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Патент USA US2404338

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_July 16, ¿1946y
Filed Aug. 20, 1941
Patented July 16, 1946
Noise sUPPREssIoN olaoUrrs
Joseph A. *Worcesten Jr., Bridgeport,z Conn., as
signor to General Electric Company, a corpora
tion of New York
Application August 20, 1941, Serial No. 407,543
23 Claims.
(C1. Z50-20) ’
My invention relates to noise suppression cir
cuits and it is particularly applicable to noise sup
pression circuits for automatically rendering the
output circuits of a frequency modulation receiver
some form of noise suppression circuits for Velim
inating _these background" noises in a frequency
modulation receiver in ‘order that the full ad
vantages of “static-less” operation may be real
inoperative when no signals are being received or 5 ized.` Accordingly, it is an important object of
' my invention to provide an improved background
when the received signals are too weak to pro
noise suppression circuitwhich is particularly
vide satisfactory operation.
adapted to the requirements of a frequency mod
It is well known that objectionable noises may
ulation receiver.
be emitted by the signal reproducer of a radio
receiver during those intervals when no signals 1Q ’ In a preferred embodiment of my invention,
' the noise voltages appearing- as amplitude modu
are being received, or while the receiver is be
ing tuned between carrier channels. rI‘hese noises
lationv in the limiter output under no-signal or
weak-signal conditions, as explained above, are
may arise from a great many causes including
cross-modulation effects, natural atmospheric
detected and utilized to provide bias potentials for -
cetera. Therefore, various squelch circuits have
the limiter'is completely blocked, and the sound
reproducer effectively muted.Y However, as soon
static, locally produced high-frequency electrical 15 reducing the transfer eiliciency of a portion of the
disturbances, “shot" effects and thermal'agitation ` transmissionchannel following the limiter. Pref
erably, one of the'amplifying "stages following
in the tubes and circuits of the receiver itselfget
heretofore been proposed for muting the sound re- -
as a signal‘of' sufficient strengthfor satisfactory
producer except when the receiver is tuned to
a signal of sufficient strength to over-ride the
operation is received (and such signal need be
only a relatively ‘weak signal just suflicient to
saturate the limiter), the "squelch -voltage au
tomatically collapses and the receiver operates
noise level and provide satisfactory reproduction.
The operation of such circuits has variously been
designated by such terms as “background noise
suppression,” “inter-channel noise suppression”
and “carrier-olf noise suppression.”
A properly designed frequency modulationv re
ceiver inherently provides a high signal to noise
at maximum sensitivity.
It is accordingly another'object of my inven
tion'to provide an improved'squelch or muting
circuit, particularly adapted to the requirements
of a frequency modulation receiver, whose >opera
ratio so long as the received signals exceed a pre
determined level. This is due in large measure to 30, tion is automatically determined by the character
the action of the amplitude nmiung circuits ` ’ and magnitude of the voltages impressed there
which transmit the desired frequency modulation
It is also a specific object of my invention to
but discriminate against amplitude modulation.
In order to realize the full possibilities of fre
quency modulation reception, it is common design
provide an improved carrier-off noise suppression
circuit for frequency modulation receivers which
practice to provide such a high degree of amplifi
is very sensitive and has a trigger action. "
Still another object of my invention is to pro
cation in the high-frequency stages of the receiver
vide an improved automatic background noise
that such effects as thermal agitation in the first
suppression circuit which is economical and read
tuned circuit and the “shot” effect in the first
tube cause voltages to be impressed on the am 40 ily adapted to existing types‘of frequency modu
plitude limiter approaching values required to
' ’
saturate it. So long as any signal strong enough
to operate the limiter is received, these voltages,
as well as other undesired static and noise volt
ages and undesired amplitude modulation present
in the signal, are reduced to a very low level at
the limiter output and do not cause objectionable
disturbances inthe sound rep-reducer. ' However,
in the absence ofsuch signals it has been found
that very substantial noise voltages appear in _the :
limiter output and that, due tothe extremely high
sensitivity of the receiver and the frequencychar
acteristics of the receiver circuits, the audible
background noise may be very severe.
A ’ .
lation receivers.
' The features of my invention which I believe
to be novel are set forth with particularity in
the appended claims. My invention, itself, how
ever, >together with further objects and advan
tages thereof, may best be understood .by refer
ence vto the following description taken in con
n'ection with the accompanying drawing, in which
Fig. 1 isla schematic representation, partly in
the form of a conventionalized one-line diagram,
of a frequency modulation receiver embodying my
invention; and Fig. 2 schematically represents
modified forms of circuitsv embodying my inven
tion which may be substituted for the correspond
It is therefore highly desirable to incorporate `5&5 >ing portions of Fig. 1 within thedashed rectangle,
Corresponding reference numerals have 'been
e. g., audio signals. They are demodulated in a’
slope filter.l or discriminator I1. Various forms of
apparatus suitable for this purpose are well
known to the art. One type of frequency modu
lation detector circuit which I have found'to be
placed upon corresponding elements in the two
figures to facilitate their comparison and simplify
the accompanying description.
Before proceeding further with the description
of the illustrated embodiments of my invention, I
wish to explain the meaning of certain terms as
satisfactory is described in Patent No. 2,121,103
Seeley,` granted June 21,1938,- to WhichV reference
used hereinafter' in the specification'- and »ap
pended claims.
maybe `made for further information. » L
vThe demodulated audio signals are supplied to
yaì-ñrst audio frequency amplifier I8, further am
pliñed in an audio power amplifier I9, and then
I intend such terms as “noise” or “noise Volt
ages,” unless otherwise'qualified, to include al1v
undesired voltages which may accompanyfthe
desired signals or be modulated upon them. `For*l
example, these voltages maybe caused by‘cross-"
*I suppliedtoV a suitable signal translating device,
_ h. Ísuoli as the loudspeaker 2U.
Y .The 'limiter |16 comprises a pentode amplifier
modulation effects, adjacent channel interference 15 30, havingan input circuit including a coupling
or non-linearities in the transmission channel;
capacitor SI and grid resistor 32 connected be
they may be caused by thermal agitation effects,
tween its control grid 33 and cathode 34, and an
“sho ” effects, and related phenomena which
output circuit including a tuned circuit 35 con
occur in the receiver itself and give rise to dis
nected to its anode 36. Anode operating poten
turbances of the soecalled' “continuous” or “hash” 20 tials are supplied in conventional manner from
a ‘suitable source of power, which is not shown but
’ typegíor they'may beA caused by natural atmos
merely indicated by the connection designated as
pher'icF static, Nignition' interference, andY 'the ` like,`
which' occur outside" the receiver and give rise'to
“To-PB” on the drawing,> through »the usual de
disturbances of the solcalled “impulse” type.
coupling resistor 33.
Í‘Á‘By‘t'he term “amplitude modulation”- I intend'
to 'i'r'iclucle> all amplitude variations in the envelope
of highl'frequency voltages appearing at speciñed
points 'in the"transmission channel, irrespective
The screen grid 39~ is' simi
larly supplied with operating potentials from the
common power supply source through vadecou
pling resistor 4U.
of the precise character of these voltages.' Thus,
for ~tl'ie'purposes of my'invention these voltages "
tailed' explanation. ” The limiter is` self-biased' by
means of the capacitor 3I and the grid resistor
32. It is so adjusted ’as to pass anode current-
may constitute periodic signal voltages, either
unm'odulated or modulated in frequency or phase
Y by desired signals, 'or irregular voltages such as
are -characteristic of any of the types of noise.v
' By the'term “frequency modulation” I mean to
The operation of the limiter I6 will be readily
understood by those skilled in the art without de
only between the >limits at which positive grid'
include' any form "of modulation produced by
current flows and
at which anode current cutoff
' In the'form of'
'v my «invention
- ' v
ï illustrated
l l
in Fig.
Varying the frequency or phase of a carrier wave
1, the anode 3'6‘ of- the limited Sil-is' also‘coupled
as 'distinguishedV from' modulation produced _by
VThe generic term “angle modulated” has been
to an amplitude detection circuit over a conductor
50.' The- circuit comprises a diode detector 5I’
' shunt‘ed by a 'load`Y resistor 52'.>> The intermediate
heretofóre employed in the art in connection with
frequency potentials occurring at theanode 36
waves'of- this character 'to indicate either'frequen
areimpressed on'the' diode 5I through Ythe con
ductor 5u anda capacitor A53. This detection
varying the amplitude of such a wave.
cy modulatedV or >phase modulated waves or waves
which 'are hybrids of the two; In the specific
example described in the following discussion of
circuit functions in a manner shortlyV to be de'
scribed ingreater detail; '» Brieiiy, it develops rpo
tentials across the load-resistor 52 in accordance
with amplitude modulation components appear
the.y drawing, the" received signal energy is Vas
sumed to befrequen'cy modulated wave energy.'
" 'In' they diagrammatic representation of a fre
quency modulation receiver Vshown in Fig. 1`, many l.
of the elements- maybe conventional and'their
details of constr'uction'are not material to my in
vention'.,` Therefore, to simplify the drawing,
such elements 'have'merely been indicated in block
form. Thus the receiver’ illustrated is'for the
ing inthe output- of the limiter I6;
Potentials developed- acrossv the diode '5IH are
impressedy upon a' second.- diode detector 54'
through a- capacitor 55, which removes direct cur
rent, and through a'low-pass filter, comprisingY a
resistor> 56 and> a capacitor- 5?-, which removes in’
termediate frequencies.
mostparta conventional superheterodyne re- '
The load for the diode detector 54 comprises a
ceiver.> Signals received 'at 'an antenna, Ii] are
resistory 5B.
amplified' in the -usual radio frequency amplifiers
I I vand combined in a'mixer. I2 with locally gener
ated waves supplied from an oscillator I3. The
carrier waves of fixed intermediate frequency are
then amplified in intermediate frequency ampli;
ners I4. ‘In frequency modulation receiversof
usual design it' is often the practice to pass the
intermediate frequency waves through two ampli
across this resistor are lfurther filtered by' a low
pass filter; comprising- a- resistor' 59j and a ca-1
tude limiters in cascade in order to secure a bet
bodimentsomeofj the amplitude modulationîis
removed in a ñrst limiter I5 and' practically 'all
of the remaining VamplitudeV modulation isf're'
moved in a second limiter I6.V
` '
pacitor G6', and> impressed- between the control
grid 1u and cathode'l-I of the first audio ampli@ii'er tube, which is illustrated asÍ a triode 12; It
will be observed that a conductive connection `is
provided betweengrid T0 and cathode 'H' through
a resistor 6 I‘v and resistors 58 and 59'. These'v'olt
ter limiting action. Thus in theillustrated' em
The intermediateV frequency carrier waves atY
the'olltput ofthe secondv limiter I6 'are therefore
of substantially constant amplitude'and contain
only'theffrequ'ency 'modulation which is repre,
Unidirectional- voltages appearing
ages are of such polarityas to bias’the `grid 7_0
negatively with respect‘to‘the 'cath‘odeïfl"and`, as
will shortly'be:explained;- under 'certain conditions »
their'magnitude is sufiicienttc‘reduce the `ampli-`
cation of the audio ampli’ñ'er tube 'I2` toa very
70 low îlevel;
Preferably tube 'I2A is Vthereby >biased
completely to cutoff, preventing transmission 'of
signals'to the power amplifier I'Sf and therloud
It has beenfoundîthat there are alwaysî noise
sentative of the intelligence being transmitted, 75 voltages present -at -the second’ limiter even'when
.the diode 54. The time constant of the network
no voltages are impressed upon the receiverin
put. It has been demonstratedlthat the Ynature
of these voltages is that of a spectrum contain
associated with this -detector‘is made very long as
compared to theperiod of the lowest; audio fre
quencyL in vthe signal channell gThe yresulting
unidirectional potentials, after being filtered to
ing an almost infinite number of component fre
quencies, some of which always coincide with the
frequencies associated with the desired signals.
It is generally accepted that irregularities in the
motion of electrons in the tubes and circuits is
largely responsible, i. e., the thermal agitation
and “shot” effects and related phenomena. The 10
first radio frequency amplifier stage undoubtedly
contributes the greatest proportion of these dis
turbances because of the high'gain' provided in
the stages between it and the limiter. Further
smooth them out further, are then utilized to-pro
vide the automatic squelch or muting bias for the
audio frequency amplifier IO, as previously ex
The modified form >of my invention represented~
in Fig. 2 differs' from that of Fig. y1 only in cer
tain particulars. Corresponding elements have
y been designated by the same reference numerals
and the function of these elements is essentially
the same. Therefore, they need not be repeated
here. In this modification the pentode 33 is it~self utilized to detect the aud-io noise voltages>
more, it has been demonstrated that extraneous
disturbances received at the antenna l0 ~ also
comprise an infinite frequency spectrum contain
present at the anode 36, permitting the first diode
ing» frequencies capable of being translated in the
detector to be eliminated. This is possible be'
receiver channel.- These disturbances further
increase -the voltage impressed upon the limiter 20 cause the time> constant of the grid circuit' 3|, 32
is such that the pentode 30 functions essentially
under no-signal conditions.
as a grid power detectorfor amplitude modula
As indicated earlier in this specification, it is
tion components within the audio frequency
preferable to provide such a high gain through
band. Therefore, it produces, audio voltages
the radio frequency and intermediate frequency
across an audio frequency load in the anode cir
amplifiers that the noise voltages impressed upon
cuit. Such a load is provided by the decoupling
the limiter grid 33 develop self-bias potentials
network comprising the capacitor 31 and decou
approaching values required to saturate the
pling resistor 38. Consequently, the audio noise
pentode 33 even- under no-signal conditions.
voltages appear at the point 13 and maybe sup
Nevertheless, under such conditions it has been
plied directly to the filter and detector-network
found that these voltages are not appreciably
5x5-'65, as illustrated.
y ’
limited in amplitude in the limiter i6 and that
In a frequency modulation receiver‘of` .com
substantial noise voltages are developedat the
limiter anode 36 including> modulation frequen
cies-within the audio band.` Reception of exter
mercially acceptable sensitivities, the audio noise
voltages at ythe point i3 have been found to `be
noise level, voltages within lthe intermediate fre-'
quency band are continuously impressed upon the
squelch voltage is automatically increased when
limiters to maintain them operative. Y Noise volt
nal “static’f disturbances at the antenna l0 mere 85 at least of the order-of lO'volts under all operat
ing' conditions. This provides a unidirectional
lyincreases their magnitude. However, as soon
>squelch voltage of > at least four volts atjthe grid
as' a carriersignalis received which is suiiicient
of the audio amplifier 12, which is sufficient Yto
to maintainrthe limiters continuously saturated,
cut off a typical high-mu triode driver tube.> vEx
these voltages at the anode 36 almostv completely
disappear.' Inl other words,` so long as the re 40 traneous noise voltages received at the antenna
I0 further increase these values so that the
ceived signal is strong enough to over-ride the
ages alone, on the other hand, apparently con
the receiver is operated in excessively'noisy loca
tain no such continuous components, but are
more like a highly over-modulated carrier vof
practically zero intensity and are not removed
by the limiters.
As soon as even a relatively weak signal is re
ceived the squelch voltage immediately collapses
and the receiver operates at full sensitivity. Such
a signal need be only of a few microvolts intensity,
i. e.`, just sufficient to render the limiters fully ef
In accordance with my invention these phe 50 fective. As previously explained, the background
noise at the loud speaker is now reduced to‘a vvery
noména are utilized to provide background noise
suppression in a manner now to be detailed.
In the embodiment of my invention illustrated
low level through the inherent capabilities of this
type of system to discriminate between signals
in' Fig. V1 the vnoisev voltages appearing at the
and noise and to reject the latter.
anode 36 are detected inthe diode'äl and appear 55
It will therefore be apparent that my inven
tion combines a highly sensitive background noise
suppression action with` a fast-acting trigger
action. Furthermore, the noise suppression cir
cuit is also `effective in removing side responses
caused by the slope characteristics of the sides
of the intermediate frequency response curve.4 lIf
the receiver is mistuned or misaligned, frequency
modulation signals will be detected in the inter
mediate frequency circuits _through Well known
-upon the load resistor 52. Preferably the time
constant of this detector circuit is such that the
noise voltages appearing on the resistor 52 lie
within the audio range, higher frequency com
ponents being filtered out. Itis desirable to vuse
the audio noise voltages for control since volt
ages of these frequencies tend to be transmitted
through the signal channel following the limiter
and to cause audible disturbances in the loud
speaker 25. Furthermore, voltages at frequen
cies within the intermediate frequency pass band
are of course not removed‘by the limiters but
, are developed at the anode 36 even when signals
slope filter action, producing amplitude modula-tion signals across the limiter anode resistor and
resulting in the production of a squelch bias in
the same manner as noise voltages.
Since noise voltages within this range cannot be f
For lcompleteness of illustration only, and not
in any sense by way of limitation, the following
readily distinguished from signal voltages, such
circuit constants are given as being typical for
higher frequency voltages must be filtered out
a frequency modulation receiver of the type rep
resented in Fig. 2. These constants'have been
found to be satisfactory yfor the circuit elements
of satisfactory strength are being received.
in order for the desired control effect to-be re
sponsive to noise voltages alone.
The audio noise -voltages are again detected in
of a frequencymodulatio'n broadcast receiver
adapted? to receive signals in the vicinity >of 42
detector also coupled to the'output of rsaid lim
iter, ;and means_for biasing said control'electrode
in response to voltages developed by said detector
and. in asense to reduce the ampliñcation of said
t'oï 5U megacycles'and having' anr intermediate fre
quency channel tuned to about. 4.3 megacycles.
In this particular receiver the .tube 30 was a type
6SJ7 pentode and the audio frequency vamplifier
12, the «diode 5I andthe diode 54 were all em
bodied »ina single duo-diode, high-mu triode, type
bination comprising a» signal ampliñer, an ampli
tudeV limiter coupled to said amplifier,` output cir
Other circuit constants follow: .
Capacitor 31 __________________ __mmf__
Resistor 3'2 .... _e _____________ __o'hms__ 180,000
Capacitor 3L` _________________ __mmf__
Capacitor 55 _____________ r.. .... „mid“
cuits coupled to. said limiter, said circuits in
10 cluding an electron discharge device having a
control electrode', an amplitude modulation de
Resistor 38 _________ __; _______ __ohms__
tector connected to the output of saidlimiter,
and means for biasing said control electrode _in
response to modulation voltages developed by said
detector- to prevent translation of saidV signals by
Resistor 56à _____________ __'__I__ohm"s__
Capacitor 57 _______ _~_ _________ __inmf"
Resistor. 58 _____ __' ________ __megohms__
Resistor 59 ____________________ __do_~_„„
Capacitor 6'0 ..... __. __________ _I__mf'du
Resistor 61 _______________ __megohms__
4. In a» frequency modulation receiver, the com
said electron discharge device.
5. In combination, a signal amplifier adapted
to amplify high frequency voltages modulated in
frequency in accordance with a band of signal
frequencies, said amplifier having such a high
gain that undesired noise voltages of. interfering
my invention, it will of course, be understood that
frequencies are continuously Ápresent in its out
I do not wish to be limited thereto since various
put,` an. amplitude limiter coupled to the output
modifications may be made, and I contemplate
of .said amplifier, said limiter being effective to
by the appended claims to. cover any such modi« 25 remove` amplitude modulation, yat signal» frequen
fications as fall Within the true spirit and scope
cies, from said 'voltages only so long.r as high fre
ofl my invention.
quency voltages exceeding a predetermined limit
' What Iv claim as new and desire> to secure by
inglevel are supplied from said amplifier, a signal
Letters Patent of the United States is:
reproducer coupled to said limiter, and means
1. In combination, a signal channel for trans 30 responsive to vamplitude modulation at signal fre
lating signal-bearing waves,.am-plitude limiting
`„quencies in the `output ofv said limiter for pre
means in said channel effective substantially to
ventingoperation of. said reproducer.
remove amplitude modulation. from waves im
6. In radio receiving apparatus for operation
pressed thereon only so long as a predetermined
on Vhiglfi-frequency Waves modulated in frequency
limiting rlevel is exceeded, and means responsive
in accordance with a band of signal frequencies,
to amplitude modulation in waves in the output
the combination comprising a thermionic ampli
of said limiting means for disabling subsequent
ner; for said waves having .such av high gain that
portions of said'channel comprising means for
undesired. noise voltages of interfering frequen
rectifying said amplitude modulation to produce
cies,. due to received disturbances,A thermal agita
a control voltage varying in accordance with the
tion,~ shot. effects,y and the like, are continuously
magnitude of said modulation.
present in its output, an amplitude limiter cou
2. In combination, a signal translating channel
pled to said. amplifier comprising a thermionic
having means therein adapted to translate fre
device-having an output electrode, said limiter
quency-modulated high-frequency waves, said
being. of` such' a character that amplitude modu
V While I have shown particular embodiments of
means also being effective to supply substantial `z. lation components. at signal frequencies are con- `
noise Vvoltages to its output, amplitude-limiting
means coupled to said translatingmeans to limit
tinuously present in potentials appearing on said
electrode so long as received high-frequency
Wavesy do not exceed a predetermined limiting
the amplitude of high frequencywaves supplied
through 'said limiting means, said amplitude-lim
level but are reduced toa negligible'value so long '
` iting means having amplitude modulation com
as said waves exceed said level, signal translating
circuits including a signal reproducer coupled. to
said. electrode, an: impedance element coupled to
said electrode> and adapted to have voltagesV of
ponents' due>y to said noise voltages developedin
itsV output when said high-frequency AWaves» sup
plied therethrough are of intensity less than a
signal. frequencies developed thereon in response
being substantially removed from its output so 55 to. said components, and means for muting said
long as the intensity of said waves exceeds said
signal; reproducer in response to said last-named
level, means for detecting said rcomponents and
deriving a unidirectional potential therefrom, and
7. Inradio receiving apparatus for translating
means responsive to said unidirectional potential
high-frequency Waves modulated. in frequency by
' for substantially reducing the transfer efliciency l60 desired signals, the combination comprising ther
. predetermined level, said‘modulation components
3. Incombination, a signal amplifier adapted
to amplify high' frequency voltages modulated in
mionic means for amplifying received waves. to
such. a .high level that substantial undesired noise
voltages of the frequencies of said signals and
frequency in accordance with .a band of signal
due to received. disturbances, thermal agitation,
of said channel.
said , amplifier
undesired 65 shot effects, and the like, are developed, ampli
noiseA voltages of interfering frequencies continu
ously present in its output, an amplitude’limiter
coupled to said ampliñer, said limiter being ef
fective to remove amplitude modulation, at signal
frequencies, from said voltages only so long as
high frequency voltages exceeding a predeter
- mined limiting. level are supplied- from said am
plifier, output circuits coupledto said limiter,
said circuits: including ‘an amplifying device have
ling‘a :control electrode, an amplitude. modulation
tude limiting means coupled to said amplifying
means, said last means being effective to reduce
said undesired voltages to a very/low level in its
output only so long as high-frequency waves ex
ceeding a predetermined minimum limiting level
`areimpressed on its input, means coupled to said
outputfor translating said high-frequency waves,
andA means responsive to said undesired voltages
appearing in said output for disabling said trans
lating means` comprising ~means for rectifying
said voltages to .produce a'control voltagevarylng
output from said receiver'in accord with'the rec
in accordance .with the lmagnitudeof said unde
tified'noise voltages.y
sired voltages."
8.'v In a frequency modulation receiver of the
type 'comprising -high frequency amplifiers for
-method which comprises controlling the‘output
vfrom-said receiver .in accord with noise voltages of
varying amplitude and of the lfrequencies of said
amplifiers provide sufliciently high amplification
that undesi?ednoise voltages ~are continuously
supplied to -said limiter, and wherein said limiter
is effective substantially to :prevent rtransmission
signals appearing 'atfthe output fof" saidlimiter s
during periods of reduced intensity ofithevre
of noise voltages therethrough only so long as said ,
ceived carrier wave.
detecting noisevoltages transmitted through said
limiter, -m'eans‘for` rectifying said detected volt
ages to provide unidirectional control potentials,
and» means for utilizing‘said control potentials to
. î. p
L_v :>
Y 13.` The combination', in a‘receiver fofrîfrequen
cy modulated carrier/waves of an .amplifier for
frequency modulated carrier waves, _ a .-limiter’to
limit-waves amplifiedbysaid amplifier to a‘ pre
determined value, saidamplifier havingi ampli
fication` s'ufñcient to produce noise` voltages " of
objectionable intensity and variable amplitudeat
prevent transmission of noise voltages from said
limiter tov said signal reproducer.
` 9. In radio receiving apparatus adapted to
translate high-frequency waves modulated in fre
quency by desired audio'frequency signals, the
combination of means for limiting the amplitude
of said frequency modulated Waves to a substan
tially constant level, said means comprising a
thermionic device'having an anode circuit, fre
quency modulation signal detecting means cou
pled to said anode circuit, an electron discharge
amplifier coupled to the output vof vsaid detecting
means, said amplifier having a _control element,
ceived carrier waves are first amplified to intensi
tyY in excess of a ‘predetermined value and then
transmitted through `a llimiter Which-limits said
waves- to a uniform. intensitylof saidî value, the
amplifying waves modulated in frequency by de
sired signals,-an amplitude limiter, a frequency
demodulator, Va lowV frequency signal amplifier and
a signal reproducer, wherein said high frequency
waves exceed a predetermined limiting level, a
noise suppression circuit comprising means for
12. In-.a `»receiver for carrier waves; frequency
modulated by desired signals in which/the' re
the output, of said limiter in the absence" of -re
ceived carrier waves, said noise voltages" disap
peeringv from said output in the presenceof re-`
ceived carrier Waves of greater thany a predeter
mined intensity, and means> to disable said? Are
ceiver in response to the amplitude variations of
said noise voltages at the' outputjof said`Ã limiter.
14. The combination, in a lreceiver for frequen
cy modulated carrier Waves,l of 'an .amplifier for
signal `modulated carrier waves,` a limiter 4to limit
waves amplified by said ampliñer to a‘pr'edeter
mined value, said amplifier' havingamplilicat‘ion
lsufficient to` produce noise- voltages of `objection
means comprisingV a diode detection circuit for
developing audio frequency voltages in response 35 able intensity at the output of said limiter in the '
absence of received carrier Waves, said noise volt
to audio-frequency amplitude modulation compo
ages disappearing from said output in the pres
nents present in Waves developed in said anode
ence of received carrier waves of greater than a
circuit, means comprising a second diode detec
predetermined intensity, a rectifier, means to
trol potential in response to said audio frequency 40 supply said noise voltages to said rectifier, said
means including means to eliminate voltages of
voltages, and means for biasing said control ele
the frequency of the carrier wave amplified by
ment negatively in accordance with said control
said amplifier, and means responsive to rectified
10. In radio receiving apparatus adapted to 45 voltages produced by said rectifier to disable said
translate high-frequency Waves modulated in fre
15. The combination, in a frequency modula
quency by desired audio frequency signals, the
tion receiver, of an amplifier for frequency mod
combination of means for limiting the amplitude
tion circuit for developing a unidirectional con
ulated carrier waves, a limiter to limit Waves am
of said waves to a substantially constant level,
plified by said amplifier to a uniform intensity, a
said means comprising a self-biased thermionic 50 rectifier connected to rectify noise voltages at the
limiter having an anode circuit, frequency modu
output of said limiter, a second rectifier, means
lation signal detecting means coupled to said an
0de circuit and an audio frequency amplifier cou
pled to the output of said detecting means, said
to supply variations in voltage at the output of
a resistance-capacitance network inrsaid anode
tified voltage produced by said second rectifier to
reduce the output from said receiver.
said first rectifier to said second rectifier and to
prevent said carrier waves from reaching said
audio frequency amplifier having a grid electrode, 55 second rectifier, and means responsive to the rec
circuit adapted to have audio frequency noise
voltages developed therein, means comprising a
diode detection circuit for developing unidirec
tional control potentials in response to said noise
voltages, said circuit having a time constant long
er than the period of the lowest desired audio fre
quency, and means for biasing said grid elec
trode negatively in accordance with said poten
16. In a receiver of angle modulated carrier
wave energy of the type provided with an ampli
tude limiter stage, a demodulator and a modu
lation signal amplifier; the method which com
prises detecting from the output of said limiter
noise voltage components when the received sig
nal-to-noise ratio is excessively low, rectifying the
65 detected noise voltage components to provide a
11. In a receiver for carrier Waves frequency
control voltage, and impairing the operation of
modulated by desired signals in which the re
said modulation signal amplifier in accordance
ceived carrier waves are first amplified to inten
with said control voltage.
sity in excess of a predetermined value and then
17. A method of receiving frequency modulated
transmitted through a limiter which limits said 70 waves which comprises limiting the amplitude of
waves to a uniform intensity of said value, the
the waves, demodulating the limited Waves, am
method which comprises rectifying noise voltages
plifying the demodulated Wave energy, detecting
of the frequencies of said signals and which ap
from the limiter output noise voltage components
pear at the output of said limiter during periods
in response to the decrease of received wave en
of reduced signal intensity, and controlling the
ergy below a usable value, deriving from the de
2,404, 338
tected -noise voltagecomponents a Vcontrol >volt
age, andpreventing said amplification of the de
modulated wave venergy in response ¿to said con
18.?¿In a receiver »of angle modulated carrier
Vvv'aveienergy `of_ the type provided with anvampli
tudeflimiter stage, the method which comprises
" detecting from the limiter output noise voltage
Vlcon‘iponents when the :received signal-toFnoise
ratio is excessively low, rectifying the detected '10
components .to provide a control voltage, and uti.;
lizing theco'ntrol Vvoltage to impair receiver re
'amplitude limiter, a detector and a modulation
amplifier; the improvement which comprises rec,
tifying amplitude modulation noise energy ap.
pearing in the limiter output when the angle
lmodulated energy decreases 4to -a level such that
peakAv carrier voltage is substantially equal to peak
noise voltage, deriving a mutingY bias voltagefrom
tlie -rectii'ied noise energy, and suppressing oper.
ation of the .amplifier with the bias voltagel
V22.111 _a frequency .modulation receiver’of the
type having an amplitude modulation limiter, a
detectorhand an audio ampliñer; »the improve
ment which includes means for detecting ampli
-„19. A method of receiving frequency modulated
tilde modulation appearing in the limiter output _
waves which .comprises -limiting the amplitude of l15 energy in response to adjustment Lof the tuning
theA Waves, detecting from the limiter ,output noise
of said'receiver between stations,î means for rec-`
-voltage components in response to the decrease of
tifying the noise modulation produced as a result
l.of said detection, and -further means lfdr muting
received" wave »energy below a usable value, de
riving `from the noise voltage components .a con
said gaudio' ampliñer with the rectified voltage
trol voltage, :and preventing reproduction of the 20 thereby y-to prevent the rush oineisercommonly
wave energy `in response to said control voltage.
produced during absence of carrier.
23. A system of receiving angle modulated .car
.20.' linv a method of frequency modulation re
ception wherein the amplitude of frequency mod
rier energy including means limiting the ,energy
ulated vcarrier Wavesvis limited in a limiter prior
to reduce amplitude modulation effects, means
tov detection; :the improvement which comprises
demodulatzing the limited energy, vmeans fOr am
deriving »a .control voltage from amplitude modu-`
plifying »the demodulated energy, means deriving
f 'lation appearing in the output of the limiter when `
a :control voltage from amplitude modulation
the 'received :signal to noise ratio is below a pre
:noise energy appearing inthe output of said lim
‘ f determined threshold level, and suppressing the
iti-ng means When the received modulated-carrier
reproduction of detected energy in response to l30 energy decreases below a `predetermined thresh
1 . .said control 4.voltage thereby to prevent reproduc
Y old, _and means >for inhibiting said ampliiication
with said control voltage.
tion of noise.
` y. :21. In. = e operation of a receiver of angle mod
ulated carrier ywave energy which >comprises an
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