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Патент USA US2120984

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June 21, 1938.
F. T. LETT ‘
Filed July 12, 1955 '
F1 6 l
An I)
. v____
.Q .
. . LE.
Patented June 21, 1938
Frederic Tom Lett, Iver, England, assignor to
Electric & Musical Industries Limited, Hayes,
Middlesex, England, a company of Great Brit
Application July 12, 1935, Serial No. 30,975
I In Great ‘Britain July 17, 1934
(Cl. 250—-20)
3 Claims.
ment being characterized in that said auxiliary
The present invention relates to wireless and
means are coupled to the control grid-cathode cir
cuit of said valve through a circuit having .a rela
tively longtime constant, for example one or
like receivers, and more particularly but not ex
clusively to receivers embodying automatic gain
In receivers embodying automatic gain con
trol, there are usually provided a high frequency
ampli?er, a signal detector, and controlling
ceived signal strength may be within predeter
strength is passed to the‘ signal detector.
ampli?er through a circuit of ‘very low damping
so that the auxiliary means are substantially
A disadvantage of receivers of this kind is that
15 over those parts of the tuning range of the re
only affected by the received signal carrier.
ceiver at which no signal is tuned in, the high
frequency ampli?er is inpits most sensitive con
dition (that is, it has its maximum gain) and
Preferably also, the receiver is provided with a
tuning indicator, which may take any known or
disturbances such as valve noises and the ‘like
may be ampli?ed to such an extent as to prove
objectionable. For the purpose of avoiding this
disadvantage, it has been proposed to provide
means for “muting” the receiver when no signal
is being received; the muting means have in some
25 cases been associated with frequency selective
means so arranged that the receiver is only
“alive” when a signal is accurately tuned in.
With such arrangements, the receiver is quiet
when no signal is being received, but in tuning
30 the receiver from one'end of the wave band over
which it is responsive to the other end thereof,
the set becomes alive each time a signal is tuned
in, and the operation of tuning is thus still a
It is an object of the present invention to provide
an improved receiver in which tuning noises are
further reduced.
According to the present invention, a wireless
receiver, which may embody an automatic gain
40 control, is provided with means arranged to mute
the receiver when no signal is being received, and
to render the receiver alive only when it is allowed
to remain tuned to a desired signal for a substan
tial period of time, for example one or more sec
According to a feature of the invention, in a
wireless receiver, which may embody an auto
matic gain control, there are provided means
for biasing the control grid of a valve. forming a
Di 0 part of said receiver to such an extent in the
negative sense relatively to the cathode thereof
that said valve is substantially prevented from
amplifying, andauxiliary means operating. auto¢
matically when a signal is received to reduce the
55 negative bias applied to said valve, the arrange
circuit. The signal is preferably fed to thisrec
ti?er from the output circuit of a high frequency
mined limits, a signal of substantially the same
noisy one.
ode circuit, and having its anode current con
trolled in accordance with the strength of the
received signal by means of a suitable recti?er and 10
load resistance associated with its ‘control grid
means operating automatically to control the
gain or ampli?cation 0f the high frequency am
10 pli?er in such manner that, whatever the re
The auxiliary means may take the form of a
thermionic valve having a resistance in its an
suitable form, but which preferably comprises a
milliammeter associated either with the high fre 20
quency ampli?er or signal detector,‘ or with a
valve forming part of the auxiliary means.
In operation, the receiver is tuned to the wanted
signal, preferably with the assistance of a tuning
indicator, and the receiver is substantially com
pletely mute throughout the whole tuning opera
tion, although it may be momentarily in tune
with several unwanted signals. Only after the
receiver has been allowed to remain in tune for
the predetermined period is the wanted signal 30
A particular embodiment of the invention will
now be described by Way of example with ref
erence to the accompanying diagrammatic draw
ing in which Figs. 1 and 2 together show a part of 35
a superheterodyne receiver constituted in ac
cordance with the invention. It is to be under
stood that the invention is not limited to the par
ticular embodiment described.
Referring to Fig. 1, an aerial-earth system A 40
is connected to, the input circuit of a high fre
quency ampli?er which is coupled through a fre
quency changer to a ?rst intermediate frequency
ampli?er; the two ampli?ers: and the frequency
changer are indicated by B. The apparatus B 45
derives anode current from a battery 31 (Fig. 2)
for which purpose terminal‘ C of Fig. 1 is con
nected to terminal C1 of Fig. 2. Similarly ter
minals D and D1 and E and 28 are connected to
> Referring to Fig. 2, the anode of the last valve
V1 of the intermediate frequency ampli?er of the
receiver is connected through two coils l and 2
to the positiveterminal 3 of the source of anode
current 31 indicted generally by a battery, the 55
negative terminal of which is earthed. The coils
I and 2 are shunted by a condenser 4, and the cir
cuit I, 2, 4 is tuned to the intermediate fre
quency. Coupled to the coil I is a further coil
Loosely coupled to the coil 2 is a resonant cir
cuit comprising a coil 29 tuned by a condenser 30
to the intermediate frequency. One end of the
circuit 29, 3D is connected to the second auxiliary
5 shunted by a condenser 6, the circuit 5, 6 being
anode 3| of the valve V3 and the other end is
also tuned to the intermediate frequency.
connected to the cathode thereof through a cir
The intermediate frequency potential - differ
ences set up across circuit 5, 6 are applied be
tween the anode 1 and the cathode 8 of a diode
10 recti?er which forms a part of diode-triode valve
V2. A load resistance 9 is connected between the
circuit 5, 6 and the cathode 8 of the valve V2, the
resistance 9 being shunted by a. condenser [8
which serves as a by-pass for currents of the in
15 termediate frequency. The diode ‘I, 8 constitutes
the second detector of the receiver, and modula
tion frequency potential differences are set up
cuit comprising a load resistance 32 shunted by
an audio-frequency by-pass condenser 33. The
control grid of the valve V3 is directly connected
to the end of the load resistance 32 remote from 10
the cathode 24, and the main anode 34 thereof
is connected through a resistance 35 and a milli
ammeter 36 in series to the point It. The resist
ance 35 and the milliammeter 36 are shunted by
a bypass condenser 31, and the anode 34 of the 15
valve V3 is connected through two resistances 38
and 40 to the grid of the valve V2. The junction
across resistance 9.
point of the two resistances 38 and 40 is con
An adjustable tapping point in resistance 9 is . nected to the cathode 24 of valve V2 through a
20 connected through a condenser II to the control condenser 39.
The operation of the circuit is as follows:
grid of the valve V2, the anode of which is con
In the, absence of a signal, the anode current
nected to the positive terminal 3 of the anode
current source 31 through a resistance l2. The
valve V2 functions as a low frequency ampli?er,
25 and is coupled through a condenser 13 to the
input circuit of a power output stage P. S. which
feeds loudspeaker L. S. The cathode B of the
valve V2 is connected through a biasing resist
ance l4 shunted by a by-pass condenser 15 to a
30 point I 6 in a potential divider constituted by
three resistances l1, l8 and 19 connected in series
across the anode current source. Resistance I9
is shunted by a decoupling condenser 20, and a
further decoupling condenser 2! is connected in
35 shunt with the resistances l8 and I9.
The junction point of the coils I and 2 is con
nected through a condenser 22 to one auxiliary
anode 23 of a double diode-triode valve V3, the
cathode 24 of which is connected to the junc
tion point of the resistances I8 and IS. The
anode 23 is connected to earth through vtwo
resistances 25 and 26 in series, the latter resist—'
ance being shunted by an audio-frequency'by
pass condenser 21.
The diode 23, 24, serves to provide a gain con
trol potential difference, oscillations at the in
termediate frequency established across coil 2
being applied to the diode through condenser 22.
For signals above’a certain strength,‘ current
50 flows in the diode, and'there is set up across the
resistance 26, that is, between the point 28 and
earth, a potential difference which is propor
tional to the strength of the signal applied to
the diode. The potential of the point 28 relative
55 to earth becomes more negative as this signal
strength increases, and the point 28 is conduc
tively connected to terminal E and thus to the
control grids of one or more of the high fre
quency and/or intermediate frequency amplify
60 ing valves, the gain of which is to be controlled.
The point 28 may be connected also to the con
trol grid of the frequency changing valve. The
controlled valves are preferably of the variable
mu type, and have their cathodes conductively
connected to earth.
The cathode 24 of the diode is maintained at a
positive potential relative to earth by the flow of
current through resistance l9, and no current
flows in the diode until the potential of the anode
70 23 exceeds that of the cathode; no automatic
gain control potential difference is therefore set
up across resistance 26 if the received signal
strength is belowa pre-determined value depend:
ent upon the potential difference across resist
ance 19.
of the valve V3 ?OWlng through resistance 35'
biases the control grid of the valve V2 with re
spect to the cathode thereof to such an extent 25
in the negative sense that substantially no anode
current ?ows in the valve V2, which is accord
ingly prevented from amplifying any noise which
may be applied to it, and the receiver is mute.
‘When a signal is accurately tuned in, current 30
?ows in the diode 3|, 24 and the sense of the
potential difference set up across the load resist
ance 32, and applied between the control grid
and cathode of the valve V2 is such as to reduce
the anode current of that valve, and to make 35
the potential of its anode more positive; 8 the
triode portion of the valve V3 thus ampli?es the
potential difference established across the re
sistance 32. As the signal is tuned in, the anode
current of the valve V3 decreases, this decrease
being shown by the milliammeter 36; a visual 4.9,
indication that a signal has been tuned in is
thus provided, and in order that the milliam
meter may adequately perform this function, the
anode and grid circuits of the vvalve V3 (that is,
circuits 32, 33 and 35, 36, 37) arearranged to 45
have time constants of the order of T16th second
or less.
The change in potential of the anode of the
valve V3 when a signal is tuned in is in such a
sense as to reduce the negative bias on the con
trol grid of the valve V2, and thus to allow it to
amplify, and the change in potential of the anode
of valve V3 is communicated to the control grid of
valve V2 with a time delay of the order of one
or more seconds.
The potential of the control grid of the valve V2
is dependent upon the state of charge of the
condenser 39 which, as far ,as changes in potential
of the anode of the valve V2‘ are concerned, 60
charges through the resistance 38. The con
denser 39 and the resistance 38 are accordingly
given relatively large values; for example, the
condenser 39 may have a capacity of 1.0 micro
farad, and the resistance 38 may be of 2.0
megohms. In this case, the time lag between the 65
tuningin of a signal and hearing the signal in
the loudspeaker is of the order of two seconds.
It may be arranged that when a signal is tuned
in, the .valve V3 is biased to such an extent in
the negative sense thatno anode current-?ows 70
therein. In these circumstances, the control grid
of thevalve V2 tends to assume the potential of
the cathode thereof, and the biasing resistance I4
is therefore provided to ensure that some nega
tive bias is at all times present.
As has been explained, for received signals
havingstrengths between predetermined limits,
‘ the effect of the automatic gain control is that
signals of substantially constant strength are ap
plied to the second detector. The effects of fad
ing, and differences in‘strength between differ
ent received signals are therefore eliminated, and
the constancy and reliability of the muting means
"are assured since a signal of substantially con
2. In a ‘signal receiving system of the type
adapted to be adjusted over a band of different
signal frequencies, a signal transmission tube
having a signal input network and a signal repro
duction output network, means connected to a
gain control electrode of the tube for impressing
a potential thereon of such polarity and magni
tude as. substantially to reduce the gain thereof
in the absence of signals of a desired amplitude,
10 stant strength is applied to the diode 3|, 24. The
said means comprising a tube having a resistor
‘ values of the various circuit elements may be so
potential, auxiliary means, responsive to signals
chosen that unless the received signals are of
‘greater than a predetermined strength, the valve
’ V2, remains unable to amplify; the receiver‘ is
then permanently mute for all signals of less than
is, said predetermined strength.
Suitable values for certain of the components
I of the‘ receiver described above (other than those
to ' which suitable values have been assigned)
20 have been found to be as follows: ‘ ‘
‘ Resistance 40 ________________ __ 1.0 megohm
Resistance 32 _______ __' _______ __ 1.0 megohm
'of the desired amplitude, for rendering the tube
of said first means ineffective, and a coupling
network connected between said resistor and said 15
gain control electrode whose constants are chosen
to impart a time delay of the order of one second
to a change in said potential whereby sounds are
not reproduced in said output network unless the
system is adjusted to a desired signal for said
time period.
3. In a signal receiving system of the type
adapted to be adjusted over a band of different
33_____' ___________ __ 0.1 microfarad
signal frequencies, a signal transmission tube
Resistance 35 ________________ __ 0.1 megohm
Condenser 3'7 ________________ __ 0.1 microfarad
production output network, means connected to
It is to be understood that the invention, al
though described as applied to a superhetero
dyne receiver, is not limited to receivers of this
in its space current path for developing said
1. In a wireless receiver, a ?rst thermionic
having a signal input network and a signal re
a gain control electrode of the tube for im
pressing a potential thereon of such polarity and
magnitude as substantially to reduce ‘the gain
thereof in the absence of signals of a desired. 30
amplitude, said means comprising a tube having
a resistor in its space current path for develop
ing said potential, auxiliary means, responsive to
ceived signals, said valve having an anode, a signals of the desired amplitude, for rendering 35
35 cathode, and a control grid, a second thermionic ‘ the tube of said ?rst means ineffective, and a
valve having an anode, a cathode and a control coupling network connected between said re
grid, a resistive impedance in the anode circuit sistor and said gain control electrode whose con
, valve for handling oscillations derived fromre
, of said second valve for developing a voltage high
stants are chosen to impart a time delay of the
.valve so ‘ as to give the grid thereof a negative
?rst means to indicate the resonance condition
enough. to render the ?rst valve inoperative in order of one second to a change in said potential
the absence of signals, a recti?er, meansfor whereby sounds are not reproduced in said out
‘feeding oscillations derived from received signals put network unless the system is adjusted to a
to said recti?er, connections for applying the . desired signal for said time period, a visual cur
recti?ed output of said recti?er to said second rent indicator electrically associated with said
bias relative to the cathode" thereof which is suf
?ciently high to reduce said voltage to a point
such that the ?rst valve becomes operative, and,
between the anode side of said impedance of said
second valve and the control grid ofv said ?rst
valve, a resistor-‘condenser networlc' having a
~ time constant of the order of one second.
of the system, and means electrically associated
with at least the resistor of said ?rst means to
impart to it a time delay constant of the order
of one-tenth second.
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