Pulse or digital communications – Cable systems and components
Reexamination Certificate
1999-04-29
2003-04-29
Liu, Shuwang (Department: 2734)
Pulse or digital communications
Cable systems and components
C375S286000, C375S288000, C375S316000
Reexamination Certificate
active
06556628
ABSTRACT:
TECHNICAL FIELD
The present invention relates generally to improved methods and systems for transmitting and receiving differential signals. More particularly, the present invention relates to methods and systems for transmitting and receiving differential signals over a plurality of conductors.
RELATED ART
Signaling is the method by which information is communicated from one part of an electronic system to another. In single-ended signaling, a single conductor, such as a wire, is used to transmit data from a transmitter to a receiver. In binary single-ended signaling, a transmitter encodes a binary digit as a voltage value or a current value on the conductor. For example, in binary single-ended signaling systems commonly used in computer communications, a logical “1” may be indicated by a voltage of about +5 volts and a logical “0” may be indicated by a voltage of about 0 volts. A receiver converts the signal on the conductor into a binary digit by comparing a received voltage or current with a local reference voltage or current.
FIG. 1
is a circuit diagram illustrating a conventional binary single-ended signaling system. In this system, a transmitter
100
receives binary data and outputs a voltage signal on a conductor
102
based on the state of the binary data. For example, if the transmitter
100
receives a logical “1”, the transmitter
100
may output a signal of about +5V on the conductor
102
. If the transmitter receives a logical “0”, the transmitter may output a signal of about 0V on the conductor
102
. The two possible values, 0V and +5V, of the signal represent two possible symbols or data units of the binary single-ended signaling system. The symbols encode the data bits “0” and “1”. A receiver
104
comprises a comparator that converts the voltage on the conductor
102
to binary data by comparing the received signal to a reference voltage V
REF
. If the voltage of the received signal is greater than the voltage V
REF
, the receiver outputs a high voltage or logical “1”. If the voltage on the conductor
102
is less than the voltage V
REF
, the receiver
104
outputs a low voltage or logical “0”.
One problem associated with conventional single-ended signaling systems is that the requirement of a local reference voltage at the receiver results in increased power consumption. For example, the power required to transmit one bit in a single-ended signaling system includes not only the power required to send the signal over conductor
102
, but also power required by a direct current voltage source to provide the reference voltage V
REF
at the receiver. Another problem associated with requiring a local reference at the receiver is that engineering a stable reference voltage may be difficult.
Another disadvantage associated with conventional single-ended signaling systems is that noise introduced in the signal transmitted over the conductor
102
may result in bit errors in the binary data output from the receiver
104
. For example, the transmitter may encode a logical “0” as a low voltage signal on the transmission line. Additive noise may be introduced into the channel between the transmitter
100
and the receiver
104
and cause the low voltage signal to rise above the threshold voltage V
REF
. As a result, the receiver
104
may decode the signal as a logical “1” when a logical “0” was intended to be transmitted.
In order to alleviate the problems of requiring a local reference and bit errors caused by additive noise, differential signaling systems have been developed. In a differential signaling system, two conductors are oppositely driven to transmit a single signal. The receiver detects the difference between the voltages or currents on the two conductors. As a result, no local reference is required at the receiver. In addition, if the conductors are physically similar and positioned close to each other, noise introduced into the channel will be common to both conductors or common-mode noise. Receivers can be easily designed, e.g., using differential amplifiers, to cancel the common-mode noise.
FIG. 2
illustrates an example of a conventional differential signaling system. In
FIG. 2
, a transmitter
200
converts an input stream of binary data bits into a pair of equal and opposite currents at its outputs. The currents are transmitted through a pair of conductors
201
and
202
to a receiver
203
. In order to transmit a logical “1” to the receiver
203
, the transmitter
200
generates current flowing towards the receiver on the conductor
201
and an equal but opposite current flowing towards the transmitter
200
on the conductor
202
. Similarly, in order to transmit a logical “0”, the transmitter generates a current flowing towards the transmitter on the conductor
201
and an equal but opposite current flowing towards the receiver
203
on the conductor
202
.
In order to decode the received signals, the receiver
203
detects the voltage difference between ends
204
and
205
of a resistive terminator
206
. If the voltage difference is positive, i.e., the voltage at end
204
is higher than the voltage at end
205
, the receiver
203
may output a logical “1”. If the voltage difference is negative, i.e., the voltage at the end
204
is lower than the voltage at the end
205
, the receiver
203
may output a logical “0”. In addition to converting the currents on the conductors
201
and
202
into voltages, the terminator
206
also reduces reflections on the conductors
201
and
202
.
In addition to the advantages of operating without a local reference voltage and rejecting common-mode noise, differential signaling systems provide a larger signal swing for detection by the receiver than single-ended systems. For example, the voltage on each of the conductors
201
and
202
may swing from +V to −V, resulting in a voltage difference of 4V volts between symbols. Applying the same voltage ranging from −V volts to +V volts to a conductor of a single-ended signaling system results in a voltage difference of, at most, 2V volts difference between symbols. As a result of the increased voltage difference between symbols, differential signals have increased immunity to noise.
One disadvantage of differential signaling systems with respect to single-ended signaling systems is that conventional differential signaling systems require twice as many conductors as single-ended systems to transmit a given signal. That is, for a signal having M different signal levels or symbols, where M=2, one bit is transmitted per symbol in both single-ended and differential signaling systems. However, in single-ended signaling systems, the number of bits per symbol per conductor is 1; whereas, in differential signaling systems, the number of bits per symbol per conductor is 0.5. Thus, conventional two-conductor differential signaling systems are one-half as efficient as single-ended signaling systems with respect to the number of bits transmitted per symbol per conductor.
In order to improve the bits-per-conductor efficiency of differential signaling systems, pairs of differential signal conductors may be connected and driven to create additional channels per conductor.
FIG. 3
illustrates one conventional method for improving the efficiency of a differential signaling channel. In
FIG. 3
, three differential channels X, Y, and Z are implemented using four conductors. Channels X and Y are conventional differential channels. The third channel, channel Z, is created without adding additional conductors. In the telephony industry, the third channel, channel Z, is referred to as a phantom channel.
In
FIG. 3
, channel X consists of a transmitter
300
, conductors
301
and
302
, a receiver
303
, and center-tapped terminators
304
and
305
. Channel Y consists of a transmitter
306
, conductors
307
and
308
, a receiver
309
, and center-tapped terminators
310
and
311
. In the illustrated circuit, the transmitter
300
transmits differential signals to the receiver
303
over the conductors
301
and
302
, based on the digi
Palmer Robert E.
Poulton John W.
Tell Stephen G.
Jenkins & Wilson, P.A.
Liu Shuwang
The University of North Carolina at Chapel Hill
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