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Патент USA US2404359

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July 23, 1946.
M. E. BOND
' 2,404,359
FREQUENCY DISCRIMINATOR
Filed June 11, 1943
2“.
0.1 0
mwbi:
0.6
INVENTOR. '
MARION E'. BOND
ATTORNEYS '
Patented July 23, 1946
‘
2,404,359
UNITED STATES- PATENT vOFFICEQ]
' FREQUENCY mscamnmr'oa
Marion E. Bond, Chicago, Ill.', vassignorf'to'Galvin
Manufacturing Corporation, Chicago, 111., a cor
poration of Illinois
Application June 11, 1943,
(01. 250-27)
Serial No. 490,467
'7 Claims.
1
2 :
discriminators and more particularly to improve
ments in frequency discriminating networks of
the character utilized in frequency modulated
radio receiving systems for detecting the modu
lation components of a received frequency modu
lated carrier.
'
.
cuit be accurately and precisely tuned to the
center intermediate frequency and that it be
accurately and precisely balanced to produce no
output voltage at this frequency. It is also
necessary that the output voltageof the circuit
accurately and faithfully respond to given de
partures in frequency from the intermediate‘fre
The present invention relates to frequency
‘
quency in either direction. These ends are, diffi
Conventional frequency discriminators, as
cult to achieve with typical prior art arrange:
used ahead of the audio frequency channel of a
' frequency modulated receiver for detecting the 10 ments, because of the difficulties inherent in ac
curate condenser tuning of, the resonant circuit
and the, difliculties inherent in accurately center
tapping, .inductances at "their 'pelectricallmide '
modulation components of a modulated interme
diate frequency carrier, conventionally comprise
a resonant circuit tuned tothe intermediate'fre
quency and designed'to respond to frequencies
which exceed or are less than the intermediate
' frequency by not moreicthan a predetermined '
i5
amount. In the usual arrangement, the signal
'modulated intermediate frequency carrier is in
to have an exact and‘ predetermined operating
troduced into the discriminating network by
tapping. the winding of the ?xed inductance 20
forming a part of the resonant circuit at its elec
trical mid-point, and connecting or coupling
the output circuit of the preceding amplitude
limiter tube to the center tap'of the inductance
element. The capacitance coupling thus pro 25
the inductance element of the discriminator to
the output inductance element of the preceding
*
V
'
‘
It is another object of the present invention to
provide an improved discriminator which ’may
be easily and accurately adjusted to produce no
output voltage at a particular frequency and ‘to
produce an output voltage which accurately con
lar frequency.
limiter stage. The tuning of the resonant cir
cuit to the desired intermediate frequency is uni
characteristic.
forms both in polarity and magnitude to ‘devia
tions in the exciting frequency from the particue
vided is supplemented by inductively coupling
versally accomplished by using an adjustable
points
‘It-‘is in
.an commercial
object of thequantities.
present invention,
' I there'
fore, to provide an improved frequency discrimé
inator which is simple in arrangement and may
be readily reproduced in production quantities
=0
'
‘
'
It is a further object of the invention to pro-'’ _
vide a frequency modulated radio receiving sys
tem which includes an improved and exceedingly
simple arrangement for accurately and faithfully
condenser connected in shunt with the induct
detecting the audio frequency components of a
ance element. The adjustable resonant circuit
received frequency modulated carrier.
thus formed is combined with additional circuit
It is still another object of the invention to
apparatus which includes a pair of rectifying 35
provide an improved discriminator or ‘network
paths and has the function of producing an out
wherein condensers of ?xed capacitance value
put voltage which varies in magnitude at the
and an adjustable inductance element are uti
signal frequency rate in accordance with de
partures of the resonant circuit exciting fre-_
lized to form the tuned circuit which determines
quency from the intermediate frequency and also 40 the: frequency response characteristic of the net
varies in polarity, depending upon whether the
According to another object of the invention,
exciting frequency is increased above or de
an inductance element of , the adjustable per‘
creased below the intermediate frequency.
meability' type is utilized to tune the resonant
The above-described conventional arrange
work.
ment is characterized by, several disadvantages
both from the standpoint of circuit performance
and also from the standpoint of commercial re
,
'
r
'
frequency of the discriminator network to the
desired center frequency. '
In accordance with still another object of the
invention, a discriminator network is provided
production of a receiver; of ,?xcd design inlarge
wherein the exciting voltage is introduced. into
quantities. Thus, in the operation of the dis
criminator it is essential that the’ resonant cir- r the network at the junction point between .twb _,
2,404,359
1
3
'_
4
series connected condensers which also serve to
is impressed upon the discriminator network 21
tune the resonant .circuit to the desired center
through a coupling condenser 86 which is con
nected at one side thereof to the junction point
frequency.
'
_
,
According to a further object of the invention,
,Bld between the two condensers 81b and 810.
capacitance coupling is alone utilized to intro 5 Actually, the condenser 86 provides the only cou
duce the exciting voltage into the discriminator
pling path between the output side of the limiter
network.
26 and the discriminator 21, a grounded conduc
,
tive shielding housing being provided for enclos
According to a still further object of the in
vention, an exceedingly simple arrangement is
provided for improving the stability of the dis
criminator
network.
'
.
,_
'
ing the circuit elements. 87a, 81b and 970 so that
mutual inductive coupling or stray capacitance‘
coupling between the discriminator and the'pre- .
-
ceding limiterris absolutely eliminated. Audio
frequency voltages detected'through ‘operation of
method of operation, together with further ob‘
:jects and advantages thereof, will best be under;
thediscriminator 21 appear across the condenser
stood-by reference to the following speci?cation; "93 andare impressed uponthe input side of the
. The invention, both as to its organization and
taken in connection with the accompanying 7
audio frequency ampli?er 28 through a coupling
drawing, in which:
‘circuit whichincludes radio frequency decoupling I
Fig. 1 represents a frequency grnodulated radiov
resistor 94. an audio'freque'ncy ?lter comprising
receiving system having incorporated therein _an* - f the'resistor;-95 and condenser 96, an audio fre~ .
‘improved discriminator network characterized by 720 quencyv coupling condenser 92. and a’ voltage
the features of the invention brie?y referred to
dividing, network comprising the two resistors
above;
98 ‘and 199 and. a direct current blocking 'con~
a’
, Fig.2 is'an equivalent circuit of the frequency‘
discriminator. network shown in Fig. 1;
Fig.3 is a graph illustrating the di?erent re
denser llll. It will be understood in this regard
that the proportion of ‘the available audio fre
25 quency voltage appearing. across the series con
sponse characteristicswhich are obtained for the
discriminator when the circuit elements of the
nected resistors 98-and'“l90 which is impressed
upon the input" circuit of the audio frequency
ampli?er 28, is determined by the settingrof the .
discriminator are arranged indifferent manners
and at least one ‘of the circuit constants of the.
networkis varied; and
.
a’
2 >
'2
*
'
‘
wiper 99 along the resistor 98. d
30
,I Fig. 4 isa circuit.diagramlillustratinga modi?
.,
-
.
l
The operation of the above described signal
receiving system, as a whole, will .be clear to
cation of, the receiving system shown in Fig. 1. r
schematically, a frequency-modulation. receiving.
those 'skilledin. the art from the foregoing'de
scription. In brief,’ a frequency-modulated car
rier signalintercepted by the antenna circuit .19 ‘
is selected and ampli?ed in theuriit '18,- further _'
system. embodying the frequency . discriminator
selected. and ampli?ed ’ in the I radio-frequency
of the invention. v1 This receiving. ‘system includes
ampli?er 29, converted in the ?rst miXeriZi into
_ Referring inowfmore-particularly 'to Fig.1 of
the drawing, there .is- represented, partially
the'following units, connected infcascade, in the
a . ‘frequency-modulated intermediateefrequency
.orderi named z.. a ' tunablefantenn‘a' circuit 1 8, 5a
waveiwhich is amplified‘ and selectediri the ampli
V tunablefradioifrequency ampli?er'20,"a_?rst mixer ‘
?erj22, converted‘ in the second mixer 23 vtof'a
onconverter. 1sta’gel2l, ai?rst intermediate fre-.
second . frequency-modulated, intermediate'efre- '
quency, ampli?er i, 22,, a -. second mixer 3or [con-v
verterv stage .23, a second‘ intermediate frequency
quency ‘signal .of a lower frequency, further se
lected and ampli?ed in the ampli?er 243, limited
ampli?er 24, a ?rst limiter 25, a second limiter . . inrlimiters 25 and 26 and applied through the
26, a frequency discriminator 2-1, anaudiofre
coupling condenser 86 to' the frequency 'dis-,
quency ampli?er 28, anda loudspeaker 29, all
criminator 2'! embodying the invention, from
connected in cascade in theorder named. -.
l
‘
which it is applied through the audio-frequency
. The frequency discriminator ‘2Tto which the
ampli?er 28 to the loud speaker 29 for reproduce
present invention more speci?cally relates 00ml
tion.
.
.
u
.
prises acircuit 81, a pair of'diode recti?er tubes 50
Referring now more particularly to theopera
88 and 89, thespace current paths of which are
tion of’ the discriminator 21,.it will be noted that
.respectively shunted by load resistors 90 .and 9|‘,
this circuit is essentially a four arm, four ter- .
a radio frequency by-pass condenser 93, having
minal bridge circuit of thev charactershown’in
’ substantially negligibleimpedance to frequencies 2.55 Fig. 2 of the drawing. In this bridge circuit the
of the order of the second intermediate frequency,
capacitances C1 and C2 are of equalvalue- and
andia-stabilizing condenser 92. More speci?cally,
represent the condensers 81b‘ and 910, respec;v
the resonant circuit 81 comprises a pairof series
tively, the capacitor C3 represents the capacitance
connected condensers .811) and 81c which. are
of the diode .88, the capacitor C4 represents the
shunted byianr adjustable inductance element
combined capacitance‘ of the‘ diode 89 and the
81a. Preferably the last mentioned elementis of
shunting
condenser 92 and the inductance L repj
the/ variablewpermeabiiity type being provided
resents the tuning inductance 81a. The diode
with'an adjustable powdered ferrous .metal'core,
recti?ers 88 and 89 are thus e?ectively individu
theposition of which may be changed to alter
ally coupled across'the two adjacent capacitance
the‘ inductance of the, element within ‘the desired.
arms C3 and C4, respectively, while the resistors
limits. The circuit constants of the resonant .cire
cuit ,8‘! together with .the associated capacitances
includingucondenser 92 are so chosen'that this
circuit is tuned to the nominal intermediate car‘
rier frequency and. su?iciently broadly tuned so
that all desired signal components of ,a frequency
modulated carrier appearing in the'second'inter
90 and 9| individually "shunt the recti?ers‘88 ‘and
89;, respectively. Since the load ‘resistors 90 andv
9| have impedances far in excess of the imped
ances' of the capacitors Q3 and C4 at the fre
quencies; involved, they may ‘be. neglected? in
‘ analyzing the circuit; Again; the capacitance of
the condenser 93 is so much greater than that
mediatefrequency channel 24, 25, 29 maybe'de- ’
tectedand impressed upon the input circuit of
of either the capacitor (her the capacitor Gig-that
the audio: ampli?er 28.‘ The voltage appearing
this condenser may also ‘be neglected in analyze“
:across the output'side of the second limiter 26 75
ing' the circuit; The signal modulated carrier
2,404,359:
voltage E, which appears at the output side of
the limiter 26, is applied to the circuit solely be
tween the terminals D and F, and the voltage
appearing at the output side of the discriminator
is the difference between the absolute .values of
the voltages to ground at the terminals A-and B.
ground is produced between the cathode of the
diode 83 and ground.
,
'
.
' More speci?cally considered the voltage be
tween the point A and ground, mathematically
expressed, is as follows:
‘
'
'
From an examination of the bridge‘, it will be
understood that if C1 equals G2, which it does,
and C3 equals C4, such that the bridge is bal
anced, the currents i1 i2 respectively travers 10
ing the’ capacitorsv C1 and C2 are equal so that
>
.
equal voltage drops c1 and e2 appear across these
capacitors;
Accordingly, nodi?’erence between
the voltages to ground is developed at the points ‘
,
.
A‘and B, regardless of the frequency of the ex 15 E=exciting voltage
Q=the Q oithe inductance element 87a
citing voltage E. In the actual circuit, however,
the capacitance C3 is less'thanthe capacitance
C4 by an amount equal to the capacitance value
of the condenser 92 such that thebridge is un
balanced. Accordingly, during excitation of the 20
circuit by the voltage E, the current i2 exceeds the
w=21rfl
Y‘
.
.4
.
EC2=C2+CCa+GC4+C3C4
>
The expression for‘the voltage between the point
B and ground is similar, viz:
v
'
current '2'1 so that a current 2‘: is caused to flow
.
a
‘
2 .
’
through capacitors C1 and C2 of the inductance
element L. The magnitude of this current ob
..=,_no? ,.
2c _
viously depends upon the reactive impedance of 25
_ n +‘7{§q2,—.L—w2} ‘
the inductance L at the particular frequency of
excitation and the direction of current’flow is
By evaluating the above voltages vfor ‘different '
such that the voltage drop across the capacitance
values of
ma circuit having"selectedcircuit
C1 is enhanced and that across the capacitance
C2 is decreased. It will be understood, therefore, 30 constants and excited at a particular voltage,
Vb EC gag +_7{(C+ on
that by suitably proportioning the impedance of
and plottingthé difference voltage'therebetween,
the inductance L relative to the reactive imped
ances of the capacitances C1 and Czat a particu~
lar' center frequency, to establish a given reiation~
ship between the currents i1, i2 and is the absolute 35
it maybe shown that at the center resonant. fre—
voltages between the points A and Band ground
' become equal. In their relationship to each other,
however, these voltages are out or‘ phase so'that
quency of‘ the discriminator network 21 the ,dif
ference voltage is zero and that'this difference
voltage varies vlinearly with'frequency changes
- above and below the ‘center resonant frequency. .
From the above‘ explanation it will be under;
stood that the extent or magnitude of the ‘dis
criminator output voltage varies in accordance
a difference voltage'actually exists between‘the
points A and B. This difference voltage is, of 10 withthe departure of‘the exciting frequency from
the center intermediate frequency to which the
course, equal to the vector sum of the absolute"
resonant circuit 81 is center tuned. ' It ‘will also
, voltages from the points A and B to the ground
point F. The particularfrequency at which these
absolute voltages become equal to balance the
be understood- that if the frequency of the carrier
appearing at the output side of the limiter 261s
modulated in accordance with a given
bridge represents the center frequency at which 45 frequency
audio signal, a correspondin'g'audio frequency
the voltage appearing at the output side of the
discriminator between the cathode of the'di'ode
88"and ground becomes zero. " In this'regard it
is pointed out that when the bridge is balanced
so that the voltages from the points 'A'and B to
ground are equal, equal direct voltages are pro;
duced'across‘the load'resistor's
and 9!. Those
. voltage is accurately reproduced across the con
denser 93 at the output 'side'of the discriminator
21. To consider "somewhat more fully the action
of vthe condenser 92 in stabilizing‘éthe'operation of
the discriminator network 21," it maybe pointed‘
out that-if theim-pedances offthe'four legs of the
bridge circuit are ‘perfectly balanced, changes in
the exciting frequency will not'produce the de
voltages are opposingly combined in aidirect cur
rent path through the inductance‘element ‘81a 55 sired differences of potential’ between ‘the ter
so that when equal, no direct voltage appears
minals A and B. By providing the condenser 92
between the cathode of the'diode 38 and around.v
connected in the manner illustrated, however,
As the exciting voltage for the resonant circuit
therebyrto insure that the over-allcapacitance
Bl'is increased above the center frequency, due
between the lower terminal B of the resonant cir
to the signal modulation thereof at an audio rate, 00 cuit 81 and ground exceeds that between the up
the reactive impedances of the circuit constants‘
per. terminal A of this circuitand ground, the
change to alter the relative magnitudes of the
desired circulating‘ current within thev resonant
currents 2'1, 2'2 ‘and is so that the voltage from the
circuit will‘ always be produced to- insure stabil
point A to ground exceeds that between the point ~ ity of circuit ‘operation. ‘In this’ regard it is
B and ground. Accordingly a voltage which is 65 pointed outthati the unbalancingior stabilizing
positive with respect to ground is producedgbeJ
‘condenser-‘.92 may be connected, either between
tween the cathode of the diode 5'3 and‘ ground.
j the lower terminal of the resonant circuit. 81 and
If, on the other hand, the exciting frequenc-yfcr
ground or between the upper terminal of‘ this
the circuit 81 is'decreased below the center fre
circuit andv ground. In either case, the desired,
quency, the reactive: impedances of the circuit 70 operation of the, network is produced. It is noted,
constants change to alter the relative magnitudes
however, that when a condenser 92 of appropri_
of the currents i1, i2 and is so that the voltage
between the point B and ground exceeds that
between the point A and ground-‘ As a result an
output voltage which is negative with respect to
ate. capacitance'value is connected between the f
upper‘terminal A of the circuit 81 and‘ ground,
the direction of circulating current ?ow'within,
the circuit ‘is reversed. l Accordingly,;the1polarlity '
2,404,359;
8
changes in the exciting frequency ofthe reso-
of the output _voltage produced across the condenser 93 incident to a given departure of the
nant circuit 81.
exciting frequency from the center intermediate
frequency isthe reverse-of that which is obtained
The curve D, as'shown in Fig. 3'of the draw-'
ing, is an accurate. reproduction of the same cir
for the same frequency departure when the con
denser 92 is connected between the lower ter
>
'
cuit when‘ equipped with a condenser'92, of .5 v
micro microfarads. connected between the“ upper
terminal of the resonant circuit 81 and ground.
- minal of the resonant circuit and ground.
It will be understood fromthe aLbbve-explanae
.tion that‘ the inventionzis‘not limited to any
By‘a'comparison-of this curye with the three
curves A, B and C, it willbe noted'that the po
I it is particularly applicable; to'the .detectionrof:
' signal frequencies within a band embracing the
to ' increases and‘ decreases in the :exciting' fre
quency of the resonant circuit'8‘l ‘is reversed ‘from
audio frequency range. In order to'illustrate
that which obtains when the condenseraSZ iscon
particular range of frequency detection, although 10 larityrof the ‘output voltage produced in response H
, the relative magnitudes of the principal circuit
nected between the lower ‘terminal of the reso
n elements of the discriminatorsn'etworkl 'Lwhen 15 nant circuit’ and ground. This is due to the fact,
designed in accordance with the present inven
7 noted-above, that when the position of this con‘
tion,.the following approximatevalues of its ele
denser in the network‘ is changed from a'connec- '
ments, together with other pertinent-informa
tion between-the lower terminal of the resonant
' ~ tion, are given for a' particular system'of. the
circuit and ground to a ‘connection between the
character shown in Figs. 1 and 2 of thedrawing'. ' 20 upper terminal of this ‘resonant circuit and
ground, the direction of circulating current flow
Center intermediatelfrequency11s A; r - -- I .
I‘ >
through
the elements of the resonant circuit'B'I
megacycles ___r ________ _= __________ __'_'
_ 2.515
Condenser"87b___r__micro microfarads__
Condenser?’ic
'
r
Condenser 93;
is
: 100 '
.
____do____ '
Condenser 92-; _______________ __do____ -
_>
->'
'
'
7
Ohms‘
'
‘50
‘.01
Condenser 96__’_j______ '
.01"
__'___'___do_;_‘_ ‘
-
criminator, and that the cathode, anode and
three grids of the tube “are used to amplify the
audio frequency voltage which is developed be
tween. the wiper 99 and ground during reception‘
‘.01
Resistorl98__'_,__1;a_‘_;__; ____ __ohms__ 500,000
.
‘
anexamination of the Fig. 4 arrangement, it will
Resistor 95 ___'__'____ ____ __,____‘__do_,__ 470,000
Condenser 97'___,___;;g_lmicrofarads__
Condenser 10,1..-" _,_
.
be seen that the diode section of the tube 89 is
utilized as one of the rectifying paths of the dis
100,000
"______do__;_ 1Q
r
the audio frequency ampli?er 28 in the manner
illustrated'in Fig. 4 of the drawing, whereinref=
erence characters corresponding'to those used in
Fig. 1 identify the same circuit elements. From
10
Resistor. 91 ___"_';__;__r__________do'_____ 220,000
Resistor 94'
'
100 25 proved discriminator 21 may be combined with '
Resistor 90__,____lv___>_______ __V__ohms__ 220,000
Inductance Telement 87a '— adjustable
from ___'____‘_V_'.___;_Q_‘_imicro"henries__ '
reversed.
Ifdesired, one rectifying section of'the' im
100:
of a selected signal. This. audio voltage is trans- '
mitted to the loud speaker'29 for reproduction
response’ characteristics shown in Fig. 3 of .
the .drawing are typically representative of the
‘improved discriminator 21. when provided with
circuit'qelements having the approximate values
indicated,,but with‘ diiferent capacitance‘ values
for the condenser 92. In the graph illustratedin
this ?gure,‘positive and-negative frequency de
viations. fromthe center intermediate frequency.
through va coupling transformer I39.
blocked under the control of the muting oscilla
tor. 33 and mode of operation of the discrimina
tor 2,1 are exactly the same as explained herein
with reference to the system, shown in Fig. 1. _In ,
fact, the circuit of Fig. 4 may be directlysubsti
tuted for the discriminator 21 and the audio fre
.7 of 2.5l5 ,megacycle's, as graduated ‘in positive vand
quency ampli?er 28 in the system of Fig. 1.
negative frequency steps .of ten kilocycleseach,
From the foregoing explanation with reference ,7
to the'discrimi'nator 21, itwill be understood that
this circuit'may be accurately reproduced accord
ing to a preoonceived?design to havethe exact
are plotted as a functionof positive and negative
voltage’ output across the. condenser 03. 7 More
speci?cally, the response curves A, B and‘ C or
this ‘graph representaccurately reproduced test
desired operating‘ characteristics. , Thus,'in the
manufacture. of this circuit‘, it'is'unnecessary to
utilize a tuning inductance ‘element 81a having
' curvesof a discriminator network of the char
acterl described when provided with a condenser
92 connected between the lower terminal of the
resonant circuit 81 and ground. 'By a compari
a center tan precisely located at the electrical '
mid-point of the inductance winding. Thisne
. son-jof these curves, it will benoted that as the
- .value 'of this condenser is increased from a value
cessity is obviated by utilizing the two condensers
81b and Y810 of equal capacitancevalues for in
.of 5 micro microiarads upward toa value of 9
troducing the carrier voltage into the discrim
inator network. Moreover, the use of ?xed con-v
densers 81b and 8lc'of matched capacitance
values is only permissible in the circuit because
an adjustable inductance element 81a is utilized
to tune the resonant circuit to the desired center
intermediate frequency. It has been found that
by utilizing available commercial inductance ele- .
ments 81a of the variable permeability type, a
~ micro microfaradsthe linear portion of the re
sulting response characteristic‘ of the network is
substantially increased. . It will also ‘be seen from
an- examination of. these curves that, as the excit
' inggifrequency of .the network is increased above
_, the. center intermediate - frequency at which the
resonant circuit‘8l is tuned; an output‘ voltage
' which is positive with respectto ground is pro- _7
duced at the output side of'the discriminator.
On' the otherhand, when the exciting frequency
resonant circuit 81 may be provided which‘ is pre-v
cisely' and accurately tuned to the desired fre
is decreased below .the centerintermediate fre- 7
' quency, arf output voltage which is negative with
' respectoto'ground is produced ‘at the output side
' of the. discriminator. Over the linear portioniof
' ‘ each curve,’ changes in the magnitude of the out
" put'voltageare obviously directly proportionallto
The man
40 ner in which the audio section of thetube is
75
quency such that no output voltage is produced 7
across the condenser 93 when the discriminator
network is excited at the center intermediate fre
quency. ' This is aniimportant consideration in
the automatic controlof the reactance-modula- ._
' 2,404,359
10
tortu'be 38 to maintain the ‘system faithfully re
means including a ?rst rectifying element for _
sponsive to a selected signal, as willfappear from
bridging one of said condensers ‘across said'sourc'e,
the explanation which follows. ~
means including a second rectifying element for
bridging the other of sai'dcondensers across said
More generally considered, the discriminator
source, load resistors respectively shunting said
network 211 is substantially more stable and less
a?ected by stray capacitance coupling paths than _
the discriminators previously available in the
rectifying elements, and an unbalancing capaci
tance element connected in shunt with one of said
prior art. Thus, the tuning slug of the variable
load resistors.
'
"
'
I
'
inductance 81a may be connected to ground so
3. In combination with a' voltage source of
that body capacity'e?ects do not affect the cir 10 variable. frequency, a pair of rectifying paths
cuit constants of the network in tuning the res-'
bridging said source and each including a recti
onant circuit 81 toa desired center frequency.
fying element and a‘ condenser arranged in series
This makes it possible to easily tune the network
circuit relationship, means comprising a two
with precision accuracy to the exact desired cen
terminal tuning inductance shunting said con
ter frequency, so that the network is perfectly 15 densers in series, means providing unequal capaci
balanced in so far as the relationship between
tance across said rectifying elements for vary
frequency deviations and voltage output is con
ing the relative magnitudes of they voltages ap
cerned. As contrasted with this improved ar
pearing across said rectifying elements in ac
rangement, a conventional discriminator utilizes
cordance with deviations in the frequency of said
an adjustable tuning condenser having its ad 20 voltage source from a particular value, load re
justable plates above ground potential. When,
sistors respectively shunting said rectifying ele
ments, and a direct current path serially includ
therefore, a tuning tool is brought into proximity
to the adjusting shaft, stray» capacitance cou
pling to the elements of the tuned discriminator
circuit is provided which makes exact tuning of
the circuit to the desired center frequency prac
tically impossible.
ing said resistors and said inductance so that a
voltage is produced thereacross which varies in
accordance with the extent of deviations of the
frequency of said voltage source from said par
ticular frequency.
.
Also, by shielding the elements of the circuit
4. In combination with a voltage source of vari
81 and the elements used at the output side of
able frequency, a pair of rectifying paths bridg
the limiter 26, thereby to eliminate mutual in 30 ing said source in parallel and each including a
ductive coupling between the discriminator and
rectifying element and a condenser arranged in
the preceding limiter 26, and by using only the
series circuit relationship, means comprising a
capacitive coupling of the condenser 85 between
two-terminal inductance shunting said condens
these stages, the discriminator network may be
ers in series and a condenser shunting one of said
much more accurately manufactured and ad 35 rectifying elements for varying the relative mag
justed to operate in accordance with given design
nitudes of the voltages appearing across said rec
speci?cations. Moreover, by capacitively cou
-tifying elements in accordance with deviations
pling the preceding limiter 26 into the discrim
in the frequency of said voltage source from a
inator 21 through the condensers 86, 81b and 810,
particular value, load resistors ‘respectively shunt
stray capacitances are minimized and the effects 40 ing said rectifying elements, and a direct current of those remaining may be more easily compen
path serially including said resistors and said
sated for in the design and adjustment of the
inductance so that a voltage is produced there
network. A further advantage of the disclosed
across which varies with deviations in the fre
discriminator 21 resides in the fact that it may
quency of said voltage source from said particular
be more easily and accurately compensated for 45 frequency.
temperature variations by making suitable pro
5. A frequency discriminator network adapted
vision for adjusting the tuning slug of the in
to be excited from a variable-frequency signal.
ductance element 81a in response to changes in
source comprising, a four-arm capacitance bridge
the temperature to which the elements of the
circuit including connections for excitation from
50 said source across one diagonal thereof, a two
network are exposed.
While one embodiment of the invention has
terminal adjustable inductor connected across the
been disclosed, it will be understood that various
the other diagonal of said bridge circuit to tune
modi?cations may be made therein, which are
said bridge circuit to the mean value of said vari
within the true spirit and scope of. the invention.
able frequency, a pair of rectifying devices indi
I claim:
'
1. In a frequency discriminator network which
is adapted to be excited from a voltage source
55
vidually coupled across two adjacent capacitance
arms of said bridge forming a branch in parallel
to said inductor, .a pair of resistors individually
of variable frequency, a pair of series connected
shunting said rectifying devices, and load imped
condensers, a two-terminal adjustable inductance
ance means coupled to said resistors for deriving
shunting said series connected condensers to tune 60 the differential of the unidirectional signals de
said network to a particular center frequency,
veloped thereacross by said recti?ers,
means including a ?rst rectifying element for
6. A frequency discriminator network adapted
bridging one of said condensers across said source,
to be excited from a variable-frequency signal
means including a second rectifying element for
source comprising, a four-arm capacitance bridge I
bridging the other of said condensers across said 65 circuit including connections for excitation
source, load resistors respectively shunting said
rectifying elements, and means providing unequal
capacitance across said rectifying elements to un
balance said network.
from said source across one diagonal thereof, a
two-terminal adjustable inductor connected
across the other diagonal of said bridge circuit
to tune said bridge circuit to the mean value of
2. In a frequency discriminator network which 70 said variable frequency, the adjacent capacitance '
is adapted to be excited from a voltage source
arms of at least one of the bridge branch cir
cuits in parallel to said inductor having unequal
of variable frequency, a pair of series connected
condensers, a two-terminal adjustable inductance
shunting said series connected condensers to tune
said network to a particular center frequency,
capacitance values, a pair of rectifying devices
individually coupled across two adjacent capaci
tance arms of said bridge forming a branch in
2,404,359
11"
parallel toisaid inductor; a pair of resistors ind1¢
rvidually jshunting :said rectifying devices, and’
means for deriving the di?erential'v of the uni
directional signals developed by said recti?ers'.
12
‘
otheridiagonalof said bridge circuitto tune-said
bridgecircuit to the‘mean va1ue_of said variable
frequency, a pair ofrectifying devices individually
coupled across two adjacent capacitance armsof
V 77; A frequency'discriminator network adapted
said bridge vforming - a branch ‘in parallelzfto'said
to be excited'from a variable-frequency signal
inducton'a pair of resistorsindividually‘shunt
ing said rectifying._ devices, andqload impedance
source comprising, a four-arm capacitancebridge
circuit including connections for excitation from
said source across one diagonal thereof, a two
terminal inductor including an adjustable ferro 10
magnetic core for adjusting the inductance valuev
thereof, said inductor being’ connected across the
means coupled to said resistorsfor deriving ‘the
di?erential of, the unidirectional signalsdevel
oped-thereacross byisaid recti?ers.
.
i. ,7
‘1:7
1 viulirtroiq E. 73014115.. I
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