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Патент USA US2406433

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Patented Aug. 27, 1946
2,406,433 '
UNITED STATES PATENT OFFICE
2,406,433
AUTOMATIC GAIN CONTROL
Lowell E. Norton, CollingsWood, N. J.,vassignor to
Radio Corporation of America, a corporation of
Delaware
Application January 28, 1942, Serial No. 428,538
1
9 Claims.
(Cl. 250-'-—-20)
2
My invention relates to automatic gain control
circuits and particularly to gain control circuits
topped pulses thus obtained may be passed
through a ?lter having a fast time constant to
containing no long time constant ?lter. The in
vention is especially adapted for use in systems
such as the rotating radio beam system described
obtain the desired gain control voltage since the .
?lter need hold a charge only for the compara
tively short time between successive ?at-topped
in Luck Patent 2,208,376, issued July 16, 1940,
pulses rather than for the time between the peaks
wherein it is desired to remove certain low fre
quency variations of the order of from 1 to 6
of recti?ed sine waves as would be the case in the
usual gain control circuit.
cycles per second, for example, and to remove,
also, comparatively high frequency variations
which may occur at ‘various random frequencies
lying between 40 cycles per second and 100 cycles
per second for instance.
10
The invention will be better understood from
the following description taken in connection‘with
the accompanying drawing in which
Figure 1 is a circuit diagram of oneembodi
ment of the invention, ,
7
~
In the system described in the above-mentioned
7 Figures 2a to 2d are curves which are referred
Luck patent, an antenna ?gure-of-eight radiation 15 to in explaining the invention,
pattern is rotated at the rate of 60 rotations per
Figure 3 is a circuit diagram of another em
second, in one particular example, and the pat
bodiment of the invention,
tern is provided with a reference mark whereby
Figure 4 is a series of curves which are referred
an airplane may determine its position by means
to in explaining the invention.
.
of a suitable receiver carried thereby. In this 20
Like parts in the several ?gures are indicated
particular system the metal propellers of the air
by similar reference characters.
plane introduced a modulation at the receiver
In Fig. 1, one embodiment of the invention is
having a frequency differing only a few cycles
shown applied to the receiver of a rotating beacon
from the 60 cycle frequency introduced by the
system. The receiver, the front end of which is
rotating antenna pattern. Thus there results a 25 indicated at 9, includes a vacuum tube 10, which
modulation component equal to this difference
may be of the pentode type for amplifying the
frequency and the cathode ray pattern on the
received signal and supplying it through a con
receiver tube tends to “bounce” at this low fre
ductor H tov a cathode ray tube indicator circuit
quency rate.
(not shown) and through a transformer I2 to my
It is an object of the present invention to pro 30 improved automatic gain control circuit. The
Vide an improved automatic gain control circuit
latter circuit includes a detector comprising di
for avoiding the above-described undesirable ef
odes l3 and it and a load or output resistor l6
fects in a rotating radio beacon.
across which appears the voltage c (Fig, 2a).
A further object of the invention is to provide
A vacuum tube ll having a plate IS, a control
an improved automatic gain control circuit which ~
grid l9 and a cathode 2! is connected across the
removes both low frequency and comparatively
output resistor It, theplate l8 preferably being
high frequency undesired signal variations.
connected through a resistor 22 to the cathode or
A still further object of the invention is to pro
positive end of resistor It, and the cathode 2|
vide an improved means for and method of de
being connected through a conductor 23 to the
veloping an automatic gain control voltage,
40 plate or negative end of resistor It. The grid 19
In one of the preferred embodiments of the
is grounded.
invention the signal having the undesired ampli
A condenser 24 is connected between the plate
tude variation is detected and applied to a clip
l8 and grid l9 and a resistor 26 is connected be
ping circuit which clips at a level that varies with
the value of the instantaneous amplitude as de 45 tween the grid !9 and the cathode 2|. The ca
pacity of the condenser preferably is small enough
to make its impedance at 60 cycles per second
large compared with that of the grid-cathode im
grounded grid which, together with a resistor-ca.
pedance of tube IT and the resistor 26 in parallel.
pacitor circuit, is so connected to the detector
that the half-wave pulses in its output periodical 50 The automatic gain control voltage may be ob
tained at the cathode 2| and applied to a grid of
ly cause a flow of grid current during which pe
riod the condenser receives a charge, and at the
the ampli?er tube H1 through a conductor 21 and
end of which the condenser partially discharges.
through a comparatively short time constant ?l-_
During the ?ow of grid current the half-wave
ter comprising a series resistor 28 and a shunt
pulses are clipped off at a certain level. The flat 55 condenser 29. In this case the voltage 21 appear
termined by the undesired modulation. This may
be accomplished by means of a triode having a
2,406,433
3
. sistor 22 may be omitted whereby the voltage m
value, the current ?ow through the resistor 26
will have a wave shape such as indicated'in Fig.
increases correspondingly up to the point where
the grid l9 goes positive with respect to'the cath
ode 2! to cause a ?owofgrid current; It may,
aid in understanding the operation to note that‘
the series connected elements 22, 24 and .26' may
be looked upon as a voltage divider connected
" .
i:
pulses e in Fig. 2a whereby any undesired am
plitude variations (either fast or slow) of pulses
e may be substantially eliminated.
For some applications of the invention the re
to Figs. 2a and 2b which show the voltages e and
ei, respectively. -As a half sine wave pulse of the
voltage e increases in amplitude from its zero
'
l
Thus the amplitudes of the ?at pulses e1 in
Fig. 2b are proportional to the amplitudes of the
ing across resistor 26 is utilized (after ?ltering
by ?lter28—29) as the gain control voltage.
The operation will be described with reference
across the load resistor l6.
2
4 .
.
'- ‘ (i
201. It will be apparent that the use of resistor
.22 will result in _a gain control output that will
‘ require a minimum amount of ?ltering.
I .iThe plate-cathode voltage 62 may be utilized
for gain control in place of voltage e, if desired.
_In that case the ground connection is removed
As soon as the grid l9 goes positive, this point
from thegrid l9 and the plate i8 is grounded
‘cathode end‘ of resistor 26 goes positive mo
mentarily with respect to ground. The gain, con
' trol voltage applied to the grid of tube do, how
plate voltage e. They are combined by passing
them through diodes t6 and {Hand through a
instead. If the time constant of condenser, 25
being indicated by the broken line 3| in Fig. _2q._,
resistor '25’is fast, the voltage e2 will/have
the current ?ow through resistor 26 becomes sub; ‘and
wave shape shown in Fig. 20; if the time
stantially constant to produce the ?at-topped the
constant is long the wave shape of voltage at
portion of a pulse of voltage e1. This current’
flow remains constant until the half-wave pulse 20v will more closelyv resemble that of voltage 61
shown in Fig. 2b.
'
'7 '
of voltage e decreases to a value where the grid
,In
the
embodiment
of
Fig.
3, a multivibrator
I9 is no longer positive with respect to the oath
A!) is utilized rather than the grounded grid
ode_2_l and the flow of grid current ceases. The
, triode of Fig. l. The detector output e is applied
condenser “meanwhile has been charged by the
to the plates of multivibrator tubes 4! and 42
flow of grid, current, and as the applied voltage
through their plate resistors 53 and M. I
e_ decreases, condenser 24 begins to discharge
multivibrator preferably is symmetrical so that it
through the circuit 22, i6, 26.
‘
produces rectangular pulses of the same width
Since the current flow through resistor 26is
atpthe
plates of tubes 4! and 427 as shown in Fig.
- reversed during this short discharge time, the
30 4; at a and b, respectively. These pulses will have
an amplitude that is proportional to the applied
. ever,>i_s alwaysnegative with respect toground
because of the smoothing action ofj‘the‘ ?lter
(‘zap-‘29. ‘ This is indicated by‘ the dotted‘line Dor- 1
tion x of Fig. 2b.
‘ ’
When voltage e drops to or close to zero, the
cathode 2| will be at the same or nearly the
_ same potential as the upper end of resistor 22
and, therefore, will be positive with respect to
the grid l9 asa result of thecharge on condenser
24 whereby the tube I1 is biased to cut-off. ‘Now,
_when thevoltage starts to rise again On the next
pulse, the tube Ii isrstill cut-off so that there is
novoltagedrop in resistor 22 due to plate cur
"rent?ow. “Consequently, the potential on con
denser 24rnever falls .to a value as low as it would
: otherwise, since ‘at plate current cut-off of, tube
common resistor 48 whereby the signal shown .at
' c in Fig. 4 is supplied over the conductor ‘21 to
the ?lter 28—29. It will be apparent that, as in
the case of the circuit shown in Fig. 1, very low
frequencies can be‘ substantially eliminated even
though the ?lter 28-29 has a fairly fast time
In some uses the ?lter 28—29, in
' constant.
either Fig. 1 or Fig. 3,, may be omitted, particu
larly since there is some ?ltering due to tube ca
pacity as indicated at Cg in Fig. l, for example.»
~From the foregoing, it will be apparent that
I’ have provided an improved gain control sys
tem that is effective to reduce or eliminate very
slow signal variations without employing any
' ?lter having a slow time constant. Thus the sys
tem is capable of removing rapid signal varia
} I'Lthe potential across condenser 24 and resistor .50
tions as well as the slow ones.
2,6 in‘ series‘is'increased by the value of the
' potential acrossresistor 22 at a time just before
plate current cut-off occurs.
_As the voltage e increases further in amplitude
during'this second half -wave pulse, the previously
described action is repeated, but the clipping
level'(or ‘point. at which grid current starts to
flow) is proportional to the amplitude of vthe
preceding half-wave pulse of‘ voltage e, Spe
ci?cally, the charge on condenser .24 is propor
tionalto the charging voltage 6, and the result
ing voltage across condenser 24 opposes theiap
plied Voltage '6. As soon as the rising voltage 6
becomes greater than the condenser voltage, then,
I claim as my invention:
'
7
'
1. In combination, an ampli?er for amplifying
asignal having an undesired modulation thereon, .
arecti?er coupled to said ampli?er to produce a
recti?ed output signal comprising voltages pulses
' that have amplitudes determined by said modula
tion, means for ‘clipping said pulses at a voltage
level which varies in accordance with variations
in the amplitudes of said pulses, and means for
60 applying said clipped pulses in negative feedback
‘relation to said ampli?er for controlling its gain.
7 " 2. The invention according to claim 1 wherein
said clipped pulses are applied to saidlampli?er
through a ?lter having a time ‘constant/that is
and onlythen, does current start to ?ow through
the circuit 24—26 so that the ‘grid [9 goes posi 65. fast with respect to the period of the undesired
tive withv respect tocathode 2|. It is evident
' 3.' In'co'mbination, an ampli?er for amplifying
' modulation.
_ that the smaller the amplitude of a pulse of volt
age 6, the smaller the Voltage on condenser1 24
‘ during .the next following pulse, and the less rise
there. is in the voltage of the said next pulsewhen
V the grid current starts to ?ow, and also with
smaller values of e, the anode-cathode potential
‘of tube vl‘l is reduced with a corresponding low
ering of grid-cathode resistance so that clip
pingstarts at a lower. value.
'
'
a signa1 having an undesired modulation thereon, I
' a, recti?er havinga load resistor coupled to said"
70 ampli?er to supply thereto a recti?ed output sig~
nal comprising voltage pulses that have ampli
tudes determined by said modulation, a vacuum
tube having‘ a‘ cathode, a plate'and 'a grounded
‘grid/said plate and cathode being connected to
the positive and negative ends of said loa'd're
5
2,406,433
sistor, respectively, a condenser connected be
tween said plate and grid, a resistor connected
between said grid and cathode, and means for ap
plying the potential appearing on said cathode
to said ampli?er for automatic gain control.
4. The invention according to claim 3 wherein
the connection from said plate to the positive end
of said load resistor includes a resistor.
5. In combination, an ampli?er for amplifying
a signal having an undesired modulation thereon, 16
a recti?er having a load resistor coupled to said
‘
ampli?er to supply thereto a recti?ed output sig
nal comprising voltage pulses that have ampli
tudes determined by said modulation, a multivi
sister to the positive end of said load resistor and
each having a cathode connected to the other end
of said load resistor, means for rectifying and
combining the rectangular voltage pulses appear
ing‘at said plates, and means for applying. said
combined‘pulses to said ampli?er for automatic
gain control.
7. The invention according to claim 6 wherein
said multivibrator is substantially symmetrical
whereby the negative and positive pulses are of
substantially the same width.
8. In combination, a ampli?er for amplifying
a signal having an undesired modulation there
on, a recti?er coupled to said ampli?er to produce
brator comprising two vacuum tubes each having 15 a recti?ed output signal comprising voltage pulses
a plate connected through its plate resistor to the
that have amplitudes determined by said modu
positive end of said load resistor and each having
lation, means for converting said pulses into ?at
a cathode connected to the other end of said load
topped pulses having peak values at a voltage
resistor, means for combining the rectangular
level which varies in accordance with‘ variations
voltage pulses appearing at said plates, and 20 in the amplitudes of said pulses, and means for
means for applying said combined pulses to said
applying said ?at-topped pulses in negative feed
ampli?er for automatic gain control.
back relation to said ampli?er for controlling its
6. In combination, an ampli?er for amplify
gain.
ing a signal having an undesired modulation
9. The invention according to claim 8 wherein
thereon, a recti?er having a load resistor cou 25 said ?at-topped pulses are applied to said ampli
pled to said ampli?er to supply thereto a recti?ed v
?er through a ?lter having a time constant that
output signal comprising voltage pulses that have
is fast with respect to the period of the undesired
amplitudes determined by said modulation, a
modulation.
multivibrator comprising two vacuum tubes each
LOWELL E. NORTON.
having a plate connected through its plate re
30
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