Telephone line interface circuit without hookswitch relay

Details Telephone line interface circuit without hookswitch relay Telephone line interface circuit without hookswitch relay

2000-07-18

2004-03-30

Barnie, Rexford (Department: 2644)

Telephonic communications

Subscriber line or transmission line interface

Reexamination Certificate

active

06714645

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention is generally in the field of telecommunications devices and circuits. More specifically, the invention is in the field of telephone line interface circuits.
2. Background Art
A hookswitch relay (also referred to as a “relay” in the present application) determines whether a telephone device is “off-hook” or “on-hook”. This determination is made by allowing or disallowing DC telephone line current to flow through the telephone line interface circuit. Off-hook describes the condition when DC line current is allowed to flow through a telephone line interface circuit which can be coupled to a communications device such as a modem. On-hook describes the condition when DC line current is not allowed to flow through the telephone line interface circuit. The use of a hookswitch relay has been thus far generally considered an essential element in a telephone line interface circuit coupled to a communications device such as a modem.
FIG. 1
shows an exemplary telephone line interface circuit
100
. System side device
102
(also referred to as SSD
102
) is shown in block diagram form. SSD
102
can be part of a communications device such as a modem.
Coupling transformer
106
comprises primary winding
120
and secondary winding
122
. One terminal of primary winding
120
is connected to resistor
104
at node
101
. The other terminal of primary winding
120
is connected directly to TXA
2
of SSD
102
. Secondary winding
122
has one terminal connected to capacitor
110
. The other terminal of secondary winding
122
is connected to the ring terminal of the telephone line at node
111
. Resistor
104
has one terminal connected to a terminal of primary winding
120
at node
101
. The other terminal of resistor
104
is connected to TXA
1
of SSD
102
.
One terminal of capacitor
110
is connected to one terminal of secondary winding
122
. The other terminal of capacitor
110
is connected to one AC signal terminal of diode bridge
114
at node
107
. One terminal of relay
116
is connected to one AC signal terminal of diode bridge
114
at node
107
. The other terminal of relay
116
is connected to the “tip” terminal of the telephone line at node
109
(the telephone line is not shown in any of the Figures). In the present discussion, telephone line terminals tip and ring can be interchanged without affecting the operation of the telephone line interface circuit.
One AC signal terminal of diode bridge
114
is connected to relay
116
at node
107
. The other AC signal terminal of diode bridge
114
is connected to the “ring” terminal of the telephone line at node
111
. The DC positive terminal of diode bridge
114
(shown as “+”) is connected to the DC positive terminal of electronic inductor
112
through line
162
. The DC negative terminal of diode bridge
114
(shown as “−”) is defined and referred to as DC ground.
The DC positive terminal of electronic inductor
112
is connected to the DC positive terminal of diode bridge
114
through line
162
. The DC negative terminal of electronic inductor
112
is connected to DC ground through line
166
. MOV
118
has one terminal connected to the tip terminal of the telephone line at node
109
. The other terminal of MOV
118
is connected to the ring terminal of the telephone line at node
111
.
Coupling transformer
106
provides isolation and impedance matching between SSD
102
and the telephone line. The value of resistor
104
is chosen to set a desired impedance of SSD
102
for properly interfacing with the telephone line. If coupling transformer
106
is assumed to be ideal, i.e., no losses due to the resistance in the transformer windings, resistor
104
is chosen to be 600 ohms so that the impedance seen by the telephone line looking into the telephone line interface circuit is 600 ohms. Capacitor
110
functions as a decoupling capacitor. Capacitor
110
essentially passes AC signals with frequencies over 10 Hz and blocks AC signals with frequencies less than 10 Hz, and, of course, blocks the DC component of the telephone line signal. This prevents any DC current from entering secondary winding
122
of coupling transformer
106
, which is generally designed for linear operation without any DC current, i.e., coupling transformer
106
is a “dry” transformer. In the circuit of
FIG. 1
, the value of capacitor
110
can be 22 &mgr;F and can have a voltage rating of 62 volts.
Diode bridge
114
rectifies the telephone line voltage and current applied to electronic inductor
112
. Since electronic inductor
112
is implemented with transistors, which are essentially polar devices, i.e., they require a DC bias of specific polarity to operate, diode bridge
114
is added to telephone line interface circuit
100
to ensure that a positive voltage and a negative voltage are always applied to the DC positive and DC negative terminals of electronic inductor
112
, respectively, regardless of the line voltage polarity present at the tip and ring terminals of telephone line interface circuit
100
. This avoids the possibility that incorrect wiring of a telephone wall jack will result in a malfunction of telephone line interface circuit
100
due to a polarity mismatch.
MOV
118
functions as a voltage surge suppressor. When the voltage across the tip and ring terminals of the telephone line exceeds approximately 300 volts, MOV
118
clamps the voltage at the tip and ring terminals of the telephone line to a maximum value, thus protecting electronic inductor
112
.
Relay
116
allows current flow from the telephone line if relay
116
is closed. In other words, the telephone line interface circuit is off-hook. If relay
116
is open, there is an open circuit and therefore no current flow. In other words, the telephone line interface circuit is on-hook. Relay
116
is turned off and on by means of a relay control in SSD
102
(the connection between the relay control and relay
116
is not shown in FIG.
1
). One reason Relay
116
is necessary to the circuit shown in
FIG. 1
is because it is required that telephone line interface circuits, such as the circuit of
FIG. 1
, must comply with certain requirements for on-hook maximum current flow and AC impedance. Generally, the standard requires that on-hook DC current flow be less than 10 &mgr;A and that on-hook AC impedance be greater than 5 kilo ohms.
To meet these specifications, assuming a typical Central Office battery voltage of approximately 50 volts, a minimum resistance of 5 meg ohms is required between the tip and ring terminals of the telephone line interface circuit (50 Volts /10 &mgr;A=5 meg ohms) when the circuit is on-hook. Relay
116
has previously been used to meet this requirement by completely disconnecting the telephone line interface circuit from the telephone line. The resistance of an open circuit is infinite and therefore there is no current flow when relay
116
is open.
In
FIG. 1
, electronic inductor
112
is shown in block diagram form. When relay
116
is closed (i.e., in the off-hook state) current is allowed to flow from the telephone line tip and ring terminals through line
107
and
162
and into electronic inductor
112
. Electronic inductor
112
sets the DC current value for the telephone line interface circuit.
The use of a relay hookswitch in telephone line interface circuits has disadvantages. One disadvantage of using a relay is its physical size. Relays are bulky and occupy a large space. Another disadvantage is that a relay requires a relatively large amount of power to be activated. In addition, the opening of a relay generally induces undesirable high-voltage spikes across its terminals. Also, a relay must completely disconnect the telephone line interface circuit from the telephone line to meet on-hook DC resistance and AC impedance requirements, which results in total signal isolation from the telephone line in the on-hook state. Moreover, a relay is a relatively expensive device.
Solid state relays have recently been introduced which are

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