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Патент USA US2115844

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May 3, 1938.
Filed April '7, 1932
D_l >l-_Hlj
2 sneèts-sheet 1
May 3, 1938.
Filed April '7, 1932
2 Sheets-Sheet 2
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Patented May 3, 1938
Paul 0. Farnham, Boonton, N. J., assignor, by
mesne assignments, to Radio Corporation of
America, New York, N. Y., a corporation of
Application April 7, 1932, Serial No. 603,849
19 Claims. (Cl. 250-20)
This invention relates to automatic control cir
which includes an automatic gain control for
cuits of the type employed in wave transmission maintaining a substantially constant output for
systems, particularly in radio receivers, and more all values of signal input above a critical value
especially to methods of and circuit arrangements and an automatic transmission control for sup
for suppressing all output from the transmission pressing all output for radio input voltages below
system when the magnitude of the received Wave that level at which the noise output becomes sig
or signal falls below a predetermined level.
nificant in comparison to the signal output.
The invention will be described in connection
These and other objects and advantages of the
with the reception of radio broadcast signals but invention will be apparent from the following
it will be apparent that the invention may be ap
specification when taken with the accompanying
plied to other types of signal transmission sys
drawings in which:
tems. Due to the high gain of receivers of the
Fig. 1 is aschematic diagram illustrative of the
automatic gain control type, the noise output is novel method of controlling the transmission
undesirably high when the receiver is tuned off through a radio receiver;
a carrier wave or when, through fading, the re
ceived signal energy falls to a low value. A large
part of this noise is “fluctuation” noise arising
directly in or from the tubes and circuits at the
voltages at the detector of the order of four or
five volts may result from this cause. In addition
to receiver noise, there is an often greater com
stages of a complete radio receiver are repre
power lines. For satisfactory reception, it Would
be desirable to effect operation of an automatic
gain control receiver in such manner that the
audio output will be completely suppressed When
ever the received signal energy falls below that
level at which the noise output is significant in
comparison to the signal output.
A negative bias on the anode of a diode de
tector will suppress all audio output until the
peak input voltage on the diode exceeds the bias
$3 Ol
voltage but, unfortunately, the continued pres
'ence of the bias voltage required for best noise
suppression may be so high as to produce some
ship between audio output and signal input for
several adjustments of the circuit of Fig. 2.
receiver, such as from electrical disturbances on
Fig. 3 is a curve sheet illustrating the relation
beginning of the amplifying system, and peak
ponent due to interference arising outside of the
lil Ul
Fig. 2 is a fragmentary circuit diagram of one
embodiment of the invention, and
In the schematic diagram, Fig. 1, the several
N) 5
3 Oi
particularly, its relationship to the automatic gain
control unit J.
distortion in the audio output.
The gain control unit J comprises an automatic
Objects of the present invention are to provide bias-developing agency taking its input from the
¿o improved methods of and circuit arrangements point l in the transmission system and furnish
for controlling the transmission through an elec
ing a control upon the radio frequency ampli
trical wave transmission system. Further objects- ñcation of the system preceding its input point I
are to provide methods of and circuit arrange
through the channels a, b and c. Since the input
ments for automatically controlling the transmis
to control unit J is in the form of a modulated
5v sion in such manner that a substantially constant radio frequency voltage, and the output is usually
and satisfactory output is obtained for all input in the form of a direct current bias suitable for
voltages above a critical value, and the output is application to the vacuum tubes of the preceding
completely suppressed for all inputs below that stages, it is apparent that the operation of the
critical value which will produce a satisfactory unit J involves the process of rectification though
output. Further specific objects are to provide it may, in addition, include amplification pre
a transmission system in two sections of which ceding or following the rectification.
the gain is automatically varied in opposite direc
In accordance with the invention, a second>
tions by the incoming signal. More specifically, bias-developing agency K is connected to the
an object is to provide a transmission system, for
.example a radio receiver of the high gain type,
sented by the units A to H which are arranged
between the antenna or input system and the
reproducer l. These stages may be combinations
of radio frequency amplifiers, a detector and audio
amplifiers, or certain of the stages may be fre
quency changers and intermediate frequency am
pliñers such as are employed in superheterodyne
receivers. The design of the several stages and
the general method of adding an automatic gain
control system, indicated as an auxiliary unit J,
to receivers of various types are well known.
The present invention relates particularly to
the provision, in such a system of the automatic
gain or transmission control unit K, and more
transmission system at some point 3 which may,v
in one special case, be at the voltage level point l _55
but which, in the general case, may precede, coin
cide with, or follow the point I. The output
from the unit K is employed through one or more
channels e, f, to control the transmission of the
. Cn
diode rectiñer of control unit K will not develop
a direct current output until the peak input Volt
age E1 exceeds the bias Voltage Eg.
The output circuit of the demodulator includes
receiver system subsequent to point I, and in
a radio frequency choke L1 and an output resist
such manner that, for radio frequency voltage
levels E1 at point I less than a critical Value, the
unit K does not permit transmission through the
remainder of the system to the audio output at
reproducer I. In general, a suppression of trans-`
mission at some point subsequent to the auto
matic gain control connection I is to be preferred
.to a suppression at a preceding point and, for
insuring a sharper cut-oí, it is desirable to have
the control unit K permit free` transmission of
ance R3, the low potential terminal of the output
resistance being grounded through an> audio fre
quency by-pass condenser I8. To suppress the
audio output for weak input voltages, a delay
bias voltage is provided between the cathode C 10
and the grid G1 of the demodulator and this bias
voltage is controlled automatically by the control
unit K. The direct current potential developed
across resistance R2 by the diode rectifier C-G2
is impressed upon the control grid of a direct 15
signals, with increasing radio frequency input,
current amplifier I9 which has output circuit
before the normal gain control of the unit J is
In the particular circuit arrangement shown
resistors R4, R5 in the plate and screen grid cir
cuits, respectively. Positive potentials are im
pressed upon the plate and screen grid by a lead
20 which connects the junction of the resistances 20
in Fig. 2, the two control units J and K are con~
nected to the transmission system at the same
point I, which point coincides with the output
voltage across the tuned circuit of unit E of the
receiver system as shown diagrammatically in
Fig. 1. Dio-de rectification is employed and, for
economy and simplicity, the rectifier elements of
.the units F, J and K are enclosed in a common
envelope, i. e., the multiple element tube I0 func
tions as three diode rectiiiers in parallel.
R4, R5 to the point Y on the direct current source
I4. The low potential terminal of the audio out~
put resistance Ra is connected to the plate of
tube I9 through a filter resistance 2|.
A‘tap 22 on the audio output resistor R3 per 25
mits any desired portion of the available audio
output .to be impressed upon the amplifier tube
23 >of the succeeding unit G of the system. The
cathode of the amplifier tube is connected by
lead 24 to a point on the direct current source I4 30A
nal through a condenser I2. Tube I9 may be a
special tube having three anodes but a commer
cial screen grid tube, with a control grid G1 and
screen grid G2 may be employed. The elements
40 will be designated as grids in the following de
of the point Y and the grid of the tube is con
nected, through a filter comprising resistances 25
and condenser 26, to the screen grid of the direct
current ampliñer I 9. A blocking condenser 21 35
is included between tap 22 and the grid to isolate
the grid from the direct current potential of the
resistance R3.
Audio frequency by-pass condensers 28 are pro
vided between the anodes of the two control unit 40
rectifiers and the anode G1 of the audio frequency
The radio frequency voltage E1 developed
across the input impedance il by the amplifier
action of the preceding stages is impressed on
tube Ill by connecting the high signal potential
terminal of impedance H to the cathode C of
35 the tube, and grounding the low potential termi
scription but it will be apparent that the elements
function independently as anodes for rectifica
Grid G1 and cathode C constitute the audio
45 frequency rectifier or »demodulator of the system,
plate P and cathode C constitute the rectiñer of
the automatic gain control unit J, and grid G2
and. cathode C constitute the rectifier of the
transmission control unit K.
The direct current circuit of the gain control
rectifier is completed by the output resistance R1
which is connected between plate P and ground,
and a connection I3 from the low potential ter
minal of the input impedance Il to a point of
positive potential with respect to-grouncl, such as
point Y on the Voltage divider resistance I4 of a
rectifier-filter system (not shown) which is en
ergized, as is common practice, from a source of
alternating current.
With this arrangement, the
60 cathode C is more positive than plate P by a
steady delay-bias voltage Ep, andno rectification
takes place until the peak input voltage E1 eX
ceeds the bias voltage Ep. When the input volt
age E1 exceeds the bias voltage Ep, the rectified
65 voltage developed across resistance R1 is returned,
through a filter comprising resistance i5 and
condenser I6, to a preceding stage or stages as
a gaincontrol voltage.
The direct current circuit of the control unit K
70 includes the resistance R2 which is connected
between the grid G2. and a point S, preferably
adjustable by tap I‘I, on the positive potential
source I4. The direct current potential of point
S is less than that of the cathode connection
point Y by lan amount Eg, andY therefore the
which is at a greater positive potential than that
In the absence of a rado frequency input at theA
antenna, the plate current flow in the direct cur
rent amplifier I9 is high and the delay bias volt 45
age on grid G1 is substantially the same as the
delay bias voltage Eg. So long as the peak input
voltage E1 to tube I0 is less than the delay bias Eg
on the grid G2, this condition obtains and the high
bias on the demodulator renders the transmission 50
system inoperative beyond the point I of the
schematic system of Fig. 1, i. e., the tube I0 of
Fig. 2. For peak values of E1 less than the delay
bias Ep on the gain control unit J, there will be
no reduction in the gain of the radio frequency 55.
amplifier system preceding the point I. For val
ues of E1 greater than Eg but less than Ep, a direct
current potential is developed across the resist
ance R2 of the control unit K, thus reducing the
plate current flow in the resistor R4 of tube I9 60
which reduces the delay bias between the demod
ulator cathode C and grid G1. This permits the
transmission system to regain its normal opera
tion beyond the point I or tube IU and up to the
blocking condenser 21.
The audio transmission is also blocked, for
Weak signals, by the high negative bias established
on thegrid of the amplifier tube 23. As the value
of the input Voltage E1 increases in the range be
tween Eg andrEp, the reduction of current flow 70
in the screen grid system of the direct current
amplifier I9 decreases the bias on tube 23 to per
mit its operation as an audio amplifier. The cir
cuit elements comprising the control unit K there
fore suppress Vall audio transmision through the
system for weak signals but restore the normal
transmission characteristics for rectiñer- input
voltages E1 above a predetermined value Eg. Ad
justment of the point S along the resistor I4
makes possible a selection of the carrier level at
the antenna below which no receiver output is
The effect of the two automatic control units
on the transmission through'the receiver system
10 will be apparent from the several curves of Fig. 3,
which curves shown the relationship between an
tenna input and audio output, at reproducer I, for
several values of delay bias voltage Eg. In obtain
ing the data for these curves, the carrier modula
tion was set at 10% and the tap I'I was adjusted
along the direct current source I4 to vary the
delay bias Eg on the rectifier `element of unit K
between zero and a maximum of 40 volts, the
bias for each curve being indicated by the legends
20 such as “Eg=10” appearing adjacent the curves.
The delay bias on the automatic gain control unit
J was such that no gain control voltage was devel
oped for antenna' input voltages less than about
15 microvolts.
The typical output-input characteristics of pre
vious types of sensitive automatic gain control
receivers are Well indicated by the curve for zero
transmission suppression bias. It will be noted
that the audio output does not fall 01T uniformly
30 as the antenna input decreases below the critical
value at which the automatic gain control comes
into operation. In the range of very Weak signals,
the total audio output decreases at a relatively
low rate, the greater part of the output being due
35 to fluctuation noises arising within the receiver.
Reception over the range of antenna voltages up
to about 10 microvolts is not satisfactory due to
the noise component and, over the greater part of
that range, it is not possible to distinguish the sig
40 nal output as it is completely masked by the noise
The total audio output in this range of signals is
reduced when the delay bias Eg on the control
unit K is adjusted to suppress transmission for
very weak signals. The curve for Eg=l0 volts
shows that the audio output decreased rapidly,
With decreasing signal strength, for carrier inputs
of less than 4 microvolts. Increasing the delay
bias Eg to l5 volts provided a much sharper cutoff
50 for inputs below '7 microvolts, the remaining
curves showing sharp cuto?fs at higher carrier in
puts. The curve for the value of Eg=30 volts in
dicates that all transmission beyond the control
point was suppressed until the automatic gain
control through the unit J was fully established,
i. e., substantially constant audio output is ob
tained for all ranges of antenna> input voltages
above that critical value corresponding to a recti
fier input voltage E1 suñicient to initiate the re
60 moval of the blocking action of the control unit.
An examination of the curves for delay bias
voltages of 30, 35 and 40 volts will show that, in
terms of increasing antenna input, the effect of an
increase of the delay bias from 35 to 40 volts has
more effect than an increase from 30 to 35 volts.
This results from the fact that the automatic gain
control unit J is functioning, in the upper half of
this range, to prevent the rectifier input E1 from
increasing at as rapid a rate as the increase in
the antenna input.
The several curves illustrate the varying trans
mission characteristics which may be obtained by
adjustment of the critical input voltage below
which the audio output is reduced or eliminated.
75~ The appropriate adjustment for any given re
ceiver will depend upon the absolute magnitude
of the iiuctuation noises arising in the receiver
and the noises arising from outside disturbances.
With the receiver for which the curves of Fig. 3
were plotted, a delay bias Eg of l5 volts was suin
cient to suppress receiver noises and the suppres
sion of additional noises due to local disturbances
may be effected by increasing Eg to 30 volts or 35
It will be apparent that embodiments of an
automatic transmission control system such as
shown in Fig. l may take other forms than that
shown in the circuit diagram oi Fig. 2. As stated
above, either or both control units may employ
ampliñcation before and/ or after rectification. 15
As shown in Fig. 2, the unit K includes the recti
I'ler C-Gz and the direct current ampliñer but it
will be apparent to those familiar with the design
of wave transmission circuits that the tube I9
may be made a rectifier by appropriate changes in 20
the circuit connections.
In its broader aspects, the invention comprises
the addition to a transmission system of two gain
control units which automatically determine the>
transmission through the system. With such 25
control units, two'separate portions- of the trans
mission system are normally biased, i. e., in the
absence of a radio frequency input, in opposite
senses, one portion being biased for maximum
gain while a second portion is biased to suppress
transmission. The control units are preferably
connected across the system at a point intermedi
ate these sections, and with increasing input volt
ages, the control units operate in opposite sense
to reduce the normal gain of one portion of the 35
system and to remove the blocking action normal
ly impressed on the other section. The net eiTect
of the two controls is to maintain a substantially
constant output for all signals of an order sum
cient to produce a satisfactory output, and to
eliminate all output for signals of lesser magni
Itis to be understood that there is a consid
erable latitude in _the design of the control units
and, obviously, in the types of transmission sys 45
tems in which such units are incorporated.
I claim:
i. In a carrier Wave receiver, the combination
with a diode demodulator, and means impressing
a bias potential on said demodulator to prevent
the development of an audio output voltage until
the impressed carrier voltage exceeds a predeter
mined value, of means for removing said bias
potential as said impressed carrier voltage in
creases above said predetermined value and ad 55
ditional means for adjusting said value.
2. In an electrical wave transmission system,
a double diode rectiñer having a cathode and two
anodes, a single input impedance for said rec
tifier, circuit elements connected to the respec 60
tive> anodes to constitute substantially independ
ent audio frequency vand direct currentv output
circuits, means for impressingy a negative poten.
tial on one of said anodes to prevent the develop- '
ment of current flow in the output circuit 'there 65
of until the impressed radio voltage exceeds a
predetermined magnitude, and means auomati
cally varying said impressed negative potential
with variations in the radio voltage impressed on
said rectifier and means for selecting the radio 70
voltage value at which said last means is op
3. In a transmission system, the combination
with a demodulator having an anode, of means
for impressing on said anode >a negativedirect 75 l
2,1 15,8442
current. potential, of means automatically reduc
ingsaid impressed potential. to a negligible value
when the received .signal energy exceeds a pre
determined level and additional means for select.
. ingthe signal energy level .at -which said auto
matic means becomes operative..
4. In. a radio. receiver, the combination with. a
radioy frequency ampli?er, andan audio fre
quency amplifier, of a demodulator connected
10: between said amplifiers, and means including a
pairof control units for automatically control
lingthetransmission through the respective am
plifiers in opposite sense and. asa function of the
received radio energy and one of said control
15; units having means to control the demodulator
5. The invention asset forth in claim 4, where
in said controlunits eachzinclude a rectifier, and
one of saidrectifiers ris in parallel with said de
.; modulator.
6. A radio receiver of thetype including a
radio. frequency amplifier, a detector, control
current. .voltage .developed by? the: second dioder
across its. associated output resistance and means
for .preselecting the radio voltage-level'at. which
said seconddiode isabletorectify.
11..A= radio :receiver as claimed in'claim 10, Ul
wherein .said last means comprises a second tube
having a cathode cooperating with a control grid
and.ianode,~-and means .impressing between the
said control grid and cathode the direct current.
voltage developed across the output resistance of 10
the second diode, the said biasing resistance being
connected between'the cathode and anode of the
second tube.
12. Aradioreceiver as claimed in claim 10,1.'
wherein saidlast means includes a direct current
amplifier, and mea-nsimpressing upon the same
the direct current .voltage developed by said sec
ond diode, thesaid biasing resistance being» in‘
eluded inthe output circuit of said direct `current
amplifier. .
;quency voltage rises above a critical value, means
normally. biasing. said detector to suppress all
ode‘for automatically controllingthe gain of said
amplifier in accordance with variations in the
strength. of received signals, and biasing means
including a third diode Yfed with signals by said
automatically to remove the suppression bias on
tude. of received signals in that sense which ren
said detector when the received radio frequency
voltagefexceeds a critical value, characterized by
meansv for adjusting .the critical voltage value at.
ders said‘demodulator diode inoperative for Weak
signals and operative for signals in excess of a
threshold value.
means operative vautomatically to adjust the gain
ofthe amplifier when the received. radio fre
13. In. a-Lradio receiver, a. radio. amplifier, a;
demodulator diode, .means includinga Ysecond di
audiooutput, and;control means independent-of amplifier for impressing .upon-said-demodulator
the action of the iirstcontrormeans operative.: diode a bias voltage that varieswith the magni
which one of said` control means becomes op
erative independently >of the critical voltage value
1 of the second controlmeans.
14. A radioireceiver `as» claimed in claim 13, in.
combination vwith means adjustable to control'the
threshold value below which said biasing means
7. The invention as set forth in claim 6, where
in. said adjusting. means. regulates the critical
value ofthe radio frequency voltage at -which
said suppression control means becomes ,opera
renders said demodulator diode inoperative.
tive to remove the suppression bias from said de.
means normally 'impressing a bias voltage on said 40
8. In a radio receiver, the combination with a
radio. frequency amplifier, a detector, an audio
frequency amplifier, means automatically con
trolling the gain of said radio frequency ampli
fier, control means rendering said .gain control
means inoperative when the received radio fre
quency voltage lfalls below a critical value, means
normally biasing said detector and audio ,fre
,1. quency amplifier to render the same inoperative,
and means operative to remove said normalbias
when .the received radio frequency voltage rises
above a critical value, of meansfor adjusting the
critical voltage value at4 which one control means
.ibecomes operative independently of the critical
voltage value of the. othercontrol means.
9. The invention as set forth'in claim 8,. where
in said. adjusting means regulates. the4 critical.
voltage value at which said second control means .
15...*In ,a radio receiver, the combination with a
radiofzfrequencyamplifier, a demodulator stage,
and a .stage'of audio frequency amplification,
amplifier to produce maximum gain, and means
normally impressing of said stages-a
bias voltage of a magnitude eiiective to prevent
transmission therethrough, of control means op
erating automatically-when the receiver input
exceeds a predetermined value to maintainthe
amplified Voltage level at a point in said ampli’
fier at a substantially ñxed'value over a wide range
of `¿received‘signal voltages, andcontrol means
operable automatically when the received radio
frequency voltage rises to a critical value to re
move .said transmission-preventing bias voltage,
wherein each of said control means includes a
rectifier and means impressing thereon a bias
voltage which rendersl the rectifier :inoperative
until the peak radio input to therectifier exceeds
the bias voltage thereon,` the biasing means of the
rectifier of usaidiirst control -means impressing
thereon a biasvoltage less than that impressed on
the .other rectifier by itsbiasing means.
said amplifier.
10. In a radio receiver, the` combination With. radio frequency amplifier, a diode detector anda
diode rectifier vonlwhich radio frequency volt
a radio amplifier, of transmission control means
comprising .a tube-having a cathode cooperating. ages are> impressed .by said amplifier, and means
Götwith a pair of anodes to function as two diodes, including a resistance in the circuit of said de 65
input circuit means impressing uponeach diode tector for applying thereto a delay bias voltage to
a radio voltage developed by said radio amplifier,` render the detector inoperative for. radio input
an output resistance for each diode, and means voltages. below acriticalvalue, of means includ
impressing upon the first of said diodes a bias ing said rectifier and a direct current amplifier for
to prevent rectiñcationthereby; establishing a current flow in said resistance to 704
reduce said applied .bias voltage when the radio
said biasingmeans comprising a biasing resist
ance between the cathode and the anode element input voltage on said »rectifier exceeds a predeter-.
of the said-ñrst diode, andmeans forxpassing mined valuefin combination with a third rectifier
through saidbiasing resistancea direct current for automatically controlling the gain-of saidam'e.
,which varies -as a .function .of the. rectified` direct pliñer in. accordance « withÀ variations in the
ûOárbecomes operative toremove >the normal biasv on
strength of the radio frequency voltage impressed
output terminals of said radio frequency amplifier,
on said receiver.
one rectifier developing an audio frequency volt
age for application to said audio amplifier, means
1'?. In a radio receiver, the combination with a
radio frequency amplifier, a diode detector and a
Ul diode rectifier on which radio frequency voltages
are impressed by said amplifier, and means in
cluding a resistance in the circuit of said detector
for applying thereto a delay bias voltage to ren
der the detector inoperative for radio input volt
10 ages below a critical value, of means including said
rectifier and a direct current amplifier for estab
lishing a current flow in'said resistance to reduce
said applied bias voltage when the radio input
voltage on said rectifier exceeds a predetermined
15 value, in combination With an audio frequency
amplifier Working out of said detector and means
impressing thereon a bias voltage to render the
same inoperative, and wherein said direct current
amplifier automatically removes the bias voltage
20 on said audio frequency amplifier when the radio
input voltage on said receiver exceeds a prede
termined value.
18. In a radio receiver, the combination with a
radio frequency amplifier and an audio frequency
25 amplifier of three rectiíiers having their input
terminals connected in parallel and across the
energized by the direct current voltages developed
by other rectiiiers for automatically controlling 5
respectively the radio frequency and the audio
frequency transmission through said receiver, said
first rectifier being normally biased to prevent
operation thereof, circuit elements cooperating
with said means to render said first rectifier op
erative when the amplified radio frequency volt
age impressed upon one of said other rectifiers
reaches a predetermined value.
19. In a radio receiver, the combination with a
diode demodulator, of means impressing a delay 15
bias voltage on said demodulator, and means for
automatically varying the said bias voltage as a
function of the radio voltage input to the de
modulator, said automatic means including a rec
tifier and direct current ampliiier for developing 20
a direct current voltage of greater magnitude than
that obtainable by rectification of the radio volt
age on said demodulator, and means for adjust
ing the radio input level at which said rectifier is
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