Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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SUBSCRIBER'S TELEPHONE CIRCUIT
Cross Reference Related Application
Canadian applications, Serial No.~.. 453,297,
453,298 and 453,299 filed on the same date as the
present application are related to the present
application~
BACKGROUND OF THE INVEN~ION
Technical Field
The present invention rela~es to transformerless
subscriber's telephone circuits and more particularly
to a subscriber's telephone circuit employing both
integrated circuit elements and discrete components in
an active gain circuit arrangement.
~ackaround Art
Since the advent of integrated circuit technology,
numerous efforts have bsen made to reduce telephone
designs hy the utilization of two to three integrated
circuit devices. The objectives of such design programs
have been to achieve low cost~ high reliability and an
increased number of usable features. The provision of
an increased number of ~eatures has to a great extent
been successful~ For example, such designs permit the
use of pushbutton dialing when connected to telephone
offices which are configured to accept only rotary dial
type telephones.
However, it should be noted that other objectives
have been only partly achieved. This is for the most
part due to very stringent circuit req~liremen~s found
in telephone circuitry. A conventional telephone
cixcuit for example, would usually be re~uired to
operate from a variable supply voltage, withstand both
current and voltage surges, provide analog and digital
functions
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and signal through its own power supply circuit. Con-
sequently, most telephone circuits commercially available
to date employ several discrete electronic components to
support the incorporated integrated circuit based designs.
While integrated circuit tec~mology has been
progressing in the direction required for telephone designs
over the last few years, substantial improvements have also
been made in the discrete semiconductor technology field.
Such improvements include a wider variety of low cost
transistors and diodes, as well as lower cost resistors
and capacitors.
Prior to the advent of semiconductor technol-
ogy~ conventional subscriber's telephones employed
^ircuitry including a hybrid transformer to perform a
two-wire to four-wire conversion. This conversion is
an essential function of all two-wire ~nalog telephones.
During this same period, voltage dependent resistors
~varistors) had been used extensively to support such
hybrid designs. Varistors were used to provide auto-
matic adjustment of transmit and receive levels andto compesate for various loop lengths (longer loops
requiring higher transmit levels and higher receive
sensitivity). An undesirable consequence of utilization
of the varistor/transformer type of circuitry is a
presence of a variable AC impedance between the ring
and tip (Ll and L2) terminals of the telephone. It is
actually much more desirable to have a fixed impedance
(600 Ohms) telephone connected to a line so as maximize
the return loss which, in turn, reduces voice signal
transmission problems within the telephone system.
One of the first portions o~ subscriber
telephone circuitry to be successfully adapted to the
utilization of integrated circuit technology is the
pulse dialing portion. Such pulse circuitry typically
employs pushbutton inputs to generate pulse outputs.
Units of this sort usually draw their power from the
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telephone line. Most designs of this type employ a
curr~nt regualtor device or current limiting device
to prevent dial pulse distortion during dialing. Such
pulse distortion is caused by supplying too much loop
current to the pulse dialer integrated circ1.1it while
trying to maintain a minimum operating vol-tage for
satisfactory integrated circuit operation. Such techniques
are disclosed in U.S. Patent No. ~,167,655 as well as
in British Patent No. 1,426,585.
Accordingly, the ob~ect of the present in-
vention is to provide a new and improved subscriber'stelephone circuit which employs an optimal combination
of discrete and integrated circuit components to over-
co~e many of the disadvantages found in prior art
subscriber telephone circuits.
SUMMARY OF THE INVENTION
The present invention consists of a subscriber
telephone circuit incorporating an electronic tone ringer
(for call alerting), integrated circuitry for loop current
interrupt dial signalling and transistorized transmit and
receive amplifiers associated with an electric microphone
and a dynamic receiver unit respectively. A three terminal
piezoelectric ringer transducer is employed in connection
with electronic tone ringer circuitry. The design of the
transmit and receive amplifier is such that the terminal
impedance of the telephone circuit is appr~ximately 600
Ohms (AC)~ The integrated circuit pulse dialer incorporates
as a standard feature re-dial of the last number dialed.
Also incLuded are a transmit mute switch and ringer out-
put level high-low switch.
The exception of the pulse dialer integrated
circuit, the present telephone circuitry is of the
bi-polar transistor type. The pulse dialer integrated
circuit is of the complementary-metal-oxide-~emiconductor
(CMOS) type which can be operated by using only a negligible
amount of current from the telephone line.
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The disclosed t~lephone circuit performs the
two-wire to four-wire conversion function by use of a unique
operational amplifier which merely cancels the side tone
signal thus separating ~he transmit and receive channels.
The result in the present design i8 the replacement of a
large iron core transformer with a smaller and lower cost
transistor circuit.
The disclosed subscriber telephone circuit
provides a terminal impedance of approximately 600 Ohms
under all loop conditions as compared to the 300-1200
Ohms we found in all loop conditions a little more
tranditional varistor/hybrid transformer type of circuitry.
The stable impedance of the present design is established
by resistor value selection and is achieved use of high
impedance transmit and receive amplifiers. Since these
amplifiers do not shunt the terminal of the telephone~ the
impedance is determined by specific shunt resistors. The
value of these resistors is not dependent on loop conditions.
I~hile the pulse dialer portion of the present
circuit is similar in many respects to those found in current
production, it does not employ a current limiting device
or current regular to prevent dial pulse distortion during
dialing. As noted above, this pulse distortion is caused
by supply of excess loop current to pulse dialer in order
to maintain minimum operating procedures satisfactory
integrated circuit operatlon. The present circuitry employs
a large (100 micro fara~) capacitor to keep voltage above
the minimum value for pulse dialer integrated circuit
operation during flial pulse interruptions of the line.
The electronic tone ringer portion of the present
circuitry employs a bi-polar tra~sistor oscillator circuit
including a three terminal piezoelectric transducer. One
of the terminals is a feedback tap which provides the
regenerative effect necessary for oscilla~ion. The oscill-
ator circuit operates from the output of a full wave rec-
tifier without a supply filter capacitor. This technique
permits the ringing voltage frequency to set the inter-
ruption rate of the oscillator which provides a more effec-
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tive alerting signal. The circuit thus is more economicalthan many found in the prior art, in that no interruption
oscillator is required. As a con-venience feature, a
"high-low-off" switch is incorporated in the present design
to select the acoustical output of the ringer unit. With
high level selected, the output level is greater than 70 dB
and is reduced by more than 6 dB when the low position is
selected.
Also included in the present subscriber's circuit
is a diode bridge which acts as a polarity guard. A hook-
switch of conventional design provides the usual "onloff-
hook" conditions. A transmit mute switch blocks voice
transmissions while still admit~ing incoming signals. A
simple re-dial switch actuates the last number dial feature
in the pulse dialer, while a PNP high voltage transistor is
employed as the actual dial pulsing switch and a metallic
oxide varistor is utilized as a voltage surge suppressor for
the protection of susceptible electronic devices.
BRIEF DESCRIPTION OF THE DRAWINGS
Figure 1 is a simplified combination block
and schematic diagram of the subscriber's telephone
circuit in accordance with the present invention.
Figure 2 is a schematic circuit diagram of
the receive circuitry included in the present in-
25 vention.
Figure 3 is a schematic circuit diagram of
the transmit circuitry included in the present invention.
Figure 4 is a combination block and schematic
circuit diagram of a dialer circuit in accordance with
the present invention.
Figure 5 is a schematic circuit diagram of
a ringer circuit in accordance with the present in-
vention.
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DESCRIPTION OF TH_ PREFERRED EMBODIMENT
Referring now to Figure 1 a simplified block
diagram of a subscriber's telephone circuit in accord-
ance with present invention is shown. The basic func-
tional elements are a receive circuit 200, a transmitcircuit 300, a pulse dialer circuit 400 and a ringer
circuit 500. For transducers, an electric microphone
310 is associated with transmit circuit 300 and an
encapsulated dynamic speaker 240 is associated with
receive circuit 200. A piezoelectric resinator attached
to an associated helm holtz resinator cavity (not
shown) may be associated with a ringer circuit 500.
Auxillary elements include voltage surge suppressor ~12
(a metallic o~ide varistor), diode bridge 600 which
provides polarity protection, hookswitch 4109 re-dial
switch 470 and transistor 430 which functions as the
dial pulse switch. As will be noted from Figure 1, all
the basis functional elements include circuit connec-
tions to terminals T and R which are adapted for connec-
tion to a telephone line.
Referring now to Figure 2 which shows thereceive circuitry. The basic portion of this circuit
consists of transistors 210, 220 and 230 which in
combination form a high open-loop-gain amplifie~. The
gain of the amplifier i~ su~ficiently high so as to permit
resistors 202, 203 and 206 to precisely establish the
closed-loop-gain. The base terminal of transistor 210 is
thP virtual ground and summing junction for the circuit.
Capacitors 205 and 231 are coupling capacitors used to block
direct current. Current source 211 provides a high im-
pedance active load for the collector of transistor 210
which is necessary to achieve a larse voltage ~ain with
the present amplifier configuration. Current source 221
isolates ~he 150 Ohm receiver impedance from the low amp-
lifier output impedance of the telephone line. This is-
olation is essential as it prevents severe inefficient
loading which results in signal power reduction.
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In a transfo~mer type of telephone circuit the
receiver impedance wou~d be matched to the telephone line
impedance by selection of the appropriate winding turns
ratio. In this manner, the current source helps to elim-
inate the transformer from the design. Resistors 207 and
222 establish the amplifier DC ou~put voltage to slightly
above that o~ a single silicon diodels forward voltage, to
maintain all of the transistors properly biased. The
present amplifier design is such that it will operate at
less than a 2 volt supply voltage, thus assuring operation
10 on low voltage loops and also when associated extension
phones are taken off/hook.
The present circuit also provides precise control
of sidetone cancellation which is a balance function.
This precision results from the summation weights of the
15 transmit and sidetone signals as determined exactly by
resistors 202 and 203.
Figure 3 shows the transmit circuitry. The basis
transmit amplifier structure consists of transistors 340,
350 and 360. Resistors 324, 343 and 352 in combinatîon with
20 transistors 340 and 350 form a current source mirror. That
is, the DC collector current of transistor 350 is approx-
imately equal to the bias current through resistor 324.
Transistor 360 services as a buffer by supplying most of the
current through resistor 352 (a low value resistor). The
25 resulting configuration presents a high impedance to the
telephone line and prevents shunting of signal currents.
Transistors 350 and 360 with resistors 352, 262 and cap-
acitors 361 and 341 form a high transconductance AC am-
plifier. A small voltage ~ignal input through capacitor 341
30 produces a relatively large signal current in the telephone
loop.
Resistor 342 presents a moderate impedance to the
microphones output. Resistor 326 matches resistor 342 to
improve tracking performance of the current mirror. Cap-
35 acitor 325 shunts AC current that flows through resistor 324to prevent signal feedback to the base of transistor 35~.
(Any such signal would reduce the transmit circuit output
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impedance.) Resistor 350 assures that transistor 350 will
bias up to the correct DC operating point. Resistor 326
isolates the microphone supply voltage filter capacitor 323
from the telephone terminals with a specific amount of
impedance. This resistor is one of the telephone impedance
establishing elements. Transistor 320, resistor 321 and the
normally open mute switch 330 provide the telephone user
with the option of momentarily disabling voice transmission
as a convenience feature.
The pulse dialer portion of the present telephone
circuitry is depictured in ~igure 4. The key element inthis circuitry is the pulse dialer integrated circuit 460
which is a commercial available integrated circuit element
manufactured by Mostek Corporation and identified as their
part number MX-50992. Integrated circui~ 460 is a CMOS
device which responds to switch closures of an associated
key pad 470 whose configuration may take any well known
form, (the details of which do not form a portion of the
present invention), which cause the pulse dialer integrated
circuit 460 to produce the approxima~e sequence of loop
current interruptions for dialing purposes. Also, when the
re-dial button 470 is operated, the last number dialed will
then be re-dialed using information stores in the memory
contained within ~he pulse dialer in~e~rated circuit
460. This memory is effective both during the off-hook and
on-hook conditions. When on-hook the telephone charges the
supply capacitor 420 through high value resistor 411. The
zener diode 421 prevents the supply voltage ~rom exceeding
the rating of the integrated circuit 460. The loop circuit
is switched on and off by transistor 430 and resist~r 432
under control of pulse dialer circuit 460. Transistor 440
serves as an on-hook condition detector. The response of
this detector is delayed by capacitor 443 to pre-
vent spurious operation. The detector is needed to reset
the re-dial function upon the telephone going on-hook.
Resistors 441, 442 and 431 are DC biasing elements for the
associated transistors 440 and 450. Resistors 462, 463 and
capacitor 461 establish the frequency of an oscillator
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included within the pulse dialer which drives all of the
timing functions of the pulse dialer integrated circuit.
Referring now to ~igure 5 which discloses
the electronic tone ringer circuitry. The basic
portion of this circuit is a 2800 Hz oscillator which
includes transistor 540 and piezoelectric transducer
560. The frequency of oscillation of this oscillator
circuit, is deterrnined by the self resonant frequency
of the piezoelectric element. This self resonant
frequency is influence by the method of mounting the
piezoelectric element and to a lesser extent by the
design of an associated Helmholtz resonator to which
it is attached. The essential oscillatory gain and
phasing is provided by transistor 540 in conjunction
with a feedback tab on the piezoelectric element which
is connected through resistor 541 to the base of
transistor 540. This oscillator turns on whenever a
voltage is supplied between the junction of resistors
531 and 532 and the emitter of transistor 540. Gapacitor
501, resistor 502 and the diode bridge 510 convert
incoming AC ringing signals when supplied across the
Tip and Ring ( T and R terminals of the telephone~ to a
DC voltage available for operation of the oscillator.
However, transistor 550 must be turned on before this
DC voltage is available for application to oscillator.
Transistor 550 and diode 521, 522 and 524 along with
resistors 523, 551 and 552 and capacitor 525 constitute
a turn-on delay circuit which prevents "dial-tapping."
This is a series of short pulses which may appear
across the terminal of a telephone when "on-hook" as
a result of someone dialing on an extension telephone.
With this ci.rcuit, the start of oscillation is delayed
by the charging time of capacitor 525. Resistor 523
discharges capacitor 525 after the input voltage sub-
sides. This re-sets the turn-on delay feature. The
switch 530 collects the supply feed resistance
through the tone oscillator circuit to provide two
different tone output levels. Also an open circuit
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(off) position is available. Different than most call
alerting configurations employed in the prior art that
employ tone ringer circuits, no tone interrupting
oscillator is employed, since the oscillator starts and
stops as the individual ringing cycles are available.
This frequent starting and stopping ~ction provides
effective tone interruptions.
A better understanding of the subscriber
telephone circuit of the present invention may be
had by reference to the following description of
operation in connection with Figures 1 through 5
inclusive of the drawings. Referring first to
Figure 1, the ringer circuit 500 which is directly
connec,ted across the leads connected to terminals T
and R which extend to the tLelephone line, offers high
AC and DC impedance until such time as a ringing voltage
appears across the terminals. At this time, the AC
impedance will drop~ allowing the oscillator portion of
ringer circuit 500 consisting of transistor 540 and
piezoelectric transducer 560 and associated components,
to absorb power from the line and produce an acoustical
alerting signal. The acoustical tone will be interruped
by voltage the magnitude dips of the incoming ringing
signal. Frequency of the interruption is equal to twice
the frequency of the incoming ringing signal.
The pulse dialer circuit 400 draws a small
amount of current from the line whenever the tele~hone is
connected to the line. This is taken through resistor 411
shown in Figure 1. This current keeps the re-dial memory
included in the integrated circuit 460 shown in Figure 4,
operative at all times. Thus any telephone number stored in
the ~emory of the pulse dialer integrated circuit 460 will
be dialed when the re-dial key 470 is depressed. Dial
pulses from this device operate the electronic switch
comprised of transistor 430 and provides loop disconnected
signalling. ~hen the re-dial feature is not used, digits are
pulsed in response to key pad key closures manually
initiated at key pad 470 wherein the subscriber utilizing
circuitry of the present invention is able to dial a
-- new number.
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When the subscriber removes the handset
from the hookswitch 410 going off-hook, closure of
the hooks~itch applies loop voltage to the
receive circuit 200 and the transmit circuit 300,
the amplifier circuitry contained which then
quickly stablize at the operating DC bias. Su~-
sequently, any sound pressure in that microphone
310 will produce a corresponding signal vol-tage at
the terminals T and R of the telephone circuit.
Also, any signal arriving at the telephone from an
associated telephone office or other switching
center, will be enhanced by the amplifier included
in receive circuit 200, to produce a corresponding
sound pressure at the output of receiver 240.
Referring now to Figure 2 the receive
lS signal is applied through resistor 202 through the
summing junction at the base of transistor 210.
An amplified receive signal appears at the emitter
of transistor 230 and through capacitor 231 is
applied to drive receiver 240. The receive gain
of the included amplifier circuitry is established
by resistors 202 and 206. Since a transmit signal
will find its way to the receive through resistor
202 ~sidetone) a controlled amount of transmit
signal is subtracted via resistors 201 and 203
from the sidetone signal to prevent undesirable
high sidetone levels at the receiver 240.
In the transmlt ~ircuit shown in Figure
3, the electret microphone 310 produces a few
millivolts of outpu~ at normal voice level. This
small signal is coupled through DC blocking ca-
pactior 341 to the base of transistor 350. Most of
the base signal also appears at the emitter since
the device is used in an emitter follower config-
uration. The result is a relatively large signal
current flow in resistor 352. This large current
then drives the telephone line and the 600 Ohm
telephone impedance. The voltage developed across
resistor 263 is used by the receive circuit for
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sidetone cancellation; the output being taken
between resistor 362 and capacitor 361 and ex-
tended through resistor 201 of Figure 2. In the
microphone portion o~ the circuit, capacitor 323
supplies a ~iltered voltage to the microphone's
power supply terminal.
In the pulse dialer circuit shown in
Figure 5, when the telelphone goes of~-hook,
hookswitch contacts 410 close applying a DC voltage
to resistor 441 and 442. This in turn causes
transistor 440 to saturate, producing a low logic
level a~ the collector and enabling the pulse
dialer integrated circui~ 460 to allow re-dial o~
the number stored in its internal memory or the
dialing o~ a new number. I~hen the telephone goes
back on-hook, the collector of transistor 440
rises to disable the pulse dialer functions,
except the memory included therein is sustained.
During out pulsing, a rectangular voltage
wave ~orm is generated by the pulse dialer integrated
circuit 460 and applied to the base of transistor
450 which in turn drives transistor 430, (the loop
current interrupt switch). Both transistors 450
and 430 are high voltage devices which can withstand
voltage transients that result from the brea~ing
of an inductive circuit. The capacitor 420 charges
either on-hook, or off-hook, supplies power to the
pulse dialer integrated circuit 460. The oscillator
which is included in the pulse dialer circuit has
a time constant determined by resistors 462 and
463 and 461 which operate only during dialing.
This arrangement minimizes power consumption.
Digit selection occurs as indicated previously
when the key pad closure contacts are made at the
key pad 470 and can output thereof is ex~ended to
the pulse dialer integrated circuit 460 as shown
in Figure 4.
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The electronic tone ringer shown in Figure 5
operates in response to the application of an alternat-
ing current (AC) ringing voltage applied across terminals
T and R. This signal is rectified by full wave diode
bridge circuit 510. The resulting DC voltage is of
sufficient magnitude to turn on zener diode 522 causing
capacitor 525 to start charging. When the voltage across
capacitor 525 reaches about seven (7) volts, transistor
550 turns on, in turn supplying operating voltage to
the 2800 Hz tone oscillator. This assumes that the
ringer volume switch is in a "low" or "high" position.
The time required to charge capacitor 525 delays any
output from the alerting device to prevent audible re-
sponse to transient voltage spi~es which may occur across
the telephone terminals.
While but a single embodiment of the present
invention has been shown it will be obvious to those
skilled in the art that numerous modifications may be
made without departing from the spirit and scope of the
present invention which shall be limited only by claims
appended hereto.
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