Download Dee. 26, 195o E. FRANK Er`AL 2,535,912

Dee. 26, 195o
E. FRANK Er'AL
VIDEO GATING CIRCUIT
Filed Dec. 8, 1948
ÜNE
2,535,912
2,535,912
Patented Dec. 26, 1950
UNITED STATES
“
ÜFFICE
2,535,912
VIDEO GATING CIRCUIT
Ernest Frank, Yeadon, Pa., and Robert Fred
Mozley, Berkeley, Calif., assignors to the United
States of America. as represented by the Secre
tary of the United States Air Force
Application December 8, 1948, Serial No. 64,138
4 Claims. (Cl. Z50-27)
1
2
This invention relates to gated viedo frequency
amplifiers particularly as used in radar systems.
In radar systems of the type in which short
pulses of high frequencyV energy are radiated and
the time intervals between the radiated pulses and
reflected pulses from outlying targets are meas
ured to determine the target ranges it is fre
gate as a pedestal in the output signal of the am
plifier or for allowing a pedestal of adjustable
height to appear, as desired.
The details of a specific embodiment of the in
vention are shown inthe accompanying drawing
in which
Fig. 1 is a schematic diagram of the gated video
quently desirable to limit the target presentation
ampliñer and
on >the screen, of the cathode ray indicator tube
Fig. 2 shows several waveforms appearing in the
circuit of Fig. l.
Referring to Fig. 1 the video ampliñer com
prises tWo ampliñer tubes, V2 and Va, connected
in cascade. The negative video signal from the
radar receiving equipment is applied to the con
trol grid of tube V2 through condenser C3. This
to those targets lying within `a given range. By
so doing the resulting presentation contains only
the targets of interest andv is,V considerably im
proved by the elimination of other target reflec
tions and a great amount of noise and background
clutter. Automatic gun laying systems for use in
-
military aircraft usually employl such limitation
thus restricting the display to only those targets
signal may be of the form shown in Fig. 2(a) in
within the usable range of the guns, say 5000
l2 and i3 represent reflections from targets. The
time interval between radiated pulses is shown in
which I0 represents the radiated pulse and Il,
`yards'.
20 this figure to be 500 microseconds which corre
'The above described limitation is usually ac
spends> to a pulse repetition rate of 2000 per
second. rThis signal is ampliñed and reversed in
phase by tube V2 and applied as a positive signal
to the grid of tube Va. Since V3 is connected as
a cathode follower the output signal taken from
across load .resistor Re is likewise a positive signal.
In order to gate the above described video am
complished by rendering the video ampliñer which
applies the received signals to the cathode-ray in-v
dicator tube operative for a period of time after
each radiated pulse just sufficient to allow reflec
tions frorn targets up to 5000 yards, or other se
lected distance, to be received. After this time
interval the video amplifier becomes inoperative
to apply signals'to the indicator tube until the
occurrence of another radiated pulso. For tar
pliñer the gating pulse` if’. in Fig. 1 is applied t0
gets up to 5000 yards this time interval is approxi- i
mately 31 microseconds which is a small fraction
the grid of tube V1 which is connected as a
cathode follower and has a load impedance R1.
This cathode follower stage acts as an impedance
changing device so that the resulting positive
of the time interval between radiated pulses
which, for a 2000 pulse per second repetition rate,
is 500 microseconds. In order to render the video
ampliñer operative for the desired interval a rec- -
tangular gating pulse is generated, the leading
gate developed across resistor R1 appears to come
from a source of very low internal impedance. A
low output impedance is one of the Well known
characteristics of a cathode follower stage and is
due to negative feedback of the entire output
voltage. The gating pulse may be generated in
edge of which is coincident with the trailing edge
of the radiated pulse ard the duration of which
any desired` way and applied at any time as re
is made equal to the time required for an elec
tromagnetic Wave to travel to and return from a 40 quired by the use to which the amplifier is to
be put. In the above described radar application
target at the maximum range desired. In Ythe
the gating pulse, as already mentioned, begins at
case of a 5000 yard maximum range the gating
the end of the radiated pulse and, in the case of
pulse would have a duration of approximately
a 5000 yard maximum range, has a length of 31
31 microseconds as stated above,
microseconds. However, apparatus for generating
It is the object of this invention to provide an
and timing the gating pulse is not a part of this
improved gated amplifier such as could be used
invention. The. tube V2 is provided with a load
in the above described application or in any ap
impedance consisting of resistance Rv and induc
plication requiring a high frequency amplifier
tance Li. The> low signal potential end of in
which may be rendered operative for short and
precise intervals of time. The ampliñer has mini- ‘ ductance L1’ is connected through condenser C1
to the cathode of Vi and through diode V4 to a
mum requirements as to the amplitude of the
source of direct potential of +150 volts. The
gating pulse due to the fact that a direct voltage
in addition to the gating voltage is applied to the
anode of the gated tube. Provisions are also
made‘fo‘r either preventing the appearance ofthe
l 5.5
output signal from amplifier V2 is applied through
inductance L2 and blccking'condenser C4 to the
grid of -cathode follower-stage Vs. The grid of
3
2,535,912
V3 is connected to an adjustable contact on poten
tiometer R5 through high resistance R1 whereby
an adjustable negative bias potential is applied
thereto. The diode V5 shunts resistor R4 and
to the sum of the anode voltage V2 and the volt
age of the tap on Rs, with the polarity of the
condenser voltage as indicated in the ligure. In
the presence of a gating pulse the anode voltage
prevents the grid from ever becoming more nega
of V5 is elevated to a higher level during which
tive than the adjustable tap on Rs. A diode used
time condenser C4 charges to a higher level;
in this manner is sometimes referred to as a
however the increase in charge will be very slight
clamping device since its clamps the applied wave
due to the large time constant of C4 and R1
to the potential of the tap on R5.
which, with C1 of the order of 0.02 microfarad
In the absence of a gating pulse on the grid 10 and R4 of the order of 1 megohm, will be of the
of V1 current flows from the +150 volt source
order of .02 second or 20,000 microseconds, and
through L1, R7 and the anode-cathode path of
to the fact that the gating pulse lasts at the
V2 to ground thus placing 2, positive potential on
most for only 122 microseconds, and for 5000
the plate of V2 somewhat less than +150 volts de
yards range for only 31 microseconds.' The gate
pending upon the drop across R1. Also current 15 pedestal appearing on the anode of tube V2 is
ñows around nthe circuit consisting of the +150
therefore for all practical purposes transferred
volt source, diode V4, condenser C1 and resistor
directly to the grid of tube Va. At the end of
R1 until condenser C1 is charged to a voltage
the gating pulse the anode potential drops to its
equal to 150 volts minus the drop across V4 and
original value and condenser C4 discharges to
the normal steady state voltage across R1 due 20 its original voltage. However the discharge cur
to the space current of V1. Since V1 has low
rent passes through the low resistance afforded
internal resistance and R1 has a low Value of
by diode V5 rather than through high resistance
5000 to 10,000 ohms, which appears still lower
R4, thus preventing any lowering of the grid po
due to the feedback action in the associated cath
tential of Vs below the value set by the position
ode’ follower stage, the time constant of the 25 of the tap on R5. Tube V5 therefore in effect
charging circuit for C1 is short and this con
restores the direct component of the rectangular
denser charges very rapidly.
wave on the anode of V2 which could not be
Due to the fact that a positive potential exists
transmitted through condenser C4.
on the anode of tube V2 in the absence of a gat
Since video signal is continuously applied to
ing pulse, as explained above, the tube acts dur 30 the control grid of V2 a resulting video signal
ing this time to amplify and reverse the phase
will appear in the output circuit of this tube
oi the video signal applied to its grid. The re
along the crest of the pedestal formed by the
sulting positive video signal on the anode of
gating pulse. This combined pedestal and video
V2 is applied to the grid of V3; however no sig
signal and its relation to the bias and cut-off
nal appears at the output of V3 due to the fact
voltages of V2 are shown in Fig. 2 (b). By pro
that this tube is biased beyond its cut-off point,
viding a bias potential from R5 of such value
by the voltage from R5, by an amount greater
that the pedestal just exceeds the cut-off point
than the voltage of the maximum video signal
of V3 a slight pedestal carrying the video signal
applied to its grid so that the video signal, in
occurring during the gate appears in the output
the absence of a gate, is unable to cause con
of V3 as shown in Fig. 2 (c). If desired, the
duction in V3. This condition is shown graphi
negative bias may be increased to the point where
cally in Fig. 2(b) .
the pedestal just reaches the cut-01T point in
The application of a positive gating pulse to
which case the video signal without the pedestal
the grid of V1 causes a pulse of substantially
appears in the output circuit.
the same amplitude to appear across resistor 45
The inductances L1 and L2 in Fig. 1 may be
R1. This voltage, acting through condenser C1
of the order of 100 and 400 microhenries respec
and impedance L11-R7, raises the potential of
tively and help to preserve the square wave form
the anode of V2 by an amount equal to the pulse
of the gate and resulting pedestal.
‘
voltage. Since the action of the gating pulse
We claim as our invention:
also raises the potential of the cathode of V1 50
1. A gated amplifier comprising a ñrst amplifier
above that of the anode this diode becomes non
tube and a second amplifier tube each having
conductive thus isolating tube V2 from the +150
an anode, a cathode and a control grid, means
volt supply during the gating pulse. Therefore,
for applying a signal to be amplified between
operating potential for the anode of tube V2
the control grid and cathode of said ñrst tube,
in the presence of the gate comes from the volt 55 means for applying a positive direct potential
ages across- R1 and C1 connected in series. Con
between the anode and cathode of said ñrst tube,
denser C1 should be large enough to hold its
means for applying a positive rectangular gating
charge and voltage substantially at full value
pulse to the anode of said ñrst tube, means for
during the gating pulse. The size of C1 there
applying the gating pulse and amplified signal
fore depends on the resistance in its discharge 60 on the anode of said ñrst tube to the grid of said
circuit which includes R1, R'z and the plate re
second tube, a source of operating potential and
sistance of V2. In a circuit of the type shown
a load impedance connected between the anode
a capacity of 0.25 microfarad for C1 is ample for
and cathode of said second tube, means for con
gating pulses up to 122 microseconds, which cor
necting anY output circuit across said load imped
responds to a range of 20,000 yards. In order 65 ance, and means independent of the signal on
to have the anode potential of V2 follow the gat
the grid of said second tube for biasing the grid
ing pulse closely the time constants of the anode
of said second tube neagtive with respect to the
circuit should be small. This will be realized in
cathode of said second tube by a ñxed amount
the circuit shown because of the comparatively
equal to the cut-oli voltage of said tube plus a
low value of Rv, which may be of the order of 70 voltage greater than the maximum signal applied
10,000 ohms, and the very small capacity be
to said grid in the absence of a gating pulse and
tween the anode of V2 and ground.
equal to or less than the amplitude of the gating
During the absence of a gating pulse and with
pulse applied to said grid from the anode of
the circuit of Fig. 1 in a stable condition the
said ñrst tube.
-
_.
"condenser C1 will be charged to a voltage equal 75. V2A gated amplifier comprising a first ampli;
2,535,912'
5
lier tube having an anode, a cathode and a con
trol grid, means connecting the cathode of said
6
the gating pulse and signal appearing on the
anode of said ñrst tube are applied to the grid
of said second tube, means including a high re
first tube to a point of reference potential, a load
sistance connected between the grid of said sec
impedance having one end connected to the
anode of said ñrst tube and the other` end con UX ond tube and a point having a, negative potential
with respect to said reference point said negative
nected to the cathode of a diode rectifier tube,
potental having a value equal to the cut-olf volt
a source of direct current having its positive
age of said second tube plus a voltage greater
terminal connected to the anode of said diode
than the maximum signal applied to the grid of
and its negative terminal connected to said point
of reference potential, a condenser having one 10 said second tube in the absence of a gating pulse
and equal to or less than the amplitude of the
terminal connected to the cathode of said diode,
gating pulse applied to said grid from the anode
means having a low internal impedance for ap
of said ñrst tube.
plying a positive rectangular gating pulse be
3. Apparatus as claimed in claim 2 in which
tween the other side of said condenser and said
point of reference potential, said condenser being 15 said rectangular gating pulse applying means is
a cathode follower stage.
of sufücient capacity that the time constant of
4, Apparatus as claimed in claim 3 in which
the circuit including said condenser, said load
a diode is connected in shunt to the high resist
impedance, the plate impedance of said first tube
ance connected between the grid of said second
and the internal impedance of said pulse apply
ing means is long compared to the duration of 20 tube and said point of negative potential and in
which said last named diode is poled to conduct
said gating pulse, means for applying a signal
toward said grid.
to be amplified between the grid and cathode
»
ERNEST FRANK.
of said first tube, a second amplifier tube having
ROBERT FRED MOZLEY.
an anode, a cathode and a control grid, means
connecting the anode of said second tube to the 25
REFERENCES CITED
positive terminal of a source of direct current
the negative terminal of which is connected to
The following references are of record in the
said point of reference potential, means con
ille of this patent:
necting the cathode of said second tube through
UNITED STATES PATENTS
a load impedance to said point of reference po 30
Number
Name
Date
tential, coupling means including a blocking con
1,912,234
Willoughby ______ __ May 30, 1933
denser connected between the anode of said first
2,132,655
Smith ____________ _- Oct. 11, 1938
tube and the grid of said second tube whereby