Note: Descriptions are shown in the official language in which they were submitted.
1036736
Field of the Invention
The present invention relates to repeaters in general, and
particularly to self-adjusting voice frequency (VF) repeaters for VF
telephone loops.
Background and Prlor Art of the Invention
The present telephone network is an interconnected multi-node
switching system, with each node serving a plurality of subscribers via
twisted pairs of wire. The further the subscriber is from the switching
node, the coarser the average aggregate gauge of his loop wire. Loops
1~ beyond a certain length having ohmic resistance larger than a certain
maximum value, if they exist within the serviced area, must, in order to
maintain the maximum permissible loss, utilize amplifying repeaters. In
the recent past, the loop length beyond which repeating becomes necessary
has been steadily declining due to the trend toward fine gauge wires, and
possibly ultimately toward a unified fine gauge or "unigauge"~ The
reasons, of course, are rising copper prices and declining cost of
electronics.
If repeaters are used in the switching centres for longer
loops, then it is most convenient for the operating companies to have a
2~ single standard repeater that self-adjusts its gain to fit the length
~f its loop. Two recent United States patents are directed to such
self-adjusting repeaters. One is U.S. Patent 3,903,378 issued
September 2, 1975 to David Q. Lee et al. It teaches the use of field
effect transistors (FETs~ as feedback elements to control the gain of
associated amplifying devices in response to a voltage that is proportional
to the loop length. The disadvantages of such simple arrangement is the
lack of temperature stability of, and possibly the necessity of selecting,
identical field effect transistors.
The second patent is U.S. No. 3,914,560 issued
October 21, 1975 to Clifford E. Greene, which patent teaches the use
of the dynamic impedance of a d-c biased diode as the gain controlling
1036736
element. Among the disadvantages of such arrangement are:
- the a-c signal no matter how small still modulates the
impedance of the diode causing harmonic distortion;
- in order to minimize the above disadvantage, the VF
signals are attenuated 40 to 50 dB before they are controlled to be
reamplified thereafter, thus requiring low-noise high-gain amplifiers
which are not inexpensive; and
- diode impedance varies considerably with temperature
and is difficult to compensate.
The present invention endeavours to solve some of the
above problems.
Summary of the Invention
The repeater of the present invention does not require
elaborate adjustments or calibrations, and it exhibits good temperature
stability.
Thus, according to the present invention the self-adjusting
voice frequency (VF) repeater for subscriber loops having ohmic resistances
between two predetermined values comprises, in combination, means for
dividing a control signal proportional to the loop resistance by a
~0 temperature-stable d-c reference signal, multiplying means responsive to
the output of the dividing means for multiplying said output by an input
signal of said repeater, and buffer means responsive to said multiplying
means for providing an output signal of said repeater, whereby a temperature
generated error of said multiplying means is substantially cancelled by an
opposite, temperature generated error of said dividing means.
Since some multiplier-dividers are capable of single quadrant
operation only, a preferred version of the present invention is to superpose
the input signal on a d-c bias, such that the total signal variation still
falls within a single quadrant.
Still a further preferred version is to utilize a
multiplier-divider having its error-producing, temperature sensitive
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1036~3~
transistors on a single, integral monolithic chip, to ensure optimal
temperature tracking and hence optimal error cancellation.
Brlef Description of the Drawings
A preferred embodiment of the present invention will now
be described in conjunction with the accompanying drawings in which:
Figure 1 illustrates the operation of a multiplier-divider
arrangement; and
Figure 2 is a block diagram of a self-adjusting voice-frequency
repeater according to the present invention utilizing the multiplier-divider
arrangement of Figure 1.
Description of the Preferred Embodiment
Referring to Figure 1 of the drawings, the divider-multiplier
arrangement comprises a single-quadrant divider 10 for producing the
quotient of the control voltage Vc by the reference voltage VR; a
single-quadrant multiplier 11 for multiplying the quotient Vc/VR by the
biased input signal (VIN + YB) to produce an output signal VOuT
VUT VR (VIN + VB) ~ IN + VB
where VB is an output d-c bias to be filtered out; and a summer 12 for
superposing VIN and a d-c biasing voltage VB at the input of the
multiplier 11.
VR is a stable reference voltage; the quotient Vc/VR,
however, due to temperature dependence of the divider 10, decreases with
increasing temperature. This decrease of Vc/VR compensates an associated
increase in the product ( ~ VIN) that cccurs in the multiplier 11. The
result is a final outpùt signal VOuT, that is substantially independent
of temperatu~e instabilities; that is if the divider 10 and the
multiplier 11 track. It is therefore opportune to utilize as the active
elements of division and multiplication integral monolithic devices. A
suitable multiplier divider circuit is fully disclosed in a book by
Jerald G. Graeme entitled "Application of Operational Amplifiers"
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103~;736
published by McGraw-Hill Book Company in 1973, pp 100-102. Such
multiplier divider as disclosed therein in Fig. 3.29, p.101, has been
found highly satisfactory when the transistors are integrated on a single
monolithic chip.
Figure 2 of the drawings is a block diagram depicting a
repeater between a switching centre 18 and a subscriber apparatus 19.
The repeater comprises for one direction of transmission; the divider 10,
the multiplier 11 and the summer 12 arranged as described above in
conjunction w~th Figure l; a buffer amplifier 13 fed by the output signal
lQ of the multiplier 11; the amplifier 13 itself feeds a hybrid 14 which
interfaces the repeater with the subscriber loop ending in the subscriber
apparatus 19; a loop sensor 16 supplies the control voltage Vc which is
proportional to the subscriber loop resistance (and hence length) between
the hybrid 14 and the subscriber apparatus 19; the loop sensor 16 senses
the length of the loop via the other hybrid 14' and the d-c continuity
path 15 that extends all the way to the subscriber apparatus 19; and a
power supply 17 providing the reference voltage VR as well as the input
biasing voltage VB, the power supply 17 may also (as in many applications)
pr~vide the loop sensor 16 with boosting d-c power for the loop via
connection 20. For the other direction of transmission, the components
designated with primed numbers provide the same functions as those just
described. Of course, the hybrids 14 and 14', the loop sensor 16 and the
power supply 17 serve both directions of repeatering. Further, while in
most cases the repeater is situated in the switching centre, this is by
no means a necessary requirement.
The summer 12 (and 12') is simply two resistors having
one common junction, which is connected to the multiplier 11 (and 11').
The output d-c bias VB, since it is not required, is
filtered out easily by a blocking capacitor and a discharge resistor
to ground at the output of the multiplier 11 (and 11') in the event
that the buffer amplifier 13 (and 13') is d-c coupled at its input.
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1(~3673~
The repeater of Figure 2 operates as follows. The loop
sensor 16 supplies a constant current to the subscriber loop between the
hybrid 14 and the subscriber apparatus 19 via the hybrid 14'; the d-c
continuity path 15 and the hybrid 14. The voltage applied to the
subscriber loop to maintain a prescribed constant current is proportional
to the resistance, and hence to the length of the loop. The control
volta~e Vc is derived from that voltage and varies between lower and
upper limits prescribed by the minimum and maximum repeater gain
required by the specifications. The control voltage Vc is thus applied
to the dividers 10 and 10' and controls the magnitude of output signal
V0uT as described in conjunction with Figure 1. The buffer amplifiers
13 and 13' provide a constant gain factor, such that at minimum control
voltage Yc~ in the case of the shortest anticipated loop, the total gain
of the repeater is the minimum required gain for such loop lengths. The
temperature stable reference voltage usually equals the minimum value of
the control voltage YC which typically varies from +4 to +9 volts. The
biasing voltage YB is +15 volts, given an input signal with a range from
0 to 4 volts peak, an input summing resistance of 200 Kohm and a bias
voltage summing resistance of 510 Kohm.
