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Jan. 7, 1947.
W. R. KOCH
2,413,977
ANGLE-MODULATION WAVE RECEIVER
F1198 Nov, 18, 1944
Am
2 Sheets-Sheet 1
INVENTOR
ATTORNEY
Jan. 7, 1947.
'w_ R, KQCH- \
2,413,977
ANGLE-MODULATION WAVE RECEIVER
~ Filed Nov. 18, 1944 ,
2 Sheets-Sheet 2
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I 712,4.
BY
ATTO'RN EY
Patented Jan. 7, 1947
attain
UNITED STATES-PATENT OFFICE
ANGLE-MODULATION WAVE RECEIVER
l/Vin?eld H. Koch, Haddon?eld, N. J., assignor to
Radio Corporation of America, a- corporation
of‘ Delaware
Application November 18, 1944, Serial No. 564,047
12 Claims. (Cl. -250—-27)
1
2
My present invention relates to receivers of
frequency modulated (FM) or amplitude modu
lated (AM) carrier waves, and more particularly
to radio receivers utilizing novel FM detector cir
cuits.
50 me. band ‘is presented by way of illustration,
since it is the FM broadcast band presently as
signed to such transmission. The 550 to 1700 kc.
band is the present AM‘ broadcast band assigned
to transmission of AM signals.
'
An important object of my present invention is
It will further be understood that in the present
to provide a novel detector of angle modulated
description and ‘following claims the generic ex
carrier waves wherein each of a pair of recti?er
pression “angle modulated” is intended to include
tubes concurrently functions as a load impedance
frequency modulation or phase modulation.
for its companion tube.
10 Froma very general viewpoint my invention re
Another important object of my invention is
lates to a demodulator network having separate
to provide an FM-AM detection network wherein
input circuits for carrier waves of different fre
no actual detector output load resistors are em
quencies and of different ‘modulation character
ployed, but instead the detector tubes per se are
istics.
employed for that purpose.
The numerals I and 2 in Fig. 1 denote respec
Another object of my invention is to provide
tively different sources of modulated‘ carrier
a switchless FM-AM detector network wherein
waves. Source I may be the usual signal ‘col
triode detection is utilized, and each triode serves
l'ector, such as a dipole, employed‘for collecting
as the output load element for another triode.
‘FM Waves.
The FM carrier waves are transmit
Still other objects of my present invention are 20 ted from FM transmitters at a center, or carrier,
to improve generally the ef?ciency of FM-AM re
frequency assigned to the particular transmitter.
ceivers, and more especially to provide economical
In the assumed FM band of 40*to 50'mc. the radi
detector circuits for FM receivers.
ated. carrier wave frequency would be in that
Still other objects of my invention will best be
range, and would be ‘a wave of variable frequency
understood by reference to the following descrip
tion, taken in connection with the drawings, in
which I have indicated diagrammatically several
circuit organizations whereby my invention may
be carried into effect.
In the drawings:
Fig. 1 shows, in partial schematic form, an FM~
AM receiver employing one embodiment of my in
vention;
Fig. 2 shows a modi?cation ‘of the detector net
work of Fig. 1;
Fig. 3 illustrates the FMv detection characteris
tic of the detector circuit of Fig. 2; and
and substantially uniform amplitude. As is Well
known, the frequency modulation of the carrier
wave would be in accordance with the modulation
signals at the transmitter. The extent of fre
quency‘deviation of the carrier frequency is a
function of the modulation signal amplitude,
while the ‘rate of frequency deviation is dependent
upon the modulation signal frequencies per se.
The permissible extreme frequency deviation in
the FM band of 40 to 50 Inc. is '75 kc. to either
as.CT side of the carrier frequency. These frequency
values are purely illustrative.
Source 2 may be a suitable grounded antenna
Fig. 4 shows a further modi?cation of the FM
circuit employed in AM broadcast reception.
detector circuit.
The allotted channels ‘are 10 kc. wide in this band.
Referring now to the accompanying drawings, an In AM transmission the ‘carrier wave is modulated
wherein, like reference characters in the different
in amplitude in accordance with the modulation
?gures designate similar circuit elements, Fig. 1
signals. The carrier frequency is maintained
shows an illustrative receiving system embodying
constant in value at the transmitter. The nu
a demodulator network adapted to provide audio
meral 3 designates a tunable radio frequency am
frequency voltage in response to FM or AM signal
plifier having ‘suitable signal selector circuits for
reception. The receiver circuits prior to the de
FM and AM reception. Switching devices 4 and
modulator are schematically represented. Those
it are provided for separate connection of the
skilled in the art of radio reception are well ac
sources I and 2 to respective selector circuits of
quainted with the nature of the circuits cus
ampli?er 3. It will be understood that when
tomarily employed in multi-band receivers.
switch 4 is in closed position, collected FM signal
While my ‘invention is readily adapted for FM
energy will be applied to selector circuits of am
and AM reception on respective bands of 40 to 50
pli?er 3 capable of selectively amplifying the FM
megacycles (me) and 550 to 1700 kilocycles (kc),
signals over a‘ band at least 150 k'c.'wide. Upon
it is to be clearly understood that the invention is
closing of switch '5,_and opening switch 4, the
not limited to such frequency bands.v The 40 to 155 same ampli?er 3 will have the FM selector cir
2,413,977
3
4
cuits thereof operatively replaced by AM selector
nized that circuits 8, I2 and I5 provide the well
known frequency discriminator network of U. S.
Patent No. 2,057,640 to Conrad. The action of
circuits.
These latter circuits will select the col
lected AM signals, and permit ampli?er 3 to am—
plify the same over a 10 kc. band.
Multi-band
selector circuits and switching devices for suit
able change-over are well known to those skilled
in the art of radio communication.
this form of discriminator circuit is well known
It functions to trans
to those skilled in the art.
late FM high frequency signals into correspond
ing AM high frequency signals.
At the 4.3 mc. frequency used for FM recep
Assuming the system is of the superheterodyne
tion the impedance of circuit I5, I8 is negligible.
type, as is the usual practice at present, the con
verter 6 and intermediate frequency (I. F.) am 10 Therefore, the low potential side of circuit I5 is
pli?er ‘I will, also, be provided with suitable FM
and AM signal selector circuits. At the converter
6 the FM signals will have the center frequency
effectively at the cathode potential of detector
triode II.
The detector triodes are shown ar
' ranged in series relation so far as the space cur
rent paths thereof are concerned. The plate 20
thereof reduced to a value which may be chosen
from a range of 1 to 20 mc., as for example 4.3 15 of triode I0 is connected to the +13 terminal (say,
for example, +200 volts) of a suitable direct cur
mc. The AM signals are reduced to an I. F. of
rent supply source. The cathode 2I is connected
455 kc., as an illustrative frequency value, the
directly to the plate 22' of triode I I. The low
latter being a commonly employed frequency in
potential side of input circuit I2 is connected to
AM broadcast receivers of the superheterodyne
type. The I. F. ampli?er ‘I, which may consist of 20 cathode ‘2L The control grid 22 of trio-dc I0 is
connected by direct current blocking condenser
one or more separate stages of ampli?cation, will
23 to the high alternating potential side of input
have an ultimate output circuit from which may
circuit I2. Resistor 24 provides a direct current
be derived, at separate points thereof, the ampli
return path for grid 22. Similarly in the case
?ed FM signals or AM signals.
The selective circuits 8 and 9 are to be under 25 of detector triode II, the resistor 25 returns grid
I3 to the grounded cathode I9.
stood as being arranged in series in the plate cir
It will be seen that a unique feature of my
cuit of the last I. F. ampli?er tube. Each of cir
present circuit is the complete absence of output
cuits 8 and 9 is resonated to its respective op
or load resistors external of tubes I0 and I I. The
erating I. F. value for PM or AM reception.
Thus, circuit 8 is tuned to 4.3 mc., while circuit 30 internal cathode to plate impedance of each of
triodes I0 and II is utilized as the load imped
9 is tuned to 455 kc. There will be developed
ance for the companion triode. The internal re
across tuned circuit 8 the FM signals at the 4.3
sistive impedance of triodes I0 and II are indi
mc. mean frequency when switch 4 is closed. and
cated by numerals 26 and 21 respectively. The
all FM selector circuits of ampli?er 3, converter
6 and I, F. ampli?er ‘I are in operative electrical ' output resistive impedances 26 and 21 are indi
cated by dotted lines to show that they are the
connection. Conversely, when switch 5 is closed,
internal resistive impedances of the triodes I0
and switch 4 is open, and all AM selector circuits
and I I. The junction of resistive impedances 26
are in operative electrical connection, there will
and 21 is connected to the audio frequency cou
be developed across circuit 9 the AM signals at
the I. F. value of 455 kc. The impedance of cir 40 pling condenser 28, the carrier bypass condenser
29 connecting the junction point to ground.
cuit 9 is negligible at 4.3 mc. Hence, the inser
Leads 30 function to feed the modulation signals
tion of circuit 9 in series with circuit 8 will not
to a subsequent suitable audio output circuit.
affect the’ development‘ of FM signal voltage
The detectors may function as grid leak de
across circuit 8. Similarly, the impedance of cir
cuit 8 is negligible at 455 kc., and circuit 8 will 45 tectors, or grid current-biased plate detectors.
Thus, if the condensers 23 and III were each as
not affect development of AM signal voltage
signed a value of 50 micromicrofarads (mmf.)
across circuit 9.
’
and each of resistors 24 and 25 a value of 200,000
The present demodulator comprises two elec
ohms, the detectors would perform as grid recti
tron discharge devices of the triode type. The
triode I0 is provided with a resonant input cir 50 ?cation devices. By assigning a value of 0.005
mmf. to each of condensers 23 and I4 and a
cuit I2 which is inductively coupled to the cir
value of 10 megohms to each of resistors 24 and
cuit 8. vTriode II has its cathode I9 established
25, the detectors perform as grid current-biased,
at ground potential, while its grid I 3 is connected
plate recti?cation. devices.
'
through condenser I4 to the high alternating po
During AM signal reception the I. F, signal en
tential side of its resonant input circuit I5. Cir 55
ergy produced in the circuit 9 will be transferred
cuit I5 is, also, inductively coupled to the circuit
to input circuit I6, I8. Each of circuits 9 and
8. The low potential side of circuit I5 is con
I6, I8 is tuned to the operating I. F. value of 455
nected to the grounded cathode I9 through the
kc. The circuit I5 and the grid to cathode path
coil I6. Coil I6 is magnetically coupled to cir
cuit 9, and condenser I8 shunts coil I6 to pro 60 of triode I I are in a series circuit with tuned cir
cuit I6, I8. The circuit I5, resonant close to
vide a resonant circuit IB-I8 tuned to 455 kc.
4.3 mc., has no appreciable effect on the series
The input circuits I2 and I5 of respective tri
circuit, since it acts as an extremely low imped
odes I0 and I I are oppositely and equally mis
ance connection at the 455 kc. frequency value.
tuned with respect to the operating I. F. value for
FM reception. In other words, if the circuit 8 65 The modulation (AM) signal voltage component
of the recti?ed I. F. energy developed across load
has an operating frequency FL: of 4.3 mc., then
resistive impedance 26 during grid leak detection
'circuits I2 and I5 will be detuned in opposite
at tube II is applied through condenser 28 and
senses by equal predetermined frequency values
leads 30 to the common modulation signal out
relative to Fe. Thus, circuit I2 is indicated as
tuned to a frequency in excess of Fe, and circuit 70 put circuit. Hence, it will be seen that during
AM signal reception the tube II functions as the
I5 is represented as being tuned to a frequency
detector tube for the AM signals at input circuit
less than Fe. For ‘example, circuit I2 could be
I6, I8. The output resistor in that case is the
tuned to 100 kc., more than 4.3 mc. (4.4 mc.)
internal cathode to plate resistance of tube I0.
whilecircuit I5 could be tuned to 4.2 mc. These
are only illustrative values. It will be recog 76 In other-words, the audio frequency voltage de
2413,07?
5
6
veloped across resistiveimpedancez?? is utilized
through'the output leads 30.
reception, andv upon grid‘ current ?owithrough
resistor 25, AVG biasis produced.
To‘ detect FM‘ signals the tuned circuits 8’
For FM‘signal reception, on the other hand,'in
ternal ‘resistances 26' and 21' are alternately used
and [2' are employed. ‘Resonant circuit 8' is
connected ; in series with circuit 9 in the I. F.
as the frequencyolf the I.‘ F; energy deviates
relative to its mean or» center frequency value.
amplifier output circuit, and is tuned approxi
Assume, ?rst, thatthe FM signal‘is at4.3 me.
mately to Fe (4.3 mc.). The resonant circuit I2’
In such case both circuits l2 and i5 apply equal
is tuned somewhat above Fe. The control grid
signal energy to the respective detector tubes.
22 of ‘triode I9 is coupled by condenser 23 to
Assuming that tubes Ill‘ and H are of like con l0 the upper end of coiLSil. The latter is magneti
struction, the'resistance values of'impedances 26
cally coupled to circuit 8’, and has its lower end
and 2'! will be equal. Therefore, there will- be
connected to the cathode 2| of triode Ill. The
equal voltage drops across impedances '26‘ and
resistor '24 connects grid 22 to cathode 2 l . Hence,
21, and theiipotential‘at junction‘ point a will
during FM reception triode II] has applied there
have a predetermined normal or mean: value.
.
to the signal energy across circuit 8’, while triode
Suppose; now, that’ the applied FM'signal
H has applied‘ to it the signal energy existing
energy at: circuit ii? deviates ini‘frequency‘ to
in the circuit including both circuits 8’ and [2’.
wards the frequency of" circuit l2l (4.4 incl).
Of course, circuit 9 has no effect at FM reception,’
“ The current flow through the tube It} would
since its impedance is negligible at 4.3 mc.
decrease, assuming the condenser 23 and resist; 2%)
The aforesaid Reid patent fully discusses‘ the
tor Hare chosen to provide grid leak‘detection.
operation of an FM discriminator of the type
There would occur a relatively larger voltage
shown in Fig. 2. By virtue of the discriminat
drop across resistiveimpedance 26. This fol—
ing action of circuits 8' and [2' the triodes l0
lows from these considerations. When a‘ signal
and l l will have amplitude modulated waves ap
is applied to the grid 22, rectification makes the
plied thereto which were derived from the re
average grid potential more negative ‘relative to
ceived FM signal waves. Resistive impedance
cathode 2|. This tends to stop electrons from
2S acts as the load for triode H, and develops
flowing to plate 28; Theelectrons start piling
recti?ed voltage thereacross. Resistive imped~
up on the cathode 2! thereby ‘making it more neg
ance 2i’ acts as the load for triode It}, and de
ative relative to the‘ plate. Therefore, the'voltage 1‘ 1 velops recti?ed voltage thereacross.
across the internal impedance 2B ‘of the tube is
The FM signal voltage across circuit 8’ is ap
greater. This would mean that‘a smaller voltage
plied to grid 22 through coil 38. Triode grid l3
drop existed across resistive impedance 21.
has applied to it the signal voltage across its
Hence, the point a would‘ be relatively. less‘posi
own input circuit. At the frequency where cir
tive relative to ground. If; now, the FM‘ signal 3*‘ cuit l 2‘ is anti-resonant the voltage across cir
cuit l2’ will be high, while that at grid l3 will
frequency deviates towards the resonance fre
be very low. At a somewhat lower frequency,
quency of circuit Hi, the reverse‘ takes place.
circuit IE’ will have an inductive reactance which
That is, the point a becomes more positive .rel
will series resonate with the capacity between
ative to ground. The sense anddegree of poten
tial change of point a will be dependent upon the 40 grid l3‘ and cathode l9 thereby making the volt
age applied to the triode l i very high. It is thus
direction. and amount respectively of'ifrequency
apparent that the radio frequency voltage ap
deviation of the FM. signaLenergy. Itwill be
plied to triode l I changes very rapidly with fre
noted that the audio output circuit is. single
quencies between th'ese points. At the ?rst fre
ended. Condenser. 29 will be chosen to bypass
quency the voltage across circuit 8’ will be high
all high frequency components, whether at .455;
thereby causing the voltage applied to triode It)
kc. orat 4.3‘ me.
>
.
to be high. At the second frequency the’ series
The detection networlsof Fig.1 mayv assume.
resonant circuit formed by circuit l2’ and the in
di?erent forms. Figs. 2 and 4, show. different
put capacity of triode ll will load‘ down circuit
modi?cations of the invention. InFig. 2 I_ have
8’ thereby causing the voltage applied to triode
I!) to be low. Hence, the recti?ed voltage output
shown a modi?cation of the invention wherein
for FM signal reception thereis used a. discrimi»
nator which is of the type. disclosed ‘and-claimed
by John D. Reidin U. S. Patent. No. 2,341,240
of triode. H changesin a direction opposite to
that of triode l0 as the instantaneous frequency
granted February 8,. 1944. In this embodiment 55 of the applied FM signals varies. Correct cir
cuit design produces the desired characteristic.
of the invention the AM- circuit 9- providesv the
In Fig. 3 there is illustrated the frequency re»
input for. triode-detector H; The condenser. Ill
sponse characteristics at grid l3 (solid line curve)
and resistor 25 function to provide grid‘; leak
and grid 22 (dotted. line curve) respectively. The
detection for AM‘ reception. Here, again,_ the
resistive impedance 26 acts as the: plate ‘load 60 cross-over frequency is at 4.3 me.
In Fig. 4 there ‘is shown a further modi?cation
for the grid‘ leak detector tube l I. It’ will be
of my invention, wherein the AM input circuit
noted that the high alternatingrpotential side of
9 is omitted and the‘I. F. ampli?er includes the
input circuit’ 9, is'icoupled’ to grid; l3'by‘a; series
F. i. circuit 8 tuned to 4.3 Inc. The latter is cou~
path consisting of .tuned‘f circuit'i8', condenser l4
pied magnetically to resonant circuit 3i tuned to
and resonant circuit 12'. Since circuits 8! and 65 4.3 me. The plate side of circuit 3 is connected
i2’ are tuned: in the:vicinity' 0f‘4;3. mo.‘ theyahave
by lead 432 tothe midpoint ofcoil 33 of secondary
negligible impedance at 455-kc; Hence;v during
AMjreception the grid l3 is-in- eflfectdirectlyv ‘ ‘
connected to the high potential side‘ of circuit‘
9. An automatic volume control '(AVC) connec-i
tion 40 isr'nade‘ to the grid‘end of'resistor' 25v
to provide AVC' bias for‘ prior-gain controlled
tubes. Filter; networkrlll isemploycd'to suppress
any. pulsating: components: of‘ thenAVC' voltage; "
circuit 3?. Control grid22 is coupled byv direct
current blocking, condenser
to one side of cir~
» cuit‘iii, and control grid‘lli is coupled by direct
70 current blocking ‘condenser l 4 to the opposite side
of circuit 3!‘. The frequency]v discriminator oi
- Fig. 4 is of the typedisclosed and ‘claimed by S.
' W. Seeley in-U. S. Patent No. 2,121,103 granted
As- the. carrier;- amplitude:- increases1 duringv AM. 75
June 21, 1938.
,
The discriminator: circuit of Fig. 4 functions
2,413,977
8
7
to provide at each of grids 22 and l 3 a signal volt
age whose magnitude is the resultant of two sig
nal voltages. The magnetic coupling between pri
mary circuit ii and secondary circuit ill results
in the application to grids 22 and I3 ofsignal
voltages in polarity opposition and in phase quad
connecting the plate to cathode impedances there
of in series relation, a frequency discriminator
input circuit, separate connections from said fre
quency discriminator circuit to a respective con
trol grid of each of said triodes, a modulation
signal output circuit connected across one of said
plate to cathode impedances, a Signal input cir
cuit tuned. to a frequency sufficiently different
age. The direct connection of circuit 8 to the
from the resonant frequency of said discriminator
midpoint of coil 33 causes signal voltages to be
applied to grids 22 and i3 in like polarity and 10 circuit to have negligible impedance effect on the
latter, and connections between the input elec
in parallel. Hence, at signal frequency Fc each
trodes of one of said triodes and said signal input
grid has applied thereto the resultant vector volt
circuit.
age of a phase-shifted signal voltage and a non
4. In a frequency modulation receiver, a pair
shifted voltage in normal phase quadrature. The
of detector tubes each including at least a cath
resultant vector voltages at grids 22 and i3 are
ode, control grid and anode, means for establish
equal at Fe. However, for signal frequencies
ing the cathode of one of the tubes at ground po
different from F0 (the operating I. F. value) the
tential, means connecting the anode of said one
resultant vectors become unequal, because the
tube directly to the cathode of the second tube,
shift due to the magnetic coupling between
rature relative to the primary circuit signal volt
circuits 8 and 3| changes.
The respective re~
sultant vector voltages at grids 22 and f3 wili
vary in sense and magnitude in response respec
tively to the direction and extent of signal fre
quency deviation.
These resultant signal voltages will cause cor
means for applying a positive potential to the
anode of the second tube whereby the space cur
rent paths of the two tubes are in series relation,
modulation signal output connections connected‘
to the anode and cathode of one of said tubes, and
a frequency discriminator network having sepa
responding voltage drops across the internal re
sistive impedances 26 and 21. The operation is
quite similar to that described in connection with
Fig. i. If desired, an AM input circuit could be
added to cooperate with one of the detector tubes
it or H. Of course, the triodes could be located
rate signal input connections to the control grids
of said pair of tubes.
in a single tube envelope, in the manner of a
source to said detector tube input and cathode
electrodes, a second tube provided with an input,
5. In combination with a source of amplitude
modulated carrier waves, a detector tube having
input, cathode and plate electrodes, an input cir
cuit tuned to the carrier frequency coupling said
twin triode, if compact construction were desired.
cathode and plate electrodes and having its inter
Grid leak detection can be made to give only a
small variation in audio frequency output volt- "P nal plate to cathode impedance in series relation
with the plate to cathode impedance of said de
age for a considerable range of input signal am
tector tube, means connecting the plate of the
plitudes. A reduction in noise and distortion
second tube to a source of positive voltage, and
is accomplished in the circuits described through
this type of detection. Low mu tubes are recom
modulation. signal output connections connected
mended where there is available sufficient signal
amplitude to provide AVC, AVC action could be
secured in either of the systems of Fig, l or Fig.
across the plate to cathode impedance of said de
tector tube.
4, in which case low mu tubes are preferred. For
rier waves, a pair of triodes Whose respective in
ternal plate to cathode impedances are connect~
ed in series between a point of relatively high
positive voltage and ground, means for varying
6. In a demodulator circuit for modulated car
maximum sensitivity it is preferred to use high
mu tubes, and will be suitable for signals too weal:
to furnish AVC bias.
While I have indicated and described several
the voltages of the control grids of said triodes
systems for carrying my invention into e?ect, it
in accordance with frequency deviation of a fre
will be apparent to one skilled in the art that
quency-variable wave to be demodulated, and
my invention is by no means limited to the par
means responsive to the voltage variations of the
ticular organization shown and described. but
junction of said plate to cathode impedances for
that many modi?cations may be made without
producing a modulation signal representative of
departing from the scope of my invention.
said frequency deviations.
What- I claim is:
7. In a detection system for frequency modu
1. In a detection system for angle modulated
lated carrier waves, a pair of electron discharge
carrier waves, a pair of electron discharge de
tubes having the space current paths thereof con
vices having the space current paths thereof con
nected in series relation, discriminator means for
nected in series relation, means for deriving from
deriving from said Waves a pair of carrier voltages
said waves a pair of carrier voltages each variable
each variable in amplitude, control grid means
in amplitude, means separately controlling the 60 separately controlling the'space current flow of
space current ?ow of each device in response to
each tube in response to the relative amplitudes
the relative amplitudes of said pair of voltages,
of said pair of voltages, and modulation signal
and modulation signal output connections con
output connections connected across the internal
nected across the space current path of one of
impedance of one of said tubes.
said devices.
8. In combination with a pair of grid leak de
2. In combination with a pair of triodes, means
tector triodes, means connecting the internal im
connecting the internal plate to cathode imped
pedances thereof in series relation, a frequency
ances thereof in series relation, a frequency dis
discriminator input circuit, separate connections
criminator input circuit, separate connections
from said frequency discriminator circuit to a re
from said frequency discriminator circuit to a
spective control grid of each of said triodes, and
respective control grid of each. of said triodes, and
a modulation signal output circuit connected
a modulation signal output circuit connected
across one of said internal impedances.
across onec‘f said internal plate to cathode im
_ _ 9. In combination with a pair of grid detection
triodes, means connecting the internal plate to
_ 3. In combination with a pair oftriodes, means 75 cathode impedances thereof in series relation, a
Dcdances.‘
‘
'
'
'
‘
"
"
’
»
7
2,413,977
frequency discriminator input circuit, separate
connections from ‘said frequency discriminator
circuit to a respective control grid of each of said
triodes, an audio signal output circuit connected
across one of said plate to cathode impedances, an
amplitude modulation signal input circuit tuned
to a frequency su?icien-tly different from the res
onant frequency of said discriminator circuit to
have negligible impedance e?’ect on the latter, and
connections between the input electrodes of solely i
one of said grid detection triodes and said sig
nal input circuit.
10. In a frequency modulation receiver, a pair
of grid leak detector tubes each including at least
a cathode, control grid and anode, means for es [5
tablishing the cathode of one of the tubes at
ground potential, means connecting the anode of
said one tube directly to the cathode of the sec
ond tube, means for applying a positive potential
to the anode of the second tube whereby the space
current paths of the two tubes are in series rela
tion, audio frequency signal output connections
connected to the anode and cathode of one of said
tubes, and a frequency discriminator network
10
having respective signal connections to the con
trol grids of said pair of tubes.
11. In a receiver circuit for frequency modu
lated carrier waves, a pair of grid leak detector
triodes whose respective internal plate to cathode
impedances are connected“ in series between a
point of relatively high positive direct current
voltage and ground, means for varying the volt
ages of the respective control grids of said tri
odes in accordance with frequency deviation of a
frequency modulated wave to be demodulated,
and means responsive to the voltage variations of
the junction of said plate to cathode impedances
for producing a modulation signal representative
of said frequency deviations.
'
12. In combination, a pair of grid leak detector ‘
tube circuits, means connecting the internal im~
pedances of said detector tubes in series relation,
means for varying the voltages of the control
grids of the tubes in accordance with received fre
quency modulated waves, and a modulation signal
output circuit connected across the internal im'
pedance of one of said detector tubes.
WINFIELD R. KOCH.
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