Note: Descriptions are shown in the official language in which they were submitted.
2~291~
A CONTROL CIRCUIT FOR A SOLID STATE TELEPHONE LINE
CIRCUIT
CROSS REFERENCE TO RELATED APPLICATIONS
Cross Reference is made to the related Canadian
Patent Applications entitled: nA Solid State Telephone
Line Circuit,~ (Attorney Docket 89-1-032), ~A High
~oltage Subscriber Line Interface Circuit,~ (Attorney
Docket 89-1-034), ~A Circuit For Synthesizing An
Impedance Across The Tip And Ring Leads Of A Telephone
Line Circuit,~ (Attorney Docket 89-1-035), ~A Tip-Ring
Short Detector and Power Shut-Down Circuit For A
Telephone Line Circuit, n (Attorney Docket 89-1-036), ~A
Thermal Protection Circuit For An Integrated Circuit
Subscriber Line Interface, n (Attorney Docket 89-1-037),
~A Thermal Protection Arrangement For An Integrated
Circuit Subscriber Line Interface, n (Attorney Docket 89-
1-038), and ~A Ring Trip Detector For A Solid State
Telephone Line Circuit,~ (Attorney Docket 89-1-040) filed
on the same date, and by the same assignee as this
Application.
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to the field of
telecommunications and, more particularly, to a control
circuit that provides the low voltage control functions
for an integrated circuit subscriber line interface.
2. Description of the Prior Art ;
~ Telephone line circuits are customarily found in the
telephone switching system or central office of a
telecommunications network. The telephone line circuit ;~
interfaces the central office, to a telephone or
subscriber station found at a location remote from the
central office. The telephone line circuit functions to
supply power or battery feed to the subscriber station
o9 1 ~ 0
via a two wire transmission line or subscriber loop and
to couple the intelligence or voice signal to and from
the telephone switching system.
In many presently known telephone line circuits the
battery feed function has been performed by using a
passive, highly balanced, split winding transformer and
or inductors which carry up to 12Oma dc. This passive
circuit has a wide dynamic range, passing noise-free
differential signals while not overloading with the 60Hz
longitudinal induced currents. The line circuit just
described, feeds dc current to the subscriber loop and
also provides the voice path for coupling the voice
signal between the subscriber station and the central
office. The electromagnetic components of passive line
circuits are normally bulky and heavy and consume large
amounts of power for short subscriber loop lengths where
the current fed to the subscriber station is more than
necessary for equalization. Active line-feed circuits
can be less bulky and require lower total power, but
meeting dynamic range and precision balance requirements
dictates an overly complex circuit design.
Recently, solid state replacements for the
electromagnetic components of the aforementioned line
circuits have been developed. Devices such as high
voltage bipolar transistors and other specialized
integrated circuits are being designed to replace the
heavy and bulky components of the electromagnetic line
circuit. Such a device is described in the IEEE JOURNAL
OF SOLID-STATE CIRCUITS, VOL. SC-16, NO. 4, August 1981,
entitled, ~A High-Voltage IC for a Transformerless Trunk
and Subscriber Line Interface.~ These smaller and
lighter components allow the manufacture of telephone
switching systems having more line circuits per circuit
card as well as decreasing the physical size of the
switching system. ~ ;
However, presently known solid state line circuits,
still suffer from deficiencies in meeting good
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transmission performance specifications. These
deficiencies manifest themselves in poor longitudinal
balance and poor longitudinal current susceptibility,
which cause the circuit to fail or to become noisy.
Other problems presently encountered are excessive power
dissipation at short loops that consume prodigious
amounts of central office power and 2 wire input
impedance circuits that are complex and that exhibit poor
return loss.
lo Accordingly, it is an object of the present
invention to provide a new and more effective control
circuit that will effectively and efficiently provide
control functions to a subscriber line interface circuit.
DISCLOSURE OF THE INVENTION
The above and other objects, advantages, and
capabilities are realized in a control circuit used in
solid state line circuit. The solid state line circuit
includes an interface circuit connected to a subscriber
loop and a PCM codec filter. The control circuit
includes, in combination, a logic interface circuit
connected via an address/data bus and a control bus to a
central controller. The logic interface circuit is
arranged to receive and store digital data representing
commands from the central controller and to transmit ~
status signals from the control circuit to the central ~ -
controller.
A ~C loop control circuit, provides loop control
voltage to the interface circuit. The DC loop control
circuit, receives from the interface circuit, a
representation of loop voltage sensed across the
subscriber loop. It then develops and feeds back to the
interface circuit, a control voltage, that adjusts the
loop current fed to the subscriber loop.
A loop sense comparator is connected to the DC loop
control circuit and to the logic interface. The loop
sense comparator receives a representation of the
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subscriber loop voltage and compares the loop voltage to
a threshold derived by a predetermined reference voltage.
When the threshold is exceeded, the loop sense comparator
transmits to the logic interface a status signal.
A ring trip circuit is connected to the subscriber
loop and the logic interface. The ring trip circuit is
arranged to monitor the subscriber loop and transmit to
the logic interface a status signal representing an
interruption in the ringing current applied to the
subscriber loop.
An AC summing amplifier amplifies voice signals
transmitted on the subscriber loop. The AC summing
amplifier also receives a subtraction signal which it
uses to subtract from the transmit voice signals a
receive voice component before sending the voice signals
to the PCM codec filter.
A switched capacitor filter, provides under control
of the logic interface, loaded or non-loaded network line
balance operation. The switched capacitor filter is
further arranged to receive a receive voice signal from
the PCM codec filter which is used to generate the
subtraction signal used by the AC summing amplifier.
Finally, a voltage reference generator is included
that provides a first reference voltage used by the loop
sense comparator as the threshold voltage. The voltage
reference generator also develops a second reference
voltage used hy the ring trip circuit for detecting the
interruption in ringing current applied to the subscriber
loop.
BRIEF DESCRIPTION OF THE DRAWINGS
A better understanding of the invention may be had
from the consideration of the following detailed ^~
description taken in conjunction with the accompanying
drawings in which:
Figure 1 is a broad level block diagram of a solid
state telephone line circuit, using the control circuit
in accordance with the present invention.
Figure 2 is a detailed block diagram of the control
circuit, in accordance with the present invention.
DESCRIPTION OF A PREFERRED EMBODIMENT
Directing attention first to Figure 1, a broad level
block diagram of a solid state telephone line circuit is
shown. The line circuit is shown driving a subscriber
station or telephone 10, via a subscriber loop 20. The
subscriber loop 20 is comprised of a twisted two wire
(2W) loop pair having a tip and a ring lead. The 2W loop
is connected from the subscriber station 10 to an
interface circuit 30. The interface circuit 30 feeds a -
48 V dc voltage to the subscriber loop across the tip and
ring leads from a central office battery (not shown). `
The interface circuit 30 further functions to superimpose
a voice signal on the dc feed voltage and also feed
ringing current to subscriber loop 20 for signalling.
The Interface circuit 30 further functions to provide the ~;
2W to 4W (four wire) hybrid function of splitting the
balanced signal on the tip and ring leads into separate -
transmit and receive paths that are ground referenced.
The control circuit 40, in accordance with the ~ ~-
present invention, works in conjunction with the
interface cir~uit 30 to provide the DC loop current
shaping and the line balance impedance portion of the 2W
to 4W hybrid function. The control circuit 40 further
controls various detection functions, such as, ring trip
detection and loop sense detection, as well as, providing
a logic interface to the central controller of the
central office switching system.
Most modern digital telephone switching systems use
Pulse Coded Modulation (PCM) digital data to convey voice
traffic through the central office switching system.
Therefore, some method of signal translation is required
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to convert the analog voice signals received by the
interface circuit 30 to PCM digital data. This is
typically accomplished by a PCM codec and filter circuit
such as shown at 50. These devices are commercially
available as a so called CODEC/FILTER COMBO~ from varisus
manufacturers. Such as the TP30XX family of COMBO~
devices manufactured by the Naticnal Semiconductor
Company. Analog voice data from subscriber station 10 is
processed by the PCM codec 50 and applied to the PCM bus
of the central office switching system for transmission
to its destination. Similarly, the codec 50 receives PCM
data from the switching system and converts the PCM data
into analog signals which are superimposed on the dc feed
voltage of the subscriber loop 20.
The three solid state circuits 30, 40 and 50 just
described, in combination embody a complete line circuit
adapted to connect a single subscriber station to a
central office switch.
Referring now to Figure 2, a more detailed
explanation of the functions of the control circuit 40
shown in Figure 1, will now be given. The control
circuit 40 shown in Figure 2, is constructed as a bipolar
integrated circuit. All signals requiring low voltages
and currents are interfaced by circuit 40 while those
signals requiring high voltages, typically central office
battery (-48v), are interfaced by device 30.
The control circuit 40 of the present invention
includes in combination, a logic interface 410, a DC loop
control circuit 401, a loop sense comparator circuit 409,
a switched capacitor filter 405, an AC summing amplifier
408, a ring trip circuit 403 and a Vref circuit 411.
The logic interface 410 functions to provide a
digital interface between the control circuit 40 and a
central controller of a central office switching system.
The control circuit operating states are defined by a bi-
directional 4-bit address and one bit data bus NDATA 413
transmitted to control circuit 40. The reading back to
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the central controller is also accomplished using the
NDATA 413 bus. Write (WR), read (RD), and chip select
signals (CEN) are transmitted to logic interface 410 via
lead 412.
The DC loop control circuit 401 is used to generate
a control voltage to drive phase splitter amplifier 309
on interface circuit 30. The feedback control voltage
generated by loop control circuit 401 controls loop
current allowing for predetermined and smoothly
decreasing current curve for increasing loop resistance
up to 2000 ohms.
The loop sense comparator 409, is used to sense the
condition of the subscriber loop 20, such as loop
closure, due to an ~off-hook~ condition, or rotary dial
pulses. ~ -~
The ring trip circuit 403, is used to detect if the
subscriber has gone ~off-hook~ during a ringing period.
AC summing amplifier 408, functions to amplify the
ac voice signals transmitted from the subscriber and to ~ ;
cancel out the receive voice signals that are fed back to
the line interface. The transmit voice signals are
output to the PCM codec 50 via the XMT lead for
transmission to the cen~ral office switch.
The switch capacitor filter 405, functions to
reshape the ac voice signals received from the PCM codec
50 on lead RCV providing echo cancellation and line
balance usually required in 2-4 wire conversions. The
capacitors are selectable under control of the logic
interface, via leads L and NL, to provide loaded (L) or
non-loaded (NL) network lines.
Vref 411 generates a 750mV voltage used by the loop
sense comparator 409 and a -36mV reference voltage used
by the ring trip circuit 403.
Since control circuit 40 works in association with
interface circuit 30, some of the functions of interface
circuit 30 will be explained in combination with those of
control circuit 40 in order to aid in the understanding
6~
of the present invention. A more complete explanation of
the functionality of interface circuit 30 may be had by
reference to applicant's copen~ing Application titled, nA
High Voltage Subscriber Line Interface Circuitn,
(Attorney Docket No. 89-1-034), filed concurrently with
the present Application.
Current on either the tip or ring lead of the
subscriber loop 20 is sensed by a network of parallel
resistors 305 and 306 whose value is approximately 50.0
ohms net on each side of the loop 20. The voltage drop
across each resistor 305 and 306 is fed back to a tip
drive and a ring drive amplifier 307 and 308,
respectively. A phase splitter amplifier 309, couples
input voltage SUMB to the tip drive amplifier 307 and the
ring drive amplifier 308 and together with the feedback
voltage from resistors 305 and 306 create a voltage to
current converter. Input voltage SUMB controls the loop
feed current, such that, a given voltage at SUMB results ;
in a given current in the subscriber loop 20 flowing from
the tip lead to the ring lead. The feed circuit of the
present invention is designed to provide a typical gain
of 20mA/Volt at SUMB.
It should be noted that the current feed in
subscriber loop 20 is not a constant current, as the
aforementioned discussion may imply. Rather, the current
is shaped to provide sufficient variation of current
versus loop resistance to ensure efficient power usage.
This i8 accomplished by providing a feedback loop between
the interface circuit 30 and a DC loop control circuit
401. As loop resistance becomes less than 2K Ohm, a
resistor network comprising resistors 310-311 apply the
voltage drop sensed across subscriber loop 20 to a XMT
Dif~erential amplifier 314. A voltage that is
approximately half, but proportional to the voltage
sensed by resistors 310-311 then appears at XMTB. The
voltage at XMTB is fed to DC loop control circuit 401,
where it is properly shaped and output to circuit 30.
~Q2~
This shaped voltage is fed back to the phase splitter
amplifier 309 as the SUMiB input voltage.
The current shaping just described, generates feed
current characteristics that exhibit smoothly decreasing
current with loop resistance, allowing for power savings
at short loops, while still providing sufficient
variation of current versus loop resistance to ensure
proper transmit levels from the subscriber station. As
the total external resistance becomes greater than 2K
Ohms, the battery feed reverts to a constant voltage
feeding scheme.
The control circuit of the present invention is
arranged to provide either loaded or non-loaded line -; -
balance, under control of logic interface 410. Switched
capacitor filter 405, includes two switched capacitor
filters, one for non-loaded lines and one for loaded
lines. The individual filters are selected by signal
lines NL and L from logic interface 410. The capacitor
filters appear between the receive side speech signal at
RCV of circuit 40, and phase splitter amplifier 309 on
interface circuit 30. The switched capacitor filter 405
combines the filtered signal with the transmit speech
signals passed on XMTA. The filters have a response
which models the gain phase of the selected (loaded or
non-loaded) line and the interface circuit 30 plus
external components. The signal output from the filters
405 is 180 degrees out of phase with the transmit speech
signals. The net result is that the ~echo~ from the
receive side voice path, is sent to the ac summing
amplifier 408 via lead -RCV. The receive volce signals
are then canceled on the transmit side voice path before
the transmit voicè signals are output via XMT for PCM
conversion by the PCM codec 50.
The AC sum amplifier 408 is used to amplify the
voice signals received from the subscriber station and to
cancel out the receive voice signals fed back from the
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subscribers loop, before they are converted to PCM
representations by PCM codec filter 50.
Differential tip/ring speech signals transmitted
from the subscriber station 10 over the subscribers line
20, are initially coupled to the interface 30 via
resistors 310 and 311. The received differential speech
signals are applied to XMT differential amplifier 314
which converts the speech signals to a single ended
signal that appears at XMTA of interface circuit 30. The
speech signals at XMTA are coupled to control circuit 40
were a high impedance amplifier in circuit 408 provides a
gain of 2.536. The amplified transmit speech signals -
then have a ~reflection replica~ of the receive side
signal subtracted from them, as explained above in the
description of the switched capacitor filter circuit 405,
the net signal is output from XMT to PCM codec filter 50.
Loop sensing, is the detection of on/off hook status
of the subscriber and the replication of dial pulsing for
dial pulsing equipped subscriber stations. All loop
sensing detection in the present invention occurs within -
the control circuit 40. This detection is accomplished
by sensing an analog representation of the loop voltage
and comparing the analog representation to a fixed
threshold with a comparator. This is accomplished in the
following manner. As explained earlier, for the battery ~--
feed function of the present invention, a voltage that is
proportional to the voltage across the subscribers line
20 normally appears at lead XMTB. This voltage is
processed by the DC loop control circuit 401 and an
output signal SUMB is generated which is proportional to
loop current. The generated SUMB signal is also passed
to the loop sense comparator circuit 409 where it is
compared to a 750mV reference voltage generated by
circuit VREF 411. Upon detection of a sensed loop
condition, such as for example, a ring trip, loop sense
comparator 409 sends an output signal to input LS of
logic interface 410. A latch internal to logic circuit
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~10 then is set and its data read by the switching system
central controller via bus 413.
The control circuit 40, is further disposed to
provide a means of applying ringing voltage to the
subscriber loop 20 for signaling the subscriber
instrument 10. A ringing relay RR associated with the
subscriber loop 20 is operated or released to provide the
desired on/off ringing period, under control of the
central office central controller. T~ ring the ~
subscribers instrument, the central controller writes to ~-
the RR control point of the logic interface 410. This
sets an internal RR latch in interface 410. The now set
control point, through the RR output lead, pulls the RR
relay via its NPN transistor driver. When the relay is
activated RR contacts 318 and 315 are closed allowing ac
ringing voltage and current from a ringing generator (not ;
shown), to be applied in series with the dc battery
potential via leads RBB and RBA. ~ -
To detect a ring trip during ringing, such as when ;
~0 the substation instrument is placed ~off-hook~, a
resistor 317 develops a voltage drop which is
proportional to the applied ringing current. The
differential voltage developed by resistor 317 is divided
down and converted to a ground referenced signal by
resistor network 319 and an operational amplifier
internal to ring trip circuit 403. The ring trip circuit
403 filters out the ac ringing component of the signal
and keeps any dc component. Normally, when a substation
is ~on-hook~ during a ringing cycle, ringing current has
only an ac component. However, when the subscriber goes
~off-hook~, DC loop current from the battery biased
ringing generator is drawn into the loop. The dc
component is then compared against a -36mV reference
voltage developed by Vref circuit 411. If the dc signal -
sensed by circuit 403 is lower than the -36mV reference,
a signal is generated and sent to the RT sense point of
logic interface 410. This resets the RR latch,
;.. ,, ., . . .".. : :.. : : : ,: ,: , :.:: :.,.. :.:.. ~:: :.. : :. : :-.. - ; . :
~291~0
deactivating relay RR and removing the ringing generator
from the loop. The RT sense point is also monitored by
the central controller to detect if a ring trip has
occurred.
The control circuit 40 just described, can be
manufactured as a large scale integrated circuit suitable
for mounting on a hybrid assembly. The integrated
circuit can thus provide the capabilities of a line
circuit control device, which in the past, occupied a
complete circuit card, to a control circuit having
greater functional capability and occupying one sixteenth
the same space. The control circuit of the present
invention also benefits from the increased reliability
inherent in integrated circuit construction as well the
economies in labor cost and manufacture which are enjoyed
by such devices. -~
It will be obvious to those skilled in the art that
numerous modifications to the present invention can be
made without departing from the scope of the invention as
defined by the appended claims. In this context, it ~ ~
should be recognized that the essence of the invention `~
resides in a control circuit for a solid state telephone
line circuit having the advantages and capabilities ~-
described herein.
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