Crosstalk compensation for electrical connectors

Crosstalk compensation is achieved by connecting coupling devices (e.g., capacitors) between different pairs of conductors of a multi-pair connector. The coupling devices are selected to offset both differential-to-differential coupling as well as differential-to-common-mode coupling that would otherwise occur between pairs of conductors when one of the conductor pair is driven with a differential signal. The present invention can be used to achieve both differential and common-mode crosstalk compensation without relying on conductor crossover techniques.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to electrical connectors, and, in particular, 
to such connectors designed to reduce crosstalk between adjacent 
conductors of different transmission paths. 
2. Description of the Related Art 
Near-end crosstalk refers to unwanted signals induced in one transmission 
path due to signals that are transmitted over one or more other 
transmission paths appearing at the end nearest to where the transmitted 
signals are injected. Near-end crosstalk often occurs when the wires 
and/or other conductors that form the various transmission paths are in 
close proximity to one another. Classic examples of near-end crosstalk are 
the signals induced during some voice transmissions that result in parties 
to one telephone call hearing the conversation of parties to another call. 
An example that would benefit from this invention is when high-speed data 
transmission is impaired due to coupling of unwanted signals from one path 
to another. 
In a conventional telephony or data application, a signal is transmitted 
over a transmission path consisting of a pair of conductors, neither of 
which is grounded. To achieve a balanced signal, one voltage is applied to 
one of the conductors and another voltage having the same magnitude but 
opposite sign is applied to the other conductor. The difference between 
these two voltages is referred to as the differential voltage and their 
sum divided by two is referred to as the common mode voltage. When the two 
voltages are exactly equal in magnitude and opposite in sign, only a 
differential voltage will exist. A balanced signal is also referred to a 
differential signal. When such a differential signal is transmitted over 
one pair of conductors, two different types of crosstalk can be induced in 
an adjacent pair of conductors: differential crosstalk and common-mode 
crosstalk. Differential crosstalk refers to a differential or balanced 
signal that is induced in the adjacent pair, while common-mode crosstalk 
refers to a common-mode or an unbalanced signal that is induced in the 
adjacent pair. 
Existing crosstalk compensation schemes for adjacent pairs of conductors in 
electrical connectors are designed to compensate for differential 
crosstalk on an idle pair induced (i.e., coupled) from an adjacent driven 
pair. In so doing, however, these schemes do not provide compensation for 
the differential-to-common-mode crosstalk between the driven pair and the 
idle pair. 
FIG. 1 is a schematic drawing representing an example of an existing 
crosstalk compensation scheme designed to compensate for differential 
crosstalk between Pairs 2 and 3 in a four-pair modular mated plug/jack 
combination, such as those typically used for telephony or data 
applications (e.g., conforming to the T568-B wiring convention of the 
Telecommunications Industry Association (TIA) 568-A Standard). If, for 
example, Pair 3 is driven differentially, any coupled differential signal 
on Pair 2 is canceled out. Unfortunately, coupled common-mode signals on 
Pair 2 are not addressed by the compensation scheme of FIG. 1. The 
presence of this common-mode signal on Pair 2 degrades the crosstalk 
performance of the connector when it is deployed in a short link (known in 
the industry as short-link resonance). It also results in unacceptable 
levels of ingress and egress of electromagnetic interference. One way to 
compensate for this differential-to-common-mode coupling is to crossover 
both pairs of conductors, as shown in FIG. 2 
FIG. 2 is a schematic drawing representing an example of a crosstalk 
compensation scheme designed to compensate for differential-to-common-mode 
coupling. While the compensation scheme of FIG. 2 effectively cancels out 
any coupled common-mode signals, it does not address 
differential-to-differential crosstalk. 
What is needed is a crosstalk compensation scheme for connectors that 
addresses both differential-to-differential crosstalk as well as 
differential-to-common-mode crosstalk. 
SUMMARY OF THE INVENTION 
The present invention is directed to an electrical connector comprising two 
or more pairs of conductors, each adapted to carry a differential signal, 
wherein one or more coupling devices (e.g., capacitors) are connected 
between the conductors of different pairs to compensate for crosstalk 
between the different pairs.

DETAILED DESCRIPTION 
The present invention is directed to a crosstalk compensation scheme for 
connectors that addresses both differential-to-differential crosstalk as 
well as differential-to-common-mode crosstalk. According to the present 
invention, a connector having two or more pairs of conductors has coupling 
devices (e.g., capacitors) that are connected between conductors of 
different pairs. Values are selected for the coupling devices to provide 
compensation for differential-to-differential crosstalk as well as 
differential-to-common-mode crosstalk. 
FIG. 3 is a schematic drawing representing a crosstalk compensation scheme 
for a modular plug/jack combination, according to one embodiment of the 
present invention. FIG. 3 shows the crosstalk compensation scheme between 
Pair 2 and Pair 3 of a four-pair connector. According to the present 
invention, capacitors are connected between conductors to form a 
compensation region for the connector. In particular, in the embodiment of 
FIG. 3, capacitor Cc1 is connected between T2 (the tip line of Pair 2) and 
T3 (the tip line of Pair 3), capacitor Cc2 is connected between R2 (the 
ring line of Pair 2) and R3 (the ring line of Pair 3), and capacitor Cc3 
is connected between T2 and R3. In one possible implementation of the 
crosstalk compensation scheme of FIG. 3, capacitors Cc1, Cc2, and Cc3 are 
implemented by routing of traces of a printed wire board that is part of 
the jack of the plug/jack combination. 
As represented in FIG. 3, the crosstalk coupling between Pair 2 and Pair 3, 
whether caused by capacitive or inductive mechanisms, can be characterized 
by four inherent capacitances Cs1, Cs2, Cs3, and Cs4 in a crosstalking 
region of the connector, the values of which are determined by the 
geometries of the conductors and the electrical properties of the medium 
material in the crosstalking region. These four capacitance values can be 
measured directly or inferred from measurements of actual crosstalk 
levels. 
If the values of capacitors Cc1, Cc2, and Cc3 are chosen correctly, all 
differential-to-differential and differential-to-common-mode couplings 
between Pairs 2 and 3 will be canceled, regardless which of the two pairs 
is driven and which is idle. 
The following analysis shows how to calculate the capacitor values for 
Pairs 2 and 3 of the modular plug/jack combination of FIG. 3 in order to 
achieve both differential and common-mode crosstalk compensation. The 
differential-to-differential and differential-to-common-mode crosstalk 
coupling effects in the crosstalking region can be represented by 
Equations (1)-(3) as follows: 
EQU Csu=--Cs1-Cs2+Cs3+Cs4 (1) 
EQU Csb23=--Cs1+Cs2-Cs3 +Cs4 (2) 
EQU Csb32=Cs1-Cs2-Cs3+Cs4 (3) 
where: 
Csu is the capacitive unbalance in the crosstalking region, responsible for 
differential-to-differential crosstalk between the two pairs; 
Csb23 is the capacitive balance in the crosstalking region, responsible for 
differential-to-common-mode crosstalk when Pair 2 is driven and Pair 3 is 
idle; and 
Csb32 is the capacitive balance in the crosstalking region, responsible for 
differential-to-common-mode crosstalk when Pair 3 is driven and Pair 2 is 
idle. 
The term "capacitive unbalance" describes the total capacitive coupling 
between two pairs contributing to differential-to-differential crosstalk, 
and the term "capacitive balance" describes the total capacitive coupling 
between two pairs contributing to differential-to-common-mode crosstalk. 
For total differential-to-differential and differential-to-common mode 
crosstalk cancellation, the three capacitors Ccl, Cc2, and Cc3 should be 
chosen to produce capacitive unbalances and balances equal to and opposite 
in polarity to those in the crosstalking region, as expressed in Equations 
(4)-(6) as follows: 
EQU --Cc1-Cc2+Cc3=--Csu (4) 
EQU --Cc1+Cc2-Cc3=--Csb23 (5) 
EQU Cc1-Cc2-Cc3=--Csb32 (6) 
Solving Equations (4)-(6) for Cc1, Cc2, and Cc3 yields Equations (7)-(9) as 
follows: 
##EQU1## 
Substituting for Csu, Csb23, and Csb32 from Equations (1)-(3) into 
Equations (7)-(9) yields Equations (10)-(12) as follows: 
EQU Cc1=Cs4-Cs1 (10) 
EQU Cc2=Cs4-Cs2 (11) 
EQU Cc3=Cs4-Cs3 (12) 
As indicated by Equations (10)-(12), knowing Cs1, Cs2, Cs3, and Cs4, the 
values of Cc1, Cc2, and Cc3 that will produce total cancellation of all 
differential-to-differential and differential-to-common-mode crosstalk in 
the combined plug/jack combination of FIG. 3 can be calculated. The same 
can be achieved by inferring Csu, Csb23, and Csb32 from 
differential-to-differential and differential-to-common-mode crosstalk 
measurements performed for the crosstalking region. 
When three capacitors are used to provide crosstalk compensation, there is 
a unique solution for a given set of inherent connector capacitances. In 
an alternative embodiment, four capacitors can be used (e.g., adding a 
capacitor Cc4 between R2 and T3). In this case, a degree of freedom is 
added to the selection of capacitor values that will achieve the desired 
result of crosstalk compensation. It will also be understood that, in 
theory, the present invention can be implemented using any type of 
coupling device (i.e., either capacitors or inductive transformers or 
both). Furthermore, these devices may be discrete or integral parts of 
printed wiring boards, lead-frames, or stamped metal conductors. 
The above derivation for the values for capacitors Cc1, Cc2, and Cc3 is 
based on the crosstalk between only Pairs 2 and 3 of a four-pair 
connector. Those skilled in the art will understand that the same 
principles can be extended to derive capacitor values that will compensate 
for crosstalk between all pairs of any multi-pair plug/jack combination. 
In general, the problem is one of solving multiple linear equations of 
multiple unknowns. 
One of the advantages of the present invention is that it eliminates the 
need for crossover of conductors. This may reduce costs of manufacturing 
at least those portions of plug/jack combinations of the present invention 
when compared with combinations that employ conventional crossover 
compensation schemes, such as those of FIGS. 1 and 2. Nevertheless, the 
present invention can be implemented in situations in which one or more 
pairs of conductors do crossover. In such situations, one or more of the 
equations in the above derivation will be changed to reflect the different 
types of capacitive coupling between pairs of conductors. In FIG. 3, the 
present invention is implemented in the context of a modular plug/jack 
combination, such as may be implemented with jack shown in FIG. 4 having 
printed wire board 402. It will be understood that the present invention 
can be generalized to apply to crosstalk compensation for any two balanced 
signal pairs that are adjacent to one another in any type of mating 
connector. 
The use of figure reference labels in the claims is intended to identify 
one or more possible embodiments of the claimed subject matter in order to 
facilitate the interpretation of the claims. Such labeling is not to be 
construed as necessarily limiting the scope of those claims to the 
embodiments shown in the corresponding figures. 
It will be further understood that various changes in the details, 
materials, and arrangements of the parts which have been described and 
illustrated in order to explain the nature of this invention may be made 
by those skilled in the art without departing from the principle and scope 
of the invention as expressed in the following claims.