Wave transmission lines and networks – Dissipating terminations for long lines – Fluid-cooling
Reexamination Certificate
2001-02-21
2002-12-10
Pascal, Robert (Department: 2817)
Wave transmission lines and networks
Dissipating terminations for long lines
Fluid-cooling
C333S02400C, C333S032000, C333S005000, C375S258000, C379S398000
Reexamination Certificate
active
06492880
ABSTRACT:
FIELD OF THE INVENTION
The current invention is directed to the delivery of a bias voltage using a pair of differential signals while it minimizing the electromagnetic interference produced by these differential mode signals in an Ethernet data port or other serial data port where a common mode transformer is employed for high potential (hipot) isolation.
BACKGROUND OF THE INVENTION
Isolation transformers and common mode transformers are commonly used in the serial interface ports in communications equipment. One such serial interface type is Ethernet, as described in IEEE standard 802.3. In this type of communications interface, it is desired to transmit differential mode voltages over long distances to a remote point, and a pair of differential transmit signals and a pair of differential receive signals are sent over twisted pair copper wire. The data rate carried over these differential pairs varies from a data rate of 10 Mbps (million bits per second) to a data rate in excess of 1 Gbps. It is often desired to reduce the high frequency common mode component of the transmitted signal, and a class of transformers known as common mode transformers is employed, whereby the opposing flux produced by common mode currents cancels the common mode voltage at the output of the transformer, while the additive flux produced by differential mode currents reinforces the output voltage. In addition, an isolation voltage requirement provides for the biasing 1500V of DC from any differential pair to ground, and an isolation transformer is commonly employed to provide this isolation.
FIG. 1
shows one such prior art termination. A device known as a SERDES (serializer-deserializer) provides a serial stream of differential output data on lines
41
and
41
′, which couples to transmitter isolation transformer
14
of transformer
12
. Isolation transformer
12
serves to provide the required hipot (high potential) isolation of typically 1500V between the data equipment associated with the serdes
40
, and the incoming data lines of connector
30
. Transmit isolation transformer
14
is followed by common mode transmit transformer
20
of common mode transformer
18
. A common mode transformer provides very little impedance to currents of opposite polarity, and a higher impedance to currents of the same polarity, thereby reducing the conduction of high frequency currents from the serdes
40
to the external conductors of the cables attached to connector
30
. This reduction in common mode transfer from the serdes to the connector conductors reduces the emissions of EMI (Electro-Magnetic Interference) from the equipment. A balancing transformer
26
provides for symmetry in the output voltages, as well as an impedance reference for the common mode transformer
20
. The output of transmit common mode transformer
20
is also furnished to an output connector
30
, which for Ethernet is pins
1
and
2
of a shielded RJ-45 connector. Similarly, the receive path comprises a differential input arriving on connector
30
pins
3
&
6
, thereafter being furnished to common mode receive transformer
22
of common mode transformer
18
, and to receive isolation transformer
16
of isolation transformer
12
, and to the receive inputs
43
and
43
′ of the deserializer section of serdes
40
. Receive balancing transformer
28
of balancing transformer
24
provides an impedance reference for common mode choke
22
, as before. The reference resistors
42
and
44
provide an impedance reference for the unbalanced voltages present in the center tap of the balancing transformers
26
and
28
. Capacitor
50
is a high voltage type that provides an AC return path for unbalanced common mode currents generated by either receive balancing tranformer
28
or transmit balancing tranformer
26
. Resistors
46
and
48
provide a high frequency termination for unused pins
4
,
5
,
7
, and
8
of the RJ-45 connector
30
, for the case where an
8
wire cable using
2
pairs of twisted pair conductors is employed. As the cable connected to connector
30
may be very long, common mode voltages appearing on the twisted pairs for transmit and receive capacitively couple to the other twisted pairs, so all wires of the cable are provided a high frequency reference to ground, including the unused pins
4
,
5
,
7
, and
8
.
OBJECTS OF THE INVENTION
A first object of the invention is to provide a bias voltage using two differential pair electrical connections while preserving the hipot characteristics of the prior art termination. A second object of the invention is to provide an operating voltage over two differential pair electrical connections while preserving the common mode EMI attenuation of the prior art termination. A third object of the invention is to couple residual transient noise energy on the twisted pair cables to a shield conductor.
SUMMARY OF THE INVENTION
A differential mode termination includes a transmit isolation transformer coupled to a transmit common mode transformer, which is coupled to an output connector. The transmit common mode transformer has a reference autotransformer with a center tap for providing a shield reference, as well as the application of a bias voltage. The differential mode termination also includes a receive isolation transformer coupled to a receive common mode transformer, which is coupled to an output connector. The receive common mode transformer is coupled to a reference autotransformer with a center tap for providing a shield reference to the common mode transformer. By providing a bias voltage to the transmit reference autotransformer center tap and a bias voltage return to the center tap of the receive reference autotransformer, it is possible to power remote equipment while minimizing common mode coupling of EMI from the serdes and other noise sources internal to the equipment to the conductors of cables attached to the output connector.
REFERENCES:
patent: 5896417 (1999-04-01), Lau
Ataee Mehran
Chiappe Jim
Lum Meilissa
Wood Dean
Chesavage Jay A.
Cisco Technology Inc.
Jones Stephen E.
Pascal Robert
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