements of the transfer=
critical (3).
T111.,
ar
quite,
First the transformer should have a primary and
secondary winding of low induotance and low distributed capacity.
This is aeoessary if the natural resonant frequency or
the horizontal sweep.
It is desirable that this frequenoy should
be high since it insures that the half cycle of free oacillation,
which results in the retrace of the beam, will be ohort.
A high
ratio of retrace to scanning velocity is consequently obtained.
The transformer, Til, should eleo have good trelueney end phase
oharacteristics.
It should be designed for the frequency renge
between the fundamental frequency of the deflecting current
and at least the tenth harmonic of this frequency.
It has been
found (5) that magnetic deflection amplifiers dive satisfactory
performance when the fifteenth harmonic is transmitted And may
serve adequately when only the tenth harmonic is included.
For 525-line scaaning with 30 frames per seoond, the transfertier
should be designed for the frequency range between 15,760
and 1604000 cycles.
The
capacitor,
C3,. and the resistors, R100 and R122,
fora a differentiating circuit across the output of the transformer, Til.
This provides the correct sawtooth control vol-
tage on the grid or V16 neaeeeary to obtain the desired
current change of 1L.
The adjustment of 12, and thus the
liner-
ity or the deflection, is accompl.:.ohea by varying the bias on
the inverse power control tube by means of resistor, 1a0L.
;,pproximately 12 percent of the hair sine wave negative
pulse voltage appearing across the horizontal deflection (toil
is taken from the junction of R120 and. R119.
to
he monoscope
This is applied
tube control grid for the purpose of blanking
out the picture durine.the: retrace time.
In this manner, the
fixed reference level is established on which the clamp circuit
of V9 is set at the end of each line.
clai7lping
If this were not done the
circuit would. operate on some erratic transient voltage
generated
the beam swept the pattern electrode on the retrace.
The resistor, R103, is the horizontal centering control.
It supplies a steady d-e current of controllable
magnitude to the deflection coils.
trol,
polarity and
By the adjustment of the con-
the beam can be centered horizontally on the pattern
eleotrode.
;lie
method employed to obtain the large current
;:e-
quired fur the horizontal centering control was to use the entire reeletanee of R103 au the ground return for the 40 and b0
microfarad filter condensers in the low voltage power supply.
R1OZ was then bypassed with a 600 microfarad condenser to provide adequate filtering of the
.i.ictually
(1.-c
deflection coil current.
R103 is a 20 ohm variable copper alloy resistor, so
that a condenser of low voltage rating can be used.
Ve:eTICAL JLPLZUTICN CIRCUITS
The vertical driving pulses, derive
from the synchron-
izing generator axe applied to the grid of V14 through the inJ4.
put jack,
amplitude.
pr
The driving pulses are critical es to width end
The duration of the pulse should correspond to 4
cent of the vertical cycle and the amplitude may vary,
within a tolerance of .5 volts, from 3.5 to
L
volts.
The vertical deflection circuits are essentially the
same as those employed throuehout the horizontal deflection
system.
Therefore, only a brief description is eziven in order
to avoid repetition.
In general,
the circuit constants em-
ployed will differ because of the difference in time between
a horizental and vertical oyole.
cy of the system also preeludes
The lower repetition frequenthe use of a damper circuit
across the output tube, since the transient voltage developed
across the coil is greatly reduced.
Amplification of the input signal is obtained from the
first triode section of V14,
amplifier.
a,
67L7 employed as the pulse
The RC product of the grid, circuit, C22 and R63,
is notably large, since the repetition freeuency is but 60
cycles.
The second triode ecction of this tube is utilized
as a sawtooth generator.
The amplitude of the sawtooth wave-
form is determined, by the height control, R69.
The two triode
sections of the tube, V13, are connected in a cascade coupled
amplifier circuit to increese the sawtooth vo)tege to the
proper driving level for the output tube, 112.
Both sections
of this output tube, a 63N 7, are connecte1 in parallel and
coupled to the vertical deflection ooil through the transformer, 110.
One half section of the
tube,
V11, is used as a vertical
feedback amplifier to improve the linearity of the sawtooth
waveform.
The grid and cathode section of V11 is coznected
through the capacitor, C27, across the resistance, N79.
This
resistance is in series with the vertical deflection coils.
The plate of this tube is connected in parallel with the plate
of the input section of V1Z.
The variable resistor, R81, is
part of the grid leak, and determines the operating bias of
the
tubes
This is the verticA. linearity control and has
effect over the ''hole raster.
Tlii;
I
LOW V0ITAGE
The low voltage power supply was constructed as a composite part of the monosoope camera.
Alile this feature may
be considered undesirable from the standpoint of the stray
magnetic fields introduced by the power transformer
it was felt advantageous
to have a
and,
choke,
self-contained unit.
1,11
precautions were takea to insure the ezeclusion of the fields
produced by the power supply from the menoscoos tube and from
the deflection circuite.
The specifioations of the
supply are given as follows:
or
input, 110-120 volts a-s 60
cycles; output voltage, 280 volts regulatS4
(1-.0;
maximum load
current, 250 milliamperes; power consumption, 75 watts.
No particular difficulty 13 eacountered in constructing
56
a power supply to meet these requiremente.
However, a dis-
cussion of the theoretical and pran.ical design considerations
is of interest.
he necessity or voltage regulation for
the d-c source
should be apparent when the ultimate purpose of the monoscopc
camera is considered.
Obviously every refinement should be
taken in the design of this equipment to insure the perfection
of the test pattern gettera.ed.
koor regulation would. react on
the deflection circuits resulting in linear displacement or
a change in picture size and on the video amplifiers oausing a
variation in signal strength.
In addition to supplying a
staple voltage under change in load,
the inherent property
of a voltage regulated supply to practically eliminate ripple
is also of marked importanee.
the
theoretical aspects of the voltage regulating section
will first be eonsidered.
14,
Reference is made to 21Lte
L,
hioh shows a generalized circuit of a degenerative or
cathode follower type of regulator.
2.ssuming ideal tube
characteristic's it is found that the output voltage can be
expressed by the following equation (1)
Ei(R2+
[
Introdueini,' the
(RE + rp2)
rp::,,, )
e-
(WI +rpi)
ulu2RrEa
+Rin(1+sa!
approximations that
J.%
az >.) 1
114141121c
"
Ail(a2
r2e)
11
Fig.
EXPLANATICE OF 2L4TE IX
Fig. 14.
Basic circuit diagram of a degenerative type
voltage regulator
Where:
El is the rectified and filteed direct
voltage to be regulated.
Zo is the regulated output voltage
be is the negative bias voltage required
for TT2
R1 is the parallel resistance of load
circuit including bleeders, control
potentiometers, and actual load
Re is the adjustable control for E
11,
.t-te
is
the load
is the voltaGe from grid tap to
ground;
of
Fig. 15.
.4.
representing a percentage
f.ctual circuit of the regulator section of the
monoscope power supply
58
PLATE IX
Fig.
V 6AS7
10
2
14
70
L
vv
10
10
riAA'
470
Ei
OFD
'27C
470
I<
V6
K
I
V:
1i`- 651_7
M FD
VR 150
r-^AAN)14-65.7
270C
V
VR 150
750C
473
K
LI
MED
Fig.
15
L,19
el+ rpi)
itilye1iu4
then the equetion will. reeuee to the form
=E
This relationship is an important design equation Ir-
respective of the fact that it predices perfect reguletioni
From an inspection of the equation several features of the
regulator become evident.
The first is that smooth (lent
of the output voltage can be obtained by manually varyine the
value of E0 or A.
Reference to the actual circuit diagram,
Plate IX, rig. 15
shows that this control is incorporated
through the use of
v
470,000 ohm potentiometer in the grid
of the second amplifier tube.
And,
This is a front panel eontrol
through it the output voltage can be varied from ZZO to
ZOO volts without the loss of regulation.
The range through which the output voltage may he varied
is
limited.
Obviously, the output voltage is the difference
between the input voltage end the drop across the pass tube,
which in turn is
current.
a,
functien of it
control grid bias end pieta
One limit is therefore reached ehen the amplifier
tube is biased to out off resulting in zero bias on the pass
tube.
The output voltage will then be at its maximum value
and will be determined by the load ourrent,
From the tube
characteristics of the peas tube, the voltage drop across it
can be found by finding the plate voltage required to sueeort
the deslred load current, whieh will be its plate current,
under zero bias conditions.
It may be noted that when the
amplifier tubs. VT2, is cut off its
its
resistance is ia-
finite.
Thus the equetion expressing the output voltage trans-
ends from that of perfect regulation to
=
RIP].
rp1
and the output voltage is now a function of the input to the
regulator.
The other
lie:lit
is reached when en attempt is made to
adjust the output voltage to
the voltage across the
very low value.
a
pass tube is high.
is required on the pass tube when
only to that of the bleeder.
suvly
or VTL.
Thus a negative bias
the output current amounts
However, the output voltage must
this bias, which is developed across
the plate potential
In this cz:se
112
in addition to
Hence it becomes iereossible to
reduce the output voltage below a certain value determinerl by
E,
bleeder load, VT1, and VT e characteristics.
The choice of the pass tube, 7T1, is an important eonsider-
ation since the characteristics of the regulator are essentially
determined by it.
The recently introduced
tiA37,
a twin
triode,
was chosen, since it was expressly designed for this service.
Their extremely low plate resistance allows the use of a lower
voltage drop across them, end thus a higher output for a riven
input voltage.
The heater cathode rating of 300 volts is ed-
vantageous in eliminating the necessity of a separate filament
transformer.
However, a separate filament transformer wee ee-
played in the power supely constructed, since the 6e37 is En
expensive tube.
Among its other advantages are its extremely
high current handling capacity.
Previously four 2e3ts vere
61
connected in parallel az pass tubes to supply a load curaent of
2.5.0
milliamps.
One &AS? with its triode sectious in parallel
will auffice.
Standard practice dictates the use
aad plate circuits when
current opacity.
Li3
ssors in grid
a
Libes are paralleled :or greater
Their elimination results in highly unatable
or erratic, operation.
It is preferable that these suppressors
be mounted with one end directly supported by the tube socket
lug. to which it connects.
Resistance values of 10 to 50 ohms
for the plate and 100 be 300 ohms far the grid prove satisfactory.
The circuit diagram, 2late II,
high
reu
lifier.
twizi
2i.
15, shows a
tL7, a'
triode, uaed in a two stage dire et-coupled amp-
The grid of the first amplifier tube should receive as
large a perceatage aa possible of the total fluctuation present
on the output,
Variations in the output voltage are developed
across the 7500 oh
resistor in the cathode of
V.
Th e 1 mi-
.
orofarad coupling aondenzer represents a low inpedanee path
for high and medium frequent:31es.
Attempts to teed back a
greater portion of the output voltage by increasing the value
of
Li
condenser will result in erratic operation or osoillat-
ions
TEE
1G 1.70
AG L
WER StWLY
The high voltages necessary for the operation of the mono-
aeope tube are obtained from a self-contained high voltage power
supply.
The high-voltage power supply is conventional in that
it consists of an iron-core stop up
transformer,
energized from
the power line,
end a rectifier circuit with a :eaothing
half-eave rectification as obtained using a
'.;e
and the filter
circuit consists of a 1500 henry choke, T9, and two 1 mierofarad
condensers, C19 and
the circuit,
The filter is in the negaeive aide of
k;e0.
the positive side of the high voltage being ground-
ed so that the signal plate of the monoseope tube may be at
ground potential.
This placed the heater and cathode of the
2.X.2
at a high potential and neceseitated the use of a high-voltaee
socket and a heater transformer with a high voltage rating.
flowever,
rasons in that
this system is desirable for safety
the high potentials are made less accessible.
It is also ad-
vantageous in that the pattern electrode can be eenneeted di-
reetly to the grid, of the following video amplifier without a
high-voltage blocking condenser intervening.
Fairly elaborate filtering of the high-voltage power supply
is necessary since the ripple voltage must be kept considerably
smaller than the video signal generated.
of 100 millivolts is about
the
AnIce a video signal
maximum obtainable under usual
operating conditions, the ripple voltaes should be no more than
Millivolts and preferably be no more than
I
millivo
The required degree of filtering is echelveu 07 the use of one
half of a 61,7, V11, as
a
voltage reguletine
ating type circuit is used, with
with the output voltage.
till
tube.
A degener-
merely inserted in series
The regulating aceion of
it
is res-
tricted. in that the voltage fed baok to its control grid is
through an
'kW
network.
If the d-c level of the output voltage
changes it will not, therefore, be corrected by the regulutor
63
tube.
This, however, is not perticulerly important since the
load on the power supply, ehieh
essentially the beam current
of the nonoscope tube, is eenstant.
Any ripple voltage appear-
ing in the output, however, eill be fed through 021, a 2 mi-
crofarad coupling condenser, to the grid of V11 and thus reduced
in magnitude through the regulating action.
The enee, V11, is
also bypessed by a 100,000 ohm resistor for safety reasonue
The output voltage
1500 volts
and. it is
()"
the power supply is approximately
capable of eupplying 5 milliamperes.
All
voltages for the monoscope tube are obtained from a tapped
bleeder resistance across the output of the power supply.
cause the beam current for the
21.'21
Be-
is small, large bleeder
current is net required.
A 991 glow tube is shunted
control potentiometer.
the control
:rid of
aerosa
1t60,
The potentiometer arm is eonnected to
the monoscope and
the positive side of the
petentiomeeer is connected to the cathode.
against excessive be
the beam current
This is a preoeution
current should the output voltage of the
power sup)l7 change, since the 991 providee a constant drop of
50 volts across R50.
The hieh-voltage power supply is ale() used to provide
the
negative hies ter the clipper tube,
'LIST
114,
in the video amplifier.
2eTTIRN ANALYSIS
The correct ieteepretation of the test pattera is
funda-
mental to the intellieent use of the monoecpee oamera in testing television equipment.
The particular test pattera coeteiaed
64
in the
2F2.1-
and refs
is shown in Plate
in the following diecussion.
e i a me de
to this
The pattern is simple, yet so de-
signed that a comprehensive analysis of the equipment under
teat say be made.
The ratio of the pattern's width to height is four to three
in accordance with the standard aspect ratio.
The outside
d'
ameter of the largest circle is, therefore, three fourths of the
width of the paetern, so that when the deflection is edjusted to
this circle, the standard aspect
give an undistorted form to
ratio will be established.
in the center of the pattern there are six concentric cir-
cles, the center of which
is.
lebled 30.
The radial spacing be-
tween the circles is the same spacing that would exist between
300 horizontal lines equelly spaced in the vertical dimension
of the pattern.
If the equipment undertest can reproduce the
ircle
pattern with these central
separate and distinct, then
the equipment is capable of resolving 390-line detail.
The four resolution we
es radiating from the central cir-
cles are calibrated in a similar manner.
The resolution lines
are of equal eldth in both the horizontal and vertical direetions.
Thus the total number of lines whieh can be contained
in the
;idth of the picture is greater than that etioh can be
contained in the height of the picture by a factor equal to the
aspect ratio or 1.33.
so that
The center line of each -wedge is dashed
the resolution corresponding to any radial distance is
sore readily determined.
The break in the center line occurs
at the same distance from the een!,er as
the arc of the resol-
LX2LANATION OF PLALL
Fig.
16.
Test pattern which i
transmitted 14 the aonoscope camera using a 2721
act
zj
0)
67
ution numbers, and the distance between the breaks in the center
line corresponds to a change in resolution of 50 lines.
The
calibration numbers should be aultiplied by 10 to obtain the
number of lines resolution.
The upper vertical and right hand
horizontal wedges vary in spacing from 300 lines at the outer
ends of the wedge to 500 lines at the inner end.
The
lower
vertical and left-hand horizontal wedges vary from 150 to 350
lines.
In all
four wedges,
varies linearly along
a
the number of equivalent lines
radius.
The point in the wedges where
distinction between individual lines just disappears, indicates
the
resolution of the system under test
In measuring resolution, distinction must be made between
the ability of
the equipment under test to resolve detail along
a horizontal line is necessarily measured by the vertical wedges
and resolution along 4 vertical line is indicates by the horizontal wedges.
If the situation should arise where the vert-
ical resolution of the system under test is comparable
to
the
nurser of scanning lines, then a spurious diamond-shaped- pattern appears in the reproduction of the horizontal wedges.
This
spurious pattern is made up of the inteesections of wedge lines
with scanning lines and the resultant pattern should not be regarded as a defect in the reproducing epuipment.
Also enclosed within the inner circle are two wedges set
on a 45 degree angle to the horizontal.
of four distinct sections,
Jach wedge is composed
the difference between each section
arising in the degree of shading.
The variation ranges from
100 per cent bleak to 25 per cent black in equal divisions.
These w
provide
a.
test for amelitude diatortion
video signal.
To the right end to the left of the central wedges are two
vertical rowe of smell rectangles.
The figure 50 above the
right hand column indicates that the top rectangle has a width
equal to 1/50 times the height of the pattern, and is thus in-
dicative of 500-line detail..
The rectanglee in the right hand
column progreseively decrease in width in steps equivalent to
25 lines up to 300-line detail, and similarily on the left hand
column from 325 lines
to
6"7elineee
These two rows of recteme
gles are useful in testing for undesired transients, since traile
ing is eometlues shown up more clearly by the rectangles than
by the wedges.
elow the central wedges is a set of 11 horizontal lines
whose length varies legarithmicelly.
is 71 per cent of the
The length of each line
lehgth of the line above it.
The short-
est line has a length 1/50 times the heieth of the pattern;
the longest line is 1/1.5
ties
the height.
These lines are
useful in obscrvin, defective low frequency responses in the
video range, and trailing in the reproduetion of these lines is
iadiaeUve of
the improper adjuetment of the low-frequency
cumpenseting cireutts.
eeeve the central wedges is an Indian head which provides
a
test of the general quality o' the reproducing system.
This
is especially true eith respect to contrast and average bright-
ness which are moat easily judged on a Pictorial subject.
The largest circle and the circle surrounding the central.
69
wedges provide a test of linearity of scanning in the horizontal and vertical direction.
outline,
If the circles have an egg-shaped
then the rate of scanning is nonlinear, in the verti-
cal direction nhen the axis of symmetry is vertical and in the
horizontal direction when the axis is horizontal.
Linearity
of scanning is also tested by the 4b degree lines and by the
grid squares into which the pattern is divided.
The fine lines
which form these squares have a width of 1/600 times the height
of the pattern and will immediately reveal any orthogonal dis-
tortion in any part of the image.
In each of the four corners of the pattern there are re-
solution we
es.
If the resolution in the corners of the pat-
tern is poorer than that given by the control wedges,
clusion is that spot defocusing is present.
defocusing is inherent
to all kinescopes,
the con-
since some spot
the resolution in the
corners as compared to the resolution of the system is a meas-
urement of the relative defocusing present.
USES OF THE UDNOSCOPE
The raonoscope camera is a coaprehensive piece of television
test equipment and may be used for a variety of purposes.
commercial af?lications,
the
In
television transaitting station may
aaploy the camera to transmit a test pattern during t,arm-up and
stand-by periods.
Station identification can be simultaneously
provided if the station call letters are printed on the pattern
electrode.
Another interesting commercial application is the use of
70
the monoeeope in obtaining a fixed background for studio works
The final signal is
combination or the video signal from an
loonoscope which might represent aotion, and that from the
monoseupe for backgroand.
The
aonoscope camera is also used in many laboratories
and factories to obtain a television signal whioh can be
to test televiaion receivers.
titled
In conjunction with a synchron-
izing generator and a distribution amplifier it produees a
sAindnrd R.IA.A. video signal for use in testing video aseplifiers and picture
tubes.
.eith the
additiorrof an I-F sweep
generator and an R-F generator it produces a complete televis
ion picture signal simulating that received off the air and
receivers under conditions
thus provides a means
eciailfaXent
to
actual use.
Plate
i.i
indicates the arrange-
ment of the necessary equipment.
Primarily
however, the main application of the mnoseope
camera remains in'testing television transmitting systems..
it furnishes the
transmitting station with an always cvailable
source of video, signal of known quality which can be substituted for the studio cameras whenever it is desired to check
or adjust the operation of the following units in the system.
In this respect the monoscope has aided materially in th
advancement of the television art.
...1A4,14:,TION
Fig, 17.
OF 2LATi;
ti
Lquipment arrangement for testing a television
receiver fed from a monoseope camera
MoNOSCO PE
DISTR1BOT ION
CArAERA
A mPLIFIER
R-F
1-F
SIGNAL
SIGNAL
GENERATOR
7
SYNCHRONIZING
GENERATOR
GENERATOR
-
V
IsT
2!4°
^RP
2Ho
sT
DET.
IF
IF
DET.
\Ape.°
2
tii)
VIDEO
73
The wrib1, e:,,teads his sincere
J. _Jdaond :solfe for his
thi;$
thesis,
areciation
to
rotcs8or
aid throughout the preparation of
°wean for his help-
r1 to Instructor Kenneth
ful sugi;estions during the cantruoiion
o..
the eamera.
74
(1)
Abate, A.
Basic theory and design of electronically regulated
eveer supplies. 2roo. IRE 33: 478-480. July, 1945.
(2)
Burnett, C. E.
The monoscope.
(3)
(4)
(5)
RCA Rev. 2(4):414.
Engstrom, E. W., and R. S. Holmes.
Television deflection circuits.
12(1):19-21. January, 1936.
Everest, F. Alton.
Wide band television amplifiers.
11(b):241. May, 1938.
April, 1938.
Electronics.
Electronics,
Fink, Donald G.
Principles of television engineering.
McGraw-Hill, 1940.
(6)
(7)
New York:
Goldsmith, Alfred N., and others, editor.
Television volume III. Prifleeen, E.
Review, 1946.
ac,
Goldsmith, Alfred N., and others, editor.
Television volume IV. Princeton, N. J.:
Review, 1947.
RCA
(8)
Liebmann, G.
Image formation in cathode ray tubes and the relation of flourescent spot size and final anodevoltage. Proc. IRE 33:381-389.
June, 1945.
(9)
Mather, N. W.
Clipping and clamping circuits.
20(7): 11. July, 1947.
Electronics.
(10) Cwen, Robert P.
Linear sweep circuits.
December, 1946.
(11) Reintjes, Francis J.,
Electronics.
19(12) :136.
editor.
Principles of radar.
New York:
McGraw-Rill, 1946.
(12 Ridenour,
Lolls J., editor.
Radar system engineering.
1947
New York:
ACCraw-Hill,
75
(1Z)
(14)
3
Karl R.
Vacuum tubes. New York:
axi 05enbere,
Zworykin, V. K., and G. A.
Television. New York:
A'eGraw-Hill, 1948
;.lorton.
Viiley,
1940.
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