Note: Descriptions are shown in the official language in which they were submitted.
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Field of the Invention
The present invention relates to the repeatering of
telephone transmission lines in general, and particularly to impedance
matching of nonloaded telephone lines to standard impedance values.
Background and Prior Art of the Invention
For various reasons, such as expense, telephone operating
companies often utilize nonloaded twisted wire pairs for the transmission
of voice frequency (VF) signals. Beyond certain cable lengths it
becomes necessary to insert repeaters in order to maintain adequate
transmission levels at all frequencies of interest. Since the cable
exhibits varying characteristic impedance with frequency while the
repeaters usually have a constant in-and output impedance7 it is
necessa~y to convert the cable impedance to match that of the repeaters
for proper operation of the total system.
United States Patent No. 3,814,867 issued June 4, 1974
to C. Wendell Boucher discloses an "Active Shunt Impedance For
Compensating Impedance Of Transmission Line" suitable for the application
at hand. However, the circuit taught therein exhibits instabilities in
the form of undesired oscillations when connected for operation in
telephone systems. Its utility for such applications is thus diminished.
In a system where more than a single impedance matching device is
necessary, spurious oscillations and instabilities, even though they
may be outside the frequency band of operation, are not acceptable.
Summary of the Invention
The present invention endeavours to provide an impedance
matching network that, in addition to being an improvement on the prior
art discussed in the patent to Boucher, supra, is stable under actual
operating conditions. As mentioned above, this is particularly important
if the system includes two or more matching networks and/or repeater.
For instance, unless the repeater is located in the switching centre of
the telephone company, where it is connected at one side thereof to
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a well-defined and matched impedance, it is necessary to use an
impedance matching network on either side of the repeater.
The present invention, thus, provides an impedance
matching network for a repeatered voice frequency transmission line
comprising in combination a first pair of terminals for connection to
a repeater, a second pair of terminals for connection to said transmission
line, means for introducing two effective series resistances at
predetermined frequencies between said first and second pairs of
terminals without substantially affecting d-c current flow, passive
impedance means coupled across said second pair of terminals, and
active impedance means a-c coupled across said second pair of terminals,
whereby said impedance matching network matches the impedance of said
transmission line to the impedance of said repeater while maintaining
stability.
From the following detailed description it will be
recognized that in the above network the three basic elements coact
to matGh the impedance of the transmission line to that of the repeater
at voice frequencies and still maintain stability at all frequencies.
In a preferable, narrower aspect of the invention, the means for
introducing two effective resistances is a balanced transformer on a
single core having two separate secondary windings interconnecting the
appropriate terminals of the network and a separate primary winding
having a resistor connected across it. It is also preferred that
the active impedance means be a-c coupled to the second pair of terminals
by means of a single isolation transformer.
Brief Description of the Drawin~s
A preferred embodiment of the present invention will now
be described in conjunction with the accompanying drawings in which:
Figure 1 is a block diagram illustrating how the impedance
matching network is used in a VF transmission line;
Figure 2 is a block diagram of the impedance matching
network.
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Figure 3 is an alternative realization of a component
of the impedance matching network of Figure 2;
Figure 4 is a detailed circuit of the impedance Zl in
Figure 2; and
Figure 5 is a detailed circuit of the impedance Z2 in
Figure 2.
Detailed Descr~ption of the Preferred Embodiment
Figure 1 of the drawings illustrates the general use of
the impedance matching network in a nonloaded VF transmission line.
Such a transmission line consists of, as is well known, a twisted pair
of conductors, one conductor designated TIP and the other RING. The
integrity of the conductors must be maintained throughout for reasons
of d-c (direct current) continuity. Repeater 10 is often a negative
impedance repeater and must be matched to the transmission line
bilaterally. Therefore~ a pair of impedance matching networks 11 and 12
are placed on either side of the repeater 10. Should the repeater 10
be in a switching office with one side connected to a matched impedance,
only one impedance matching network on the transmission line side
would be necessary.
Turning now to Figure 2 of the drawings, the impedance
matching network is shown with terminals 20 and 20' for connection to
the repeater 10 and terminals 21 and 21' for connection to the
transmission line. The network comprises a resistance insertion
network 22 which consists of a balanced transformer Tl with the
windings as shown in the drawing, and a resistor Rl connected to the
primary winding of Tl while each of secondary windings connects the
terminals 20/21 and 20'/21', respectively. The transformer Tl has
sufficiently low inductance so as not to affect voice frequencies
(i.e. ca. 300 to 3.4 KHz) substantially, yet enough to insert an
effective series resistance (Rl transformed) at frequencies of 5 KHz
and higher with each of the TIP and RING conductors. A suitable
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transformer ratio for Tl is 2:1 with the primary having an
inductance of ca. 24mH and each of the secondaries being 6mH. A
suitable value for Rl is 1 KOhm. The network in Figure 2 further
comprises a complex passive impedance Zl coupled across the
terminals 21 and 21' for matching the transmission line to the
repeater 10 at the lower portion of the VF spectrum from appr. 200 Hz
to 1 KHz. An isolation transformer T2 with preferably a 1:1 ratio
of primary and secondary windings is connected with one of its
windings across the terminals 21 and 21' and serves to couple an
10 active complex impedance Z2 at voice frequencies thereacross. The
impedance Z2, in cooperation with the insertion network 22, matches
the transmission line to the repeater 10 at the higher portion of the
VF spectrum without creating a device or total system instabilities
either within or outside of the VF spectrum.
Figure 3 of the drawings depicts an alternative to
the insertion network 22 shown in Figure 2. Insertion network 22' in
Figure 3 comprises a resistor R2 and a thereto shunted inductor L
in series with the TIP conductor, and similarly R2' and L' in series
with the RING conductor. The inductor L has sufficiently low
20 inductance to act substantially as a short for VF frequencies.
Figure 4 shows in detail the passive complex
impedance Zl, which comprises a fixed inductor Ll, a variable
resistor R3, a fixed resistor R4 and finally a capacitor C. Suitable
values are: Ll = 184mH; R3 = 10 KOhm; R4 = 510 Ohm; and
Cl = 0.47 microfarads.
Turning now to Figure 5, the active complex impedance Z2
comprises a negative impedance converter 23 for converting the combined
impedance of the series connection of a variable resistor, a
capacitor C2, an inductor L2 and a fixed resistor R5, to its output
3Q leads 24 and 24' connected to the winding of the transformer T2. Two
series capacitors C3 and C3', serving as impulse noise suppressors,
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the junction of which is grounded, are connected between the leads
24 and 24'. Also connected therebetween are two zener diodes D and D'
in series for surge protection. The negative impedance converter 23
is the subject of a copending patent application serial number 287,826
in the name of T. Lewandowski filed September 30, 1977 and assigned to
the same assignee as the present application. Component values in the
negative impedance converter 23 suita~le for the present preferred
embodiment are given in Figure 5 of the drawings. Reference is give~
to the above-mentioned copending application for understanding of the
improved negative impedance converter 23. However, it is necessary to
give here the condition for the stability of the active complex
impedance Z2. The feedback factor F of the negative impedance
converter 23 and associated circuitry is as follows:
R I Z ~ Zfl (R + R )
A(¦ Zsl + ¦Zfl~ (Ra + Rb)
where: Ra~ Rb and Rc are resistors associated directly with the
negative impedance converter 23 as shown in Figure 5; A is the open
loop gain factor of the operational amplifier MC1458; Zs is the
equivalent source impedance appearing between the leads 24 and 24';
and Zf is the total impedance of the series feedback elements R4, C2,
L2 and R5. Hence stability is always guaranteed if ¦Zf¦ tRa + Rc) >
Rb¦ Zsl For a better understanding of stability criteria of negative
impedance converters reference is given to Chapter 7, p. 208 in a book
by S.S. Hakim titled: "Junction Transistor Circuit Analysis",
published by John Wiley & Sons, Inc., 1~62.
Since such devices tend to become unstable at
frequencies above VF, the inclusion of the inductor L2 (lmHenry is
sufficient) in Zf fulfills the above condition at such frequencies,
without major effect in the VF range.
Suitable values are as follows:
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C3, C3' = 1000 picofarads;
R4 = lKOhms;
R5 = 200 Ohmsi and
L2 = lmH.
C2 is chosen practically to yield the best match
(highest return loss) measured across the terminals 21 and 21'
(TIP and RING) under actual operating conditions. The value, depending
on the length and gauge of the VF transmission line to be matched would
vary from a few thousand picofarads to fractions of microfarads. The
variable resistor R4 is also adjusted for best match with the impedance
matching network in situ. With the herein given component values a
match is easily achieved to 900 Ohms plus 2.15 microfarads in series.
