Multiplex communications – Diagnostic testing – Determination of communication parameters
Reexamination Certificate
1997-09-04
2002-01-29
Olms, Douglas (Department: 2661)
Multiplex communications
Diagnostic testing
Determination of communication parameters
C370S276000
Reexamination Certificate
active
06343069
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to an apparatus and method for automatically selecting an amount of build-out capacitance to be applied to a hybrid in a telecommunications circuit to balance or substantially balance the hybrid. More particularly, the present invention relates to an apparatus and method for measuring the voltages of signals being transmitted on telecommunication lines to which the hybrid is coupled, and based on the measured values, automatically selecting any number of capacitive elements to be coupled to the balance port of the hybrid as the build-out capacitance.
DESCRIPTION OF THE RELATED ART
In the field of telecommunications, it is common to use a hybrid function as an interface between the external tip and ring lines of a “plain old telephone service” (POTS) line and any other signal processing circuitry within a unit. That is, the tip and ring lines of the POTS line are capable of handling two-way signal transmission. However, most processing circuitry requires the transmission to be split into one-way paths before the signals can be processed. Such an interface is commonly used in telephones, modems, and so on, to couple traffic coming out of the device into the POTS line (receive as viewed from the POTS line), and traffic from the POTS line (transmit as viewed from the POTS line) into the device.
A hybrid can be a single-transformer type, as shown in
FIG. 1
, or a dual-transformer type, as shown in FIG.
2
. In a telecommunications system using either a single-transformer hybrid or a dual-transformer hybrid, the hybrid couples the tip and ring lines of the POTS line (identified as the “2-W
Line
” in
FIGS. 1 and 2
) to a pair of transmit lines (“N
Tx
” in FIG.
1
and “4-W
Tx
” in
FIG. 2
) and a pair of receive lines (“N
Rx
” in FIG.
1
and “4-W
Rx
” in FIG.
2
), which can be coupled to the transmit and receive ports, respectively, of a telephone, modem, private branch exchange unit (PBX), or the like. For example, the two transmit lines can be coupled to a microphone or transmitter of the telephone, while the two receive lines can be coupled to a speaker or earphone of the telephone. The signal being transmitted by the microphone will appear between the two transmit lines, while the signal being received by the speaker will appear between the two receive lines.
In other words, the signal being transmitted on the transmit lines is represented by the difference in potential between the two transmit lines, while the signal being received on the receive lines is represented by the difference in potential between the two receive lines. Of course, one of the receive lines and one of the transmit lines could be coupled in common to ground. This is generally done in the single-transformer type by grounding the external connection on Z
o BAL
. In the dual-transformer type, one side of each port is connected to ground to accomplish the same.
The hybrid converts a signal being transmitted on the two transmit lines so that it is transmitted between the tip and ring lines of the POTS line, while the hybrid also converts a signal being received on the tip and ring lines of the POTS line into a signal that is received over the two receive lines. It is again noted that the signals being received and transmitted can appear simultaneously on the tip and ring lines of the POTS line.
When employing a hybrid in the manner described above, it is important that the signal being transmitted over the two transmit lines is essentially isolated from the signal being received over the two received lines, and vice-versa, so that minimal electrical coupling between the paths occurs. Preferably, none of the signal being transmitted over the two transmit lines should appear on the receive lines, and likewise, none of the signal being received on the two receive lines should appear on the two transmit lines.
The degree to which the signals on the transmit and receive lines are isolated from each other is dependent on the trans-hybrid loss of the hybrid. If the trans-hybrid loss of the hybrid is low, some of the signal being transmitted over the transmit lines may appear on the receive lines and vice versa. However, if the hybrid has a very high trans-hybrid loss, the transmit and receive lines will be essentially isolated from each other so that their respective signals do not appear on each other's lines.
The amount of trans-hybrid loss of the hybrid is affected by the impedance characteristics of the tip and ring lines of the POTS line. In particular, the amount of trans-hybrid loss of the hybrid is influenced by the amount of capacitance that is present at the interface of the hybrid and the tip and ring lines. The amount of capacitance between and the amount of resistance of the tip and ring lines depends on the length of the tip and ring lines, as well as the gauge of wire that is used for those lines. In standard practice, telephone companies use twisted pair cables having about 0.083 &mgr;F of capacitance per mile, regardless of gauge.
The capacitance appearing at the interface of the hybrid and the tip and ring lines will tend to place the hybrid “out of balance”, which reduces the amount of trans-hybrid loss in the hybrid and thus allows coupling to occur between the transmit and receive lines. However, that capacitance can be compensated for to “balance” the hybrid by adding additional capacitance to a balance port of the hybrid, which is usually configured as a tap on one of the coils of the transformer or transformers in the hybrid.
For example, a circuit card developed by Pulse Communications, Inc., which functions as an interface circuit to a central office switch (e.g., a switching unit to which telephones are connected), includes a hybrid for coupling the tip and ring lines of a POTS line to transmission and receive lines in the manner described above. This circuit card includes a plurality of capacitors, which are each coupled to a manual switching device that enables the capacitors to be selectively coupled to the balance port of the hybrid.
Specifically, the circuit card includes 2 nF, 4 nF, 8 nF, 16 nF, 32 nF and 64 nF capacitors, which are each coupled to the hybrid by a respective switch of the manual switching device. By opening or closing the switches as desired, a capacitance of 0-126 nF in 2 nF steps can be applied to the balance port of the hybrid. Accordingly, an appropriate amount of capacitance can be applied to the hybrid as a “build-out capacitance” to compensate for capacitance that is applied to the hybrid by the tip and ring lines. For instance, if the installer of the circuit card determines that 10 nF of build-out capacitance is sufficient to compensate for the capacitance that is applied to the hybrid by the tip and ring lines, the switches coupled to the 2 nF capacitor and 8 nF can be selected, so that a total of 10 nF of capacitance will be applied to the hybrid circuit.
This manual switching configuration of the circuit card described above has certain disadvantages. For example, when this type of circuit card is installed in an existing telephone system, a skilled technician must determine the capacitance applied to the hybrid by the tip and ring lines by using a trial-and-error measurement technique. He attempts to maximize trans-hybrid loss by guessing the best setting, activating the appropriate switches, measuring trans-hybrid loss, then adjusting his guess repeatedly. As he zeros in on the best setting, the trans-hybrid loss improves. This measuring and setting process is very time consuming. Therefore, if a large number of circuit cards are being installed, a technician may have to work for many days to complete the installation. Furthermore, error factors associated with the technician's equipment, the technician's abilities, and so on will influence the measurements and thus could result in an inaccurate amount of build-out capacitance being selected.
To overcome these disadvantages associated with a manually adjustable circuit card, Pulse Communications, Inc. has developed a circuit card, for
'Hubbell Incorporated
Buczynski Joseph J.
Olms Douglas
Presson Jerry M.
Vanderpuye Ken
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