Pulse or digital communications – Equalizers
Reexamination Certificate
1997-06-30
2001-05-29
Pham, Chi (Department: 2631)
Pulse or digital communications
Equalizers
C375S350000
Reexamination Certificate
active
06240131
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention is directed toward the field of transmission line filters.
2. Description of the Related Art
In data communications systems, data is transferred over transmission lines at high frequencies. For example, in a data communications network that complies with the Institute of Electrical and Electronics Engineers (“IEEE”) 802.3u Standard for data communications, differential three level analog baseband signals are transferred over transmission lines at a rate of 125 megahertz (“MHZ”).
The IEEE 802.3u Standard supports both a 100 Base-T4 standard and a 100 Base-TX standard. In 100 Base-T4, Category 3 type twisted pair wire having lengths up to 100 meters is used to transmit data. In 100 Base-TX, either Category 5 shielded or Category 5 unshielded twisted pair wire having lengths up to 100 meters is used to transmit data.
During a high frequency transmission of signals, such as the signal transmissions in IEEE 802.3u 100 Base-T4 and 100 Base-TX networks, signals become severely attenuated and undergo significant phase shifts.
FIG. 1
shows two graphs
100
and
110
. Graph
100
illustrates the loss of amplitude that high frequency signals suffer on different lengths of the Category 5 unshielded twisted pair cable. The vertical axis in graph
100
measures signal amplitude loss in decibels (“db”), and the horizontal axis measures the frequency of the signal on the transmission line.
Curve
101
shows the characteristics of a 100 meter Category 5 unshielded twisted pair cable. For signals in the range of 100 MHZ, the amplitude loss is severe at −20 db. Curves
102
,
103
, and
104
in graph
100
illustrate the characteristics of Category 5 unshielded twisted pair cables having lengths of 50 meters, 25 meters, and 1 meter, respectively. In the case of curves
102
and
103
, the amplitude loss is also shown to be fairly significant at frequencies in the range of 100 MHZ.
Graph
110
illustrates the phase shift that signals undergo at different frequencies on Category 5 unshielded twisted pair cable. Curves
111
,
112
,
113
, and
114
show the phase shift characteristics of Category 5 unshielded twisted pair cables having lengths of 100 meters, 50 meters, 25 meters, and 1 meter, respectively. At a frequency in the range of 100 MHZ, the phase shift for the 100 meter cable
111
exceeds 100 degrees. Similarly undesirable phase shifts are shown in curves
112
and
113
.
FIG. 2
illustrates the distortion that is suffered by a differential three level analog signal in a 100 Base-TX IEEE 802.3u compliant network.
FIG. 2
shows two streams of bits
120
and
121
each being transmitted on an IEEE 802.3u Standard 100 Base-TX Category 5 unshielded twisted pair cable at a frequency of 125 MHZ. A measure of time is provided on a horizontal axis below the signals in each bit stream
120
and
121
.
Bit stream
120
is a set of bits represented by differential three level analog signals afer traveling a distance of 1 meter on a 100 Base-TX Category 5 unshielded twisted pair cable. Bit stream
121
shows the same bits from bit stream
120
after traveling a distance of 100 meters on the same cable. As can be seen from
FIG. 2
, signals being transferred at 125 MHZ over 100 meters of 100 Base-TX Category 5 unshielded twisted pair cable become very distorted due to both amplitude attenuation and phase shift.
In order to properly receive signals that are transferred over a transmission line at high frequencies, a filter is placed at the receiving end of a transmission. The filter provides compensation to the signal being received, so that the distortions caused by the transmission line are removed. Ideally, the filter has a transfer function that substantially offsets the transfer function of the transmission line. As a result, the filtered signal is substantially the same as the signal provided at the input of the transmission line.
In data communications applications, such as IEEE 802.3u compliant networks, it is further desirable for the transfer function of the filter, to compensate for the different distortions provided by different lengths of transmission line. As shown in
FIG. 1
, different length transmission lines provide different transfer functions affecting signal amplitude and phase shift.
In the case of filters for data communications applications, it is also desirable for the filter to be implemented using complimentary metal oxide semiconductor (“CMOS”) technology that is targeted for digital applications. This will enable the filter to be designed for low power operation. Further, the filter could be integrated onto a single wafer die along with other digital circuits required for implementing an IEEE 802.3u Standard network, such as a transceiver, data terminal equipment node, or repeater.
FIG. 3
illustrates a filter
130
that has a transfer function with poles and zeros that are dependent on specific values of resistors and capacitors employed in the filter
130
. The filter
130
includes an operational amplifier (“op-amp”)
135
having an output (VOUT) which provides the output of the filter
130
. A first input (VPOS) of the op-amp
135
is coupled to ground, while a second input (VNEG) is coupled to two different sets of resistors and capacitors.
One set of a resistor and capacitor includes a resistor
131
having a resistance of R
1
connected in parallel to a capacitor
132
having a capacitance C
3
. Resistor
131
and capacitor
132
each have one end connected to an input signal VIN of the filter
130
and another end connected to the second input (VNEG) of the op-amp
135
. The other set of a resistor and a capacitor includes a resistor
133
having a resistance R
2
coupled in parallel to a capacitor
134
having a capacitance C
4
. Resistor
133
and capacitor
134
each have one end connected to the second input (VNEG) of the op-amp
135
and another end connected to the output of the op-amp
135
.
The transfer function of a filter is the ratio of the filter's output to the filter's input. Transfer functions for filters are typically expressed in terms of their s-domain equivalent, where s is equal to j&ohgr; and a capacitance is equal to s times the capacitor's capacitance. The transfer function of the filter
130
in
FIG. 3
is equal to the following s-domain expression:
VOUT/VIN=(
C
3
/C
4)*(
s+
1/(
R
1
*C
3))/(
s+
1/(
R
2
*C
4)) (Equation 1)
The filter in
FIG. 3
therefore has the following pole and zero:
Pole=1/(R
2
*C
4
)
Zero=1/(R
1
*C
3
)
In order for the filter
130
in
FIG. 3
to provide adequate compensation for the transmission line distortion that a signal suffers, the value of the filter's transfer function will have to be set to offset the transfer function of the transmission line. In the case of filter
130
, this requires selecting precise values for R
1
, R
2
, C
3
, and C
4
. However, it is very difficult, and sometimes not possible, to form resistors and capacitors in integrated circuits with precise resistance and capacitance values.
Further, there is no mechanism in filter
130
to provide for adjusting the transfer function to account for different lengths of transmission line once the resistors
131
,
133
and capacitors
132
,
134
are selected. It is also very difficult in CMOS technology targeted for digital applications to provide an op-amp with sufficient high gain bandwidth for operating at frequencies of 125 MHZ.
FIG. 4
illustrates a filter
140
that is not dependent on the specific values of components employed in the filter
140
. Instead, the filter's transfer function is dependent upon the ratio of capacitors that are switched into the filter
140
. The filter
140
includes a network of capacitors
141
,
142
,
143
,
144
and a set of switches
145
,
146
,
147
,
148
. The switches
145
-
148
may be implemented by using transistors. The switches
145
-
148
couple and decouple capacitors to the inputs and output of an op-amp
149
to set the filter's tran
Cheng Yi
Holt Kris M.
Advanced Micro Devices , Inc.
Burd Kevin M
Fliesler Dubb Meyer & Lovejoy LLP
Pham Chi
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