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Патент USA US2410149

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Oct. 2g, 1946.
R. A. CLARK, JR
SIGNALING-~ -SYSTEM
Filed. Feb. 17, 1944
l2,410,149
2 Sheets-Sheet l
` ROBERT A. CLARK JR.
ATTORNEY
Oct, 29: 1946.
R. A. CLARK, JR
¿£110,149
SIGNALING- SYSTEM
Filed Feb. 17,' 1944
2 Sheets-Sheet 2
NGE,@
NJ
3..
d
INVENTOR.
ROBERT A. CLARK JR. '
ATTORNEY
Patented Oct. 29, 1946
2,410,149
UNITED STATES PATENT OFFICE
'
-
`
2,410,149
A
'
l
SIGNALING SYSTEM
Robert A. Clark, Jr., Chicago, Ill.
Application February 17, 1944, Serial'No. y522,742
7 Claims.
(Cl. 179-84)
2
1
The present invention relates to signaling sys
tems and more particularly to improved appara
tus for discriminating between two different sig
provide an improved circuit network which is
simple in arrangement and is positive and reli-v
able in its operation to discriminate between a
signal having different frequencies within a given
over a signal channel of a telephone or similar 5 band and a second signal having a ñxed frequency
nals which may be non-concurrently transmitted
signaling system.
~
within the given band.
In conventional telephone systems it is usually
necessary that each voice channel be utilized
Y
'
'
The invention, both as to its organizationand
method of operation, together with further ob
jects and advantages thereof, will best be un
derstood by reference to the' specification taken
not only for voice signal transmission, but also
for the transmission of supervisory signals, such
for example as ringing signals, flashingrecalls
in'connection with the accompanying drawings,
signals and the like. Further, in long lines net
» in which Figs. 1 and 2, when laid end to end in
works, utilizing repeaters for voice current gain,
the order named, illustrate a portion of a tele
it is highly desirable to use alternating super
phone system having embodied therein a signal
visory signals having a ñxed frequency within 15. discriminating network characterized by the fea
the voice frequency rangezto provide the desired
tures of the present invention.
signal indications. This has previously been ac
Referring now to the drawings, the equipment
complished by modulating a voice frequency car
there illustrated may be provided in the toll
rier at a low frequency, and utilizing the low fre
exchange of a metropolitan central exchange area
quency component lof the signal for supervisory 20 or alternatively may be located in an isolated
signaling purposes. It has been found, however,
private branch exchange which is connected with
that with such arrangements, voice signals trav
a central exchange area by means of trunks re
ersing the signals channel may produce a false
quiring voice current amplification for satisfac
response of the supervisory signal detecting
tory communication thereover. In brief, this
equipment, if the level of voice signal trans 25 equipment comprises an operator switchboard III
mission is sufficiently high.
having a jack Illa, a ringing signal in the form
It is an object of the present invention, there
fore, to provide improved and exceedingly reliable
methods and apparatus for` discriminating be
of a drop I 0c, and a pad switching key Ißd as
sociated with a signal channel to a distant oilice.
This channel is divided within the exchange into
tween voice and supervisory signals which may be 30 an incoming signal channel II and an outgoing
transmitted over a voice signal channel.
signal channel I2 in order to permit suitable filter
More generally stated, it is an object of the
and .amplifier facilities to be provided for insur
present invention to provide improved methods
ing high level, high fidelity transmission between
and apparatus for discrimination between a ñrst
the switchboard I 0 and the distant oflice.
If car
signal having different frequencies Within a band 03 Ol rier transmission is utilized between the illus
and a second signal having a fixed frequencytrated exchange equipment and the distant oñice,
within the band when the signals of different
the bracketed terminals of the two signal chan
types are non-concurrently received overl the
nels II and I2 will of course extend to conven
saine
signal channel.
.
tional composite equipment through which they
'
According to a further object of the invention, 40 are coupled to a specified carrier channel. On
those components of a received first or voice
the other hand, if a two conductor trunk is
signal which have frequencies other than the
utilized to connect the illustrated exchange equip
fixed frequency of the second orl supervisory sig
ment with the distant office, the identiñed chan
nal are utilized lto` prevent the'apparatus for
nels will of course be , coupled to the trunk
detecting the second or supervisory signal from
through a conventional hybrid system. Within
responding to a component of a received first sig
nal which has the same frequency as the second
the exchange, the two channels I I -and I2 are
coupled to a two-way link extending tothe call
and answer jack Illa by means oiga conventional
signal.
»
In accordance with a further and more specific
hybrid system I4 and pad or transmission loss
object of the invention, _a four terminal bridge
circuit, which is balanced only at the fixed fre
quency of the second yor supervisory signal, is
utilized invdiscriminating between the signals of
network I3, both of which are entirely conven
tional in arrangement.` The network I4 is, of
course, utilized to determine the level of voice
current transmission over a connection set up by
way of the illustrated equipment. The imped
It is still another object -of the invention to 55 ance of the network I3, as combined with the
different characters.
`
2,410,149
3,
equivalent impedance of an operator cord cir
cuit connected with the tip and ring contacts
of the jack lila, is balanced by means of a bal
ancing network I5, thereby to effectively isolate
the two channels l! and I2 and thus minimize
the transmission of signals from the channel II
to the channel I2 through the hybrid system I4.
Switching relays 60 and Til arrange for control
by the pad key Illd are respectively provided
d
ing upon the incoming signal channel I I and im
pressed upon the bridge circuit I9 in the manner
just explained, detecting means are provided
which comprise a three electrode thermionic tube
213 and a signal repeating relay 4€) shunted by a
delay release condenser dilo. More specifically,
the output circuit of the tube 20 includes the wind
ing of the relay ¿li and the upper part of a volt
age dividing resistor 29. The input electrodes of
for switching in` and out signal loss resistors 10 the detecting tube 2i) are bridged across the res
onant circuit 2d in series with a decoupling re
within the network I3 and correspondingly
changing the impedance of the balancing net
work 'Ill to maintain the hybrid system I4 in a
balanced condition. A iilter and amplifier. net
work 9, which may be entirely conventional in
character, is provided in the outgoing signal
channel I2 for the purpose of raising the level
of voice transmission to the desired value. Sim
ilarly, the incoming signal channel II includes
an amplifier and filter network I6 and a iinal
audio amplifier stage lf‘I, arranged in tandem in
the order named. The final amplifier stage Il
includes an electron discharge tube I8 provided
with an output circuit which is directly coupled
to the input terminals of the hybrid system Id.
For the purpose of discriminating between
voice signals and supervisory signals traversing
the incoming signal channel II, a bridge circuit
indicated generally at I9 is coupled to this
sistor 26a, a biasing resistor 2l and a source of
biasing potential represented by the voltage ap
pearing> across the section 29a of the voltage di
vidingvresistor 23. With this arrangement and as
will appear later,'when the bridge circuit I9 is en
ergizedat the particular frequency of 1500 cycles
at which it is balanced, the voltage drop across
the resonant circuit 29 is detected by the tube
2li and utilized to effect operation of the signal
repeating relay> @di For the purpose of utilizing
a' received> voice signal to develop a bias voltage
which may be applied to the tube 2€) to prevent
the relay di) from operating, a biasing network is
provided which is coupled to the terminals IBa
and läd of the bridge circuit, and is utilized to
develop a bias voltage across the biasing resistor
2l. More specifically, this network comprises a
transformer 26 having a primary winding 26a
channel through an adjustable potentiometer 30 bridged between the terminals ISa and läd, and a
secondary winding 2Gb which is bridged across
resistor 22T which~ shunts `the channel at the
the biasing resistor 2l in series with the space
output side of the tube I3 and is paralleled by
a ñlter condenser 23; More speciñcally, the
current path of a diode rectifier 3U. The resistor
2l is shunted by a smoothing condenser 28 which
bridge circuit IQ'incl'udes a voltage step up trans
former 2l` having a» primary winding 2id bridged 35 is utilized to maintain a direct voltage across the
across an adjustable portion of the potentiometer
resistor 21 when the primary winding 25a of the
resistor 22 by means of the wiper 22a which is
transformer is energized.
In' the illustrated arrangement of the equip
adìustable along this resistor. The transformer
2l also includes a secondary winding having a
ment, incoming supervisory signals are utilized
tapped midpoint I'Sa, such that equal voltages are 40 only for the purpose of controlling the drop Iûc
developed in the two sections 2lb and 2I'c there
associated with the jack I lia. To this end, the
of when the primary wind 2 Ia is energized' at any
relay Il@ is arranged directly to control a ringing
frequency within the audio frequency range. The
relay 5t having the function of cutting off the
two identified sections ofthe transformer sec
padV network I3 from the jack Illa, opening the
ondary winding respectively comprise two legsA of> I
incoming signal channel Il, and connecting the
the described'. bridge circuit. A third leg of the'
bridge circuit is made up of a parallel resonant
circuit 2'4 which comprises the shunt connected
inductance and capacitance elementsY 24u and
tip and ring contacts of the jack Ilia to the source
of ringing current which is provided in the ex
change. In this regard it will be understood that
the exchange in which the illustrated equipment
is located includes a conventiona1 exchange bat
tery having a grounded positive terminal and a
negative terminal connected to each of the illus
2`4b.
This resonant circuit is tuned exactly to .
the fixed frequency within the audio frequency
range at which supervisory signals are trans
mitted to the input side of the amplifier and
filter network I6. Thus if a ñxed frequency of
trated relay terminals identified by the negative
polarity sign.
150'0 cycles is to be utilized as the supervisory ..
vBrieiiy to consider the operation of the above
signaling frequency, the circuit 24 is tuned ex
described equipment, it will be understood that
actly to this frequency. For the purpose of
when this equipment is seized in the distant oñice
matching the terminal impedance of the circuit
on a call to the operator switchboard II), a ring
24v at this frequency, the fourth leg of the bridge
ing signal of the fixed frequency selected for su
circuit is made up of an adjustable resistor 25 (in pervisory signaling and originating at the distant
having a wiper 25a movable therealong for the
end of the connection is impressed upon the input
purpose of varying the encircuited portion of the
side of the signal channel II. More speciñcally,
resistor. It will thus be apparent that the bridge
this signal has a frequency exactly equal the par
circuit I9 comprises» four legs` 2Ib, 2Ic, 24 and
25 having four terminals. lea, IQb’, ISc and Idd,
and that a voltage appearing upon the- incoming
signal channel Il is equally'impressed.. upon the
two legs 2lby and l¿lo thereof through the in
ductive coupling between these legs and the pri
mary winding 2Ia. It will also be understood
that at resonance the impedance of the tuned
circuit 24is substantially resistive and hence may
be entirely balanced by suitable adjustment of
the wiper 25a along the resistor 25.
In order to detect a supervisory signal appear
~ ticular frequency to which the resonant circuit
24 istuned. It is amplified through the network
I6 and the final amplifier stage I'I and appears
across the potentiometer resistor 22. That por
tion thereof which appears across the lower part
of the resistor 22 serves to energize the winding
21a', thereby to produce equal voltages across the
two legs 2'Ib and 2Ic of the bridge circuit i9.
Since at the particular frequency indicated, the
resistive impedance of the resonant circuit 2li is
'. matched by the resistance of the potentiometer
2,410,149 .
6
5
I4 and the' amplifier network 9 for transmission
to the distant operator. Thus a two-way communication channel is set up between the calling
resistor 25, substantially-no _voltage appears b_e‘
tween the bridge circuit terminal I'Ba' andy lad, so
that the Voltage input to .the biasing- network.
and answering operators over which instructions
may be transmitted to the answering operator for
the extension of the call. The manner in which
the call may be further extended to the desired
comprising ithe" diode. _rectifier 3U ' is negligible.>
The >alternating voltage yappearing across the res-f'
onant circuit 24'is obviously impressedv between
the input electrodes of the detector tube 20 and
during thealternate'half cycles thereof opposesy
the bias voltage appearing across vthe resistor sec- '
ftion 29a, so >that the current iiow through this.
tube and the winding of the signal repeating relay
40 is substantially increased'. As a result, the re
lay 40 is caused to operate. In operating, thisV
relay opens its contacts 42 to interrupt the path
normally short circuiting the winding of the ring 15
ing relay 3B, and closes its contacts 4I‘to energize
the relay 5i! from the exchange battery. The re
lay 50 upon operating. opens its contacts 5I to in
terrupt the connection between the hybrid sys
tem I4 and the linal ampliiier stage I'I of the in 20
coming signal channel II. At its contacts 52 and
54, the relay» opens the connection between the
jack Ilia and the pad network I3. At its contacts
destination is exactly the same as that followed
in conventional manual oiiice or toll ofñce prac
tice. In this regard it is pointed out that de
pending upon the destination of the call, a
variable amount of voice current gain may be re
quired for satisfactory communication between
the calling and called parties. Thus if the call
is rto be extended over a long high loss line, con
siderable voice current gain may be required in
the illustrated equipment to insure an adequate
level of signal reproduction at the calling and
called ends of the connection. In such case the
_pad key IIld may be operated for the purpose of
energizing the two switching relays 60 and 1I),
thereby to cut out thetransmission loss pads in
the network I3 and to correspondingly change
the impedance of the network I5 to provide for
53 and 55, the relay 50 completes a circuit for
energizing the winding of the drop IUc from the 25 a continued balance of the-hybrid system I4. If,
ringing current source. When thus energized,
the drop Illc is actuated to provide an indication
on the other hand, the call is'to be extended over
a short line having good voice current transmis
sion characteristics such that only the normal
Vamount ‘of signal gain through the illustrated
Upon termination of the ringing signal appear 30 equipment is required, the loss pads of the net
that an incoming call has been routed to the jack
Illu which should'be answered.
'
ing at the input side of the‘signal channel I I,‘ the
bridge circuit I9 is obviously de-energized, per-l
mitting the bias voltage across the resistor sec
work I3 are retained in the circuit.
In this regard it will be understood that voice
signals originating at the distant end of the con
tion 29a to so reduce the Vspace current iiow
through the detector tube 20 as to eilect the re-v
signal channel II may comprise different com
lease of the signal relay 40. Upon restoring, this
posites of all frequencies within the audio fre
nection and appearing at the input side of the
quency range. The received voice signals may
relay opens its contacts 4I to deenergize lthe re
or may not include a component having the par
lay 5D and closes its contacts 42 to' recomplete
ticular frequency to which the resonant circuit
the path for short-circuiting the winding of the
relay 50, thereby to render the latter relay slow 40 24 is tuned. Moreover, any received voice signal
having the particular frequency t0 which the
to release. After a short interval, the relay 53
resonant circuit 24 is tuned will always be accom
restores and opens its contacts 53 and 55 to de- v
energize the winding of the drop IEic.-
panied by other components having frequencies
differing from this particular frequency. Also, at
At'its
contacts 5I, the relay 50 recompletes the connec
tion between the hybrid system I4 and the final
amplifier stage I1'. At its contacts 52 and 54 the
relay 5i] recompletes the connection between the
- all frequencies withinv the voice frequency range
jack Ilia and the pad network I3. Thus the illus
trated equipment is conditioned for» two-way
In order to extend the call from the jack Illa
resonant circuit 24 is -diiïerent from that of the
resistor 25. Thus, the bridge circuit I9 is un
balanced at such frequencies, so that a Voltage
is developed between the terminals I9a and I9d
thereof. It will be understood, therefore, that
to the desired point, the operator attending lthe
so long as vvoice 'current transmission over the
voice communication over a connection set up
by way of the jack Illa. _
’
"
other than the particular frequency to which the
circuit 24 is tuned, the terminal impedance of the
channel I I continues, a substantial voltage is im
switchboard I0 or that portion thereof at which
the identified jack is provided, may insert the 55 pressed upon the primary winding 26a of the
answer plug of her cord circuit in the jack Illa
to establish a connection between her cord cir
transformer 2B.
This voltage appears in in
creased magnitude across the secondary winding
2Gb and is rectified by the diode rectifier 30 to
cuit and the calling operator. Incident to this
operation, the jack contacts Ißb and Ille _are
appear as a direct current biasing voltage across
opened to disconnect the winding of the drop Ille 60 the biasing resistor '21. Moreover, the direction
from the two sides of the signal channel. Voice
of current flow through the diode 30 is such that
signals originating at the distant end of the con
nection and appearing at the input side of the
signal channel II are amplified through the net-V
work I6 and the final amplifier stage I'I, andare 65
transmitted through the hybrid system I4 and
the pad network I3 to the receiver of the opera
tor’s head set for reproduction. In this regard
it will be understood that the hybrid system I4
the Voltage developed across the resistor 21 is
additive with respect to the voltage across the
resistor section 29a, whereby the total Ibias volt
age impressed between the input electrodes of the
tube 2B through ythe inductance element 24a is
substantially increased. This increased bias >volt
age as applied to the detector tube 2li opposes
acts to prevent signal energy transfer from the 70 the voltage appearing across the resonant circuit
channel II to the channel l2, thereby to elimi-- ' 24, thereby to prevent .the relay 40 from operating
even though a voice signal component of the par
nate any possibility of singing. Voice currentsl
ticular frequency to which the circuit 24 is tuned
originated by the operator handling the call at
appears yat the input side of the signal channel
the switchboard ID, are similarly transmitted
through the pad network I3, the hybrid system 75 I I. Thus ,Y the relay 40 is positivelyr prevented
f
agrar/ref
a
fromfoperating. when thechannel Il. is being
utilized for voice signal transmission.
l
After ahconnection hasy been set up between
two- substations through the illustrated equip
ment in the. manner explained above, it maybe
released under the control of the calling operator
andthe operator handling the call at the switch
lboard iii in an entirelyr conventional manner.
Further, the manner in which calls may be routed
'from the switchboard le through the-illustrated
equipment to the distant cnice is entirely con
ventional and will be readily apparent to those
skilled in the art.. In this regard it is pointed out
that in the interests of> simplifying the disclosure,
the equipment for transmitting supervisory sig
nals from the switchboard lll through the illus
trated circuit equipment to the distant office' has
not Ibeen illustrated in the drawings.
From the above explanation it will be under
channel and balanced.> at> said .fixed frequency,
said circuit including a parallel resonant circuit.
in one leg thereof, a biasing resistor, thermionic
means, including inputv electrodes bridging said
resonant circuit in series with said resistor to
detect a second' signal- appearing on said chan
nel, and means coupled to two opposed terminals
of said bridge circuit and responsive to the Volt
tage developed acrossA said two terminals when
said bridge circuit is unbalanced for developing
a bias voltage across said resistor which serves
to bias said thermionic' means against signal de
tection when a first signal appears on said chan
nel.
2; In a signaling system Iwhich includes a sig
nal channel over which may be transmitted a
first signal having different frequencies within
a frequency band or a second signal having a
`fixed frequency- within said band, a four termi
stood that in order to obtain an exact balancing 20 nal bridge circuit coupled to said channel, a res
of the circuit iii. atl the particular frequency to
onant circuit tuned to said fixed’ frequency and
included in one leg of said bridge circuit, an ad'
which the resonant circuit 24 is tuned, the im
pedance across the terminals of the circuit 24 at
justable impedance included in a second leg of
this frequency must balance the impedance of
said bridge circuit to balance the impedance of
the resistor 25. The tuned circuit 24 is, however, 25 said resonant circuit at said fixed frequency,
shunted by the grid-cathodel impedance ofthe
whereby the v-oltage developed across two opposed
tube 20, which impedance is variable with the
terminals-of said bridge circuit is substantially
grid voltage, and if not decoupled from the tuned>
negligible when a'second signal appears on said
circuit tends to prevent the proper bridge circuit
signal channel and becomes appreciable when a
balance from being obtained. By providing the 30 first signal appears on said channel, a biasing
resistor, thermionic means including input elec
series resistor 20a, however, havin'ga resistance
substantially greater than'the grid-cathode im
trodes bridging said resonant circuit in series
with said resistor and operative to detect a sec
pedance of the tube 20, the latter impedance is
ond Isignal appearing on said channel, means
rendered substantially non-effective to upset the
balance of the circuit I9 at the particular fre 35 coupled to said two terminals of said bridge cir
cuit and responsive to the voltage developed
quency‘ to which' theV resonant> circuit is tuned,
thereacross for biasing said thermionic means
regardless of the voltage input to the bridge cir
against signal detection, thereby to prevent said
cuit. It has been determined that under condi-V
thermionic means'from detecting a component
tions of resonance> of thecircuit 24, theV voltage
appearing across the secondary winding 26h is 40 of a transmitted ñrst signal having said fixed
frequency, and means for preventing the input
less than five percent of the voltage across the
tuned circuit 24. When, on the other hand, the
bridge circuit is excited at any frequency-substan
tially removed from the resonant frequency of
the tuned circuit 24, the voltage across the wind
ing 2th- may exceed' by ten times the voltagev
across the tuned circuit 24. These relative values
indicate'the extreme sensitivity of the network
to frequency deviations from the particular value
to which the circuit 2li' is tuned.
'
electrode impedance of said thermionic means
from disturbing the balance of said bridge cir
cuit at said fixed frequency.
3. In a network for discriminating between a
receivedY first signal having different frequencies
within a frequency band and a received second
signal having a fixed frequency within said band,
a four terminal bridge circuit adapted to be ex
50 cited by said signals> and balanced at said fixed
frequency, a resonant circuit tuned to said fixed
Although the improved signal discriminating
frequency and included in one leg of said bridge
network has been disclosed in its operation to
circuit, an impedance element included in a
discriminate between voice and ringing signals
second leg of said bridge to balance the im
appearing upon the same channel of a telephone
circuit, it will be understood that this network is 55 pedance of said resonant circuit' at said fixed
frequency, whereby the voltage developed across
adapted for use in any signaling system where
two opposed terminals of said bridge circuit is
fixed frequency and multi-frequency signals are
substantially negligible when said network is ex
to be received over the same signal channel, but
cited by said second signal and becomes appre
only the fixed frequency signal is to be detected.
llt` will also be understood that regardless of the 60 ciable when said network is excited by a first sig
nal, a thermionic tube provided with a grid and
particular system in which it is used, the action
cathode bridged across said resonant circuit,
of the network is such that a false response of
means for preventing the grid-cathode imped
the detecting equipment during transmission of
ance of said tube from disturbing the balance of
the multi-frequency signal is postively prevented.
While one embodiment of the invention has 65 bridge circuit at said ñxed` frequency, means in»
cluding said tube for detecting a second ysignal
been disclosed, it will ‘be understood that various
impressed on said network, and means coupled to
modification may be made therein, which are
said two terminals of said bridge circuit and re
within the true spirit and scope of the invention.
sponsive to the voltage developed thereacross for
Il claimt
1. In a signaling system whichincludes a sig
nal channel over which may be non-concurrently
transmitted a first signal having different fre
quencies within a frequency band or a second
biasing said tube against signal` detection, there
by to prevent, said detecting means from detect
ing a component of a first signal having said
fixed frequency.
4. In a signaling system which includes a sig
signal having a fixed frequency within said band,
a four terminal bridge circuit coupled to 'said' 75 nal channel over which maybe non-concurrently
2,410,149
9
transmitted a first signal having different fre
10
quencies within a frequency band or a second
quencies within a frequency band or a second
signal having a fixed frequency within said band,
signal having a fixed frequency within said band,
a` four terminal bridge circuit coupled to said
channel and balanced at said fixed frequency,
said circuit including a parallel resonant circuit
in one leg thereof, a biasing resistor, thermionic
means including input electrodes bridging said
>resonant circuit in series with said resistor to de
said circuit including a parallel resonant circuit
in one leg thereof, a biasing resistor, thermionic
means including input electrodes bridging said
resonant circuit in series with said resistor to
tect a second signal appearing on said channel,
means coupled to two opposed terminals of said
bridge circuit and responsive to the voltage de
veloped across said two terminals when said
bridge circuit is unbalanced for developing a bias
voltage across said resistor which serves to bias
said thermionic means against signal detection
when a first signal appears on said channel, and
means for preventing the input electrode imped
ance of said thermionic means from disturbing
the balance of said bridge circuit at said fixed fre
quency.
v
5. In a signaling system which includes a sig
nal channel over which may be non-concurrently
transmitted a ñrst signal having diiferent fre
a four terminal bridge circuit coupled to said
channel and balanced at said ñxed frequency,
detect a second signal appearing on said chan
nel, a transformer provided with a primary wind
ing bridged across two opposed termina-ls of said
bridge circuit and including a secondary wind
ing having a voltage developed thereacross in
response to excitation of said bridge circuit at
frequencies otherk than- said ñxed frequency,
means including a thermionic rectifier connect
ed in series with said secondary winding and
said resistor for converting said last-named volt
age into a bias voltage across said resistor which
20 serves to bias said thermionic means against de
tection of a ñrst signal appearing on said chan
nel, and means for preventing the input electrode
impedance of said thermionic means from dis
turbing the balance of' said bridge circuit at said
quencies within a frequency band or a second 25 ñXed frequency.
7. In a network for discriminating between a
signal having a fixed frequency Within said band,
a four terminal bridge circuit coupled to said
channel and balanced at said vfixed frequency,
said circuit including a parallel resonant circuit
in one leg thereof, a biasing resistor, thermionic
means including input electrodes bridging said
received iirst signal having different frequencies
within a frequency band and a received second
signal having a fixed frequency within Said fre
quency band, a four terminal bridge circuit
adapted to be excited by said signals and bal
anced at said nxed frequency, means included in
resonant circuit in series with said resistor to
said circuit for unbalancing said circuit at fre
detect a second signal appearing on said chan
quencies other than said fixed frequency within
nel, a transformer provided with a primary
winding bridged across two opposed terminals of 35 said frequency band, thermionic means including
input electrodes coupled to one leg of said bridge
said bridge circuit and including a secondary
circuit, ‘said thermionic means being operative to
Winding having a voltage developed thereacross
detect excitation of said bridge circuit at said
in response to excitation of said bridge circuit
fixed frequency, means coupled to two opposed
at frequencies other than said ñxed frequency,v
and means including a thermionic rectiñer con 40 terminals of said bridge circuit and responsive
to the voltage developed across said two termi
nected in series With said secondary winding and
nals when said bridge is unbalanced for biasing
said resistor for converting said last-named volt
said thermionic means against signal detection,
age into a bias voltage across said resistor which
and means for preventing the input electrode
serves to bias said thermionic means against de
tection of a first signal appearing on said chan 45 impedanceof said thermionic means from dis
turbing the balance of said bridge circuit at said
nel.
ñxed frequency.
6. In a signaling system which includes a sig
ROBERT A. CLARK, JR.
nal channel over which may be non-concurrently
transmitted a first signal havingdifferent fre
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