Telephone line feed

The direct current line feed for a telephone subscriber's line is derived from a variable voltage source connected to the line. In one case, the source is series connected in one leg of the line while in the other case it is connected across the line. In the first case there is a constant current source across the line while in the second case the voltage source is a constant current device. In both cases a pair of matching resistors is connected across the line with the center tap grounded to provide a high resistance balanced termination for the line. In both cases a high impedance monitor is connected across the line to monitor the voltage conditions on the line, and this, via a control circuit controls the voltage source and the separate constant current device if provided. This enables line voltage to be adjusted to take account of line length without introducing undesirably large dissipation.

This invention relates to a telphone subscriber's line interface, and 
especially to the supply thereto of the line current. 
The conventional arrangement for feeding a line uses a centre-tapped 
transformer arrangement with the two sides of the direct current supply 
connected to the centre of the line side winding of the transformer, which 
winding is in two parts coupled to AC but not DC via a capacitor. This is 
cumbersome, uneconomical from the power consumption aspect, and has 
difficulties when lines of different lengths have to be supplied. 
The present invention seeks to overcome the difficulties of such known 
arrangements. 
According to the present invention there is provided a line feeding 
arrangement for a telephone subscriber's line, which includes a high 
valued balancing impedance connected between the two wires of the line, 
said impedance having a centre-tap which is earthed for speech signals, a 
variable voltage source supplied by the exchange power supply and whose 
output forms the voltage drive for the line, a high-impedance monitor 
circuit which is also connected across the line so as to monitor the 
voltage between the two wires thereof, and a control circuit to which the 
output of said monitor is connected and which generates a reference 
condition which is applied to the voltage source so as to control the 
output voltage thereof, so that variations in line conditions cause 
compensatory variations in the line voltage supply.

The arrangement of FIG. 1, which avoids the necessity for a line 
transformer, uses a matched pair of resistors R1, R2 to achieve line 
balance. These resistors perform the dual function of terminating the line 
and providing the necessary balance about earth. Thus the termination 
impedance of the line is equal to the value of these two resistors. The 
termination impedance can, if desired, be rendered complex by connecting 
other components across these resistors R1, R2. 
The power feed to the subscriber's line is generated by a programmable 
voltage source PVS1 connected in one leg of the line. In the present 
context, the term "programmable" means that the voltage generated is 
adjustable in accordance with control conditions applied, including the 
actual line condition. This voltage source generates any required direct 
voltage needed to drive the line, and it has dynamic negative feedback 
with high gain so that its voltage output is maintained constant. Hence 
the voltage source has negligible impedance to alternating current, and 
hence to speech, provided there is sufficient gain in its feedback loop. 
The principle on which this source is based will be described below with 
reference to FIG. 8. The source PVS1 is so controlled that there is a 
potential of approximately 4 volts DC across the parallel combination of 
R1-R2 and a constant current circuit CC. 
The circuit CC just mentioned directs the loop current so that it does not 
flow in the balance/terminating resistors R1-R2, thus avoiding excessive 
dissipation in those components. This programmable (i.e. adjustable) 
constant current source has inherently high impedance to speech, and the 
principles on which it is based will be described with reference to FIG. 
7. 
Since the source PVS1 is so controlled that only 4 volts is dropped across 
the constant current source CC and the resistors R1-R2, the absolute 
potential on the A wire with respect to earth is -2 volts. Similarly the 
absolute potential of the B wire to earth is (-V.sub.o +2) volts, where 
V.sub.o is the output voltage of the source PVS1. Since V.sub.o is always 
greater than 4 volts, both the A and the B wires are always at a negative 
potential to earth. This eliminates problems due to galvanic corrosion in 
the subscriber line plant. 
The outgoing speech signal from the 2 wire -4 wire connector circuit is 
driven to the line via the constant current circuit, by modulating the 
steady state line current with this signal. The principle on which this is 
based is described below with reference to FIG. 10, from which it will be 
seen that a single driving transistor achieves a balanced speech drive 
from a source impedance of 2R.sub.t (where R.sub.t is the value of R.sub.1 
or R.sub.2). In principle it would be possible for the speech signal to be 
driven to line via the source PVS1, effectively modulating the steady 
state output voltage thereof. However, this would make the speech drive to 
line unbalanced, which is inferior from the cross-talk aspect. 
The 2 to 4 wire hybrid uses active components based on an operational 
amplifier. The incoming speech from the line is sensed by an input 
amplifier with a high input impedance connected across the terminating 
resistors R1-R2. Due to the circuit configuration used, the potentials at 
this point, which are within a few volts of earth, are compatible with low 
voltage electronic technology, regardless of the higher voltage (of to 
about 80 volts) generated by the source PVS1 to drive the line. 
In the arrangement shown in FIG. 2, line balance is achieved by matched 
resistors R3-R4 centre-tapped via battery to earth. The value of each of 
these resistors is in the region of 20 K ohms, the value being such as to 
ensure that there is no significant DC dissipation or AC speech loss in 
these resistors. The programmable source PVS2, which is connected across 
the line, has dynamic feedback so that it is effectively a constant 
current source and so has high impedance to speech. Its principles will be 
described later with reference to FIG. 9. As the centre-tap of R3-R4 is 
returned to the negative side of the exchange battery, the absolute 
potentials to earth of the line wires are: 
A wire: -48 volts+V.sub.out 
B wire: -48 volts-V.sub.out 
so the potentials on these wires are always negative, provided V.sub.out 
(the voltage across PVS2) is less than 96 volts. This gives enough range 
to drive very long lines, and there is no problem due to galvanic 
corrosion of line plant. 
Due to the isolating capacitor C1, the transformer T carries no direct 
current, and hence can be small. Thus complete electrical isolation is 
provided, so there is no restriction on the realization of the line 
terminating impedance Z.sub.T, connected across the secondary of the 
transformer T. The 2 to 4 wire converter can be an active hybrid using 
operational amplifiers. 
As in the case of the arrangement of FIG. 1 the outgoing speech signal may 
be driven via the programmable voltage source PVS2 if it is considered 
desirable. This gives a balanced speech drive as PVS2 is effectively in 
parallel with the line terminating impedance. 
FIGS. 3 and 4 show the additions needed to FIG. 1 and 2 respectively to 
realise any desired line feed characteristic. In each case a 
high-impedance monitor HIM is connected across the line wires, and this 
responds to line conditions to control the voltage source PVS1 or PVS2 via 
an intermediate control circuit. In the case of FIG. 3, the constant 
current source CC is also controlled in this manner. This is similar in 
some respects to the arrangements described in U.S. Patent Application 
Ser. No. 208,559 filed Nov. 20, 1980. Thus both circuits can provide 
either a constant voltage or a constant current type of line feed 
characteristic, in which in the idle (on-hook) condition the line voltage 
is in the region of 15-20 volts. This gives significantly lower line 
leakage than would be expected with a fixed 48 volt battery. As a 
subscriber's instrument is on-hook for the majority of the time, and in an 
exchange there are more lines on-hook than off-hook this provides a useful 
reduction in power consumption. 
In both FIGS. 3 and 4, the monitor HIM monitors line voltage and/or 
current, and the control circuitry has a time constant such that its 
output is unresponsive to the speech signals on the line so that it only 
controls direct current and voltage line levels. See also the arrangements 
described in U.S. Patent Application Ser. No. 212,347 filed Dec. 8, 1980. 
We now turn to FIGS. 5 and 6, which show how an arrangement embodying the 
principles of FIGS. 1 and 3 can be embodied into a complete line circuit. 
In FIG. 5 a relay contact network is used to switch ringing current to line 
and to connect the line to the test bus. These relays are controlled from 
a Control Interface, which is itself controlled over the connections shown 
from the remainder of the exchange. The hybrid is shown as consisting in 
fairly conventional manner of three operational amplifiers OA1, OA2 and 
OA3. The operations of the relays R, T, I, V and RV under control of the 
Control Interface are not described as it is felt that they are clear from 
the drawing. 
The high-impedance monitor HIM controls the constant current generator and 
the voltage source PVS1 via a feedback control circuit: this performs the 
functions of the CONTROL block of FIG. 3 and also exerts an influence on 
the Control Interface, so that the latter is influenced by line 
conditions. 
In FIG. 6, the relay contact network of FIG. 5 is replaced by a switch chip 
including an array of high voltage DMOS switches controlled from a control 
logic chip, which also acts as a control interface with the rest of the 
exchange. The feed control (which in FIG. 5 included the monitor HIM, 
Feedback Control, Constant Current Source and voltage source PVS1) and the 
2 to 4 wire hybrid are realized on another chip which interworks with the 
Switch Chip. Thus the transistor which drives the voltage source PVS1 is 
one of a group of high voltage transistors on DMOS chip. 
Similar circuits to FIGS. 5 and 6 but derived from FIG. 2 and FIG. 4 can 
also be envisaged, but are not shown to avoid unnecessarily complicating 
the drawings. 
FIG. 7 shows a simple way to realise a constant current circuit using an 
operational amplifier OA4 and a bipolar transistor BT. The actual value of 
the output current I.sub.c is controlled by a reference voltage V.sub.Ref, 
which is itself controlled by the control block (FIG. 3). 
FIG. 8 is a realisation of the voltage source PVS1, showing a dynamic 
feedback loop in which the output voltage between terminals 1 and 2, based 
on an "energy pumping" circuit including a capacitor C2, diode D1, 
transformer T2 and field effect transistor FET1 is compared with the 
reference voltage V.sub.Ref. The output of the comparator OAE is used to 
control the pulsed energy delivered into the pumping circuit such that the 
output voltage is controlled by V.sub.Ref. The amount of energy delivered 
into the pumping circuit is controlled by various means, e.g. by changing 
the frequency or the width of the pulses applied to the gate G1. 
FIG. 9 shows the principle of the circuit of FIG. 8 used for PVS2, the 
voltage output appearing between the terminals 3 and 4. Here the sample 
voltage is that across R6, which is proportional to the output current 
from the pumping circuit. Hence, as in FIG. 8, the control circuitry 
controls the output voltage by controlling the value of V.sub.Ref. 
FIG. 10 shows how the constant current circuit of FIG. 7 may be modified so 
that the speech signal can modulate the output current I.sub.c delivered 
to the line loop. In all cases, the circuits described enable line voltage 
to be kept relatively low and to be adjusted to take account of line 
conditions, including line length.