DC restoration circuit for multi-level transmission signals

A DC restoration circuit to correct for baseline wandering in a data receiver is provided. A voltage correction circuit is connected to the received data line to adjust the voltage level of the received data dynamically. The voltage correction circuit is controlled by a feedback circuit which includes a voltage detection circuit configured to detect the peak voltage levels or envelope of the received data. This detected level is then compared to a reference level, and the result of the comparison is used as a control signal for the voltage correction circuit.

BACKGROUND OF THE INVENTION 
The present invention relates to DC restoration circuits for data 
transmissions, and in particular for multi-level transmission-3 (MLT-3) 
signals in Ethernet 100Base-TX systems. 
In multi-level transmission systems, rather than a digital high and low 
value for data, multiple levels can be used to encode data. In an MLT-3 
system, three levels can be used, a high, middle and low. The high level 
could be considered a positive, the middle a zero and the low a negative, 
for instance. The use of multiple levels, rather than two levels, makes 
the levels closer to each other and thus more susceptible to noise and any 
variation in the DC reference or level value. 
One phenomenon that is a problem in Ethernet systems is that of "baseline 
wander." The transformers used for the data lines can introduce baseline 
wander into a received signal. This baseline wander can vary over time, 
and can vary the low frequency content of the received signal at rates up 
to 100 KHz. Such a variance can cause errors in the detected data. 
One technique for correcting for baseline wander involves converting the 
data into digital form, then performing a low pass filtering of a copy of 
the data and adding the filter version back to the original signal for 
correcting. 
It would be desirable to have a simpler form of correction for baseline 
wandering that also does not require conversion into digital form, and is 
not dependent on gain, temperature or other IC process variations. 
SUMMARY OF THE INVENTION 
The present invention provides a DC restoration circuit to correct for 
baseline wandering in a data receiver. A voltage correction circuit is 
connected to the received data line to adjust the voltage level of the 
received data dynamically. The voltage correction circuit is controlled by 
a feedback circuit which includes a voltage correction circuit configured 
to detect the wander of the voltage levels or envelope of the received 
data. This detected level is then compared to a reference level, and the 
result of the comparison is used as a control signal for the voltage 
correction circuit. 
In a preferred embodiment, the peak detector has a response time which is 
greater than a DC wander rate of 100 KHz, and lower than a slowest 
expected data rate of approximately 2 MHz. The comparison circuit is 
preferably an operational transconductance amplifier that compares the 
peak detected voltage to the reference voltage, and outputs a difference 
current. The difference current is used to charge a capacitor which is 
buffered and resistively added to a node on the input data line. The input 
data line passes through another resistor, and thus the ratio of the 
resistors can be adjusted to provide the desired summing weight. 
For a fuller understanding of the nature and advantages of the invention, 
reference should be made to the following description taken in conjunction 
with the accompanying drawings.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
FIG. 1 is a block diagram of one embodiment of a physical layer transceiver 
circuit 10 incorporating the present invention. A transmitter driver 
circuit 12 and a receiver circuit 14 are connected to a cable 16. These 
are provided to a switching circuit 18, which chooses a 10Base-T circuit 
19 for 10 megabits transmissions, and a 100Base-TX circuit 20 for 100 
megabits per second applications. The 100 megabits application also 
includes an MLT-3 decoder circuit 22 which incorporates the present 
invention. 
A management circuit 24 couples to the MAC layer through its transmitter 26 
and receiver 28. Management circuitry 30 through registers 32 controls 
N-WAY circuit 34, which selects the path through switch 18. 
FIG. 2 is a block diagram of one embodiment of a DC restoration circuit 
according to the present invention, which would be incorporated into MLT-3 
block 22 of FIG. 1, in one embodiment. Input data on a line 40 is provided 
to an equalizer circuit 42 which corrects for high frequency variations in 
transmission speed over the transmission medium. The output of the 
equalizer is a data value indicated by voltage v.sub.eq (t). This signal 
passes through a first resistor R to produce a signal v(t) which is 
provided as an input to a slicer circuit 44. Circuit 44 clamps the data 
signal to remove any high-frequency noise. 
The present invention provides a feedback loop through a baseline wander 
(peak) detector 46, a comparison circuit 48 and a voltage correction 
circuit 50. Circuit 50 is composed of a capacitor C and a buffer 52. A 
second resistor, R.sub.cor is used to sum the feedback signal with the 
input signal through resistor R. The use of a feedback loop eliminates 
dependencies on temperature and other IC process variations. 
Peak detector 46 detects the low frequency wander, preferably by detecting 
the peaks or envelope of the received signal and provides it as one input 
to comparison circuit 48. Circuit 48 is preferably an operational 
transconductance amplifier which compares the input 54 from peak detector 
46 to an input 56, which is preferably a reference voltage of 3.5 volts. 
The output is a current, i.sub.cor (t), which is used to charge capacitor 
C. Thus, the DC value of the received input signal is adjusted to 
compensate for baseline wander. 
FIG. 3 is a diagram illustrating baseline wander. An MLT-3 data signal 60 
is shown as having peaks, mid-levels and negative levels to indicate three 
different encoding possibilities. A dotted line 62 illustrates the 
variation of the baseline or mid-level value from the ideal, zero level. 
As this varies, the positive and negative peaks vary accordingly. Dotted 
line 64 shows the variation of the positive peaks, which would be detected 
by peak detector 46 of FIG. 2. The DC voltage added back to correct for 
the baseline wander is shown by a dotted line 66, which would be the 
voltage set by voltage correction circuit 50 of FIG. 2. 
In one embodiment, equalizer 42 adjusts the voltage level of the received 
data so that some DC level always needs to be added back, even when there 
is no baseline wander. This enables capacitor C to either be charged or 
discharged to correct in two directions for baseline wander. Alternately, 
two different circuits could be used according to the present invention, 
one for detecting positive peaks, and another for negative peaks, with a 
reversal of the connections shown in FIG. 2. 
The equations describing the operation of the feedback loop according to 
the present will now be described. 
##EQU1## 
It can be shown the ota (operational transconductance amplifier 48 in FIG. 
2) has a transfer function as follows: 
EQU i.sub.cor (t)=(-3.51 tanh (11.3 (v.sub.p (t)-3.5 V))) .mu.A (4) 
If we let 
##EQU2## 
after some algebra, we have 
##EQU3## 
So, in the steady state (when the peak of v(t)=3.5), we have 
EQU v(t)=(1-.alpha.) v.sub.eq (t) (7) 
To understand the operation of this feedback loop, imagine that v.sub.eq 
(t) varies slowly enough so that v.sub.p (t)=v(t). Then Eq. 6 becomes: 
##EQU4## 
The detailed circuit diagrams illustrating one embodiment of the circuit of 
FIG. 2 are shown in FIGS. 4-10. FIG. 4 illustrates the overall circuit, 
while FIG. 5 shows the two blocks which make up dcr.sub.-- z of FIG. 4. 
The dcr.sub.-- 1 block of FIG. 5 is shown in FIG. 6 in more detail, while 
the dcr.sub.-- 2 block is shown in FIG. 7. Indicated by dotted line box 
46, which extends across FIGS. 6 and 7, is the peak detector of FIG. 2. 
The operational transconductance amplifier 48 of FIG. 2 is also indicated 
in FIG. 7. FIGS. 8-10 include additional detail of blocks from FIGS. 6, 7 
and 8. 
In a preferred embodiment, v.sub.cor (t) of FIG. 2 is set to be 
approximately 1.5v when no correction is required. This level allows 
movement up or down to correct as needed. 
In a high speed application, data can be transferred at 125 MHz, with a 
fundamental frequency of one-quarter of that, 31.25 MHz. Four bits of 
information can be encoded at this rate. The encoding scheme ensures that 
no more than 60 bits will pass without a transition. Thus, for the slowest 
expected data, approximately 2 MHz would be the slowest rate at which 
transitions would occur. Thus, the DC restoration circuit should not 
follow transitions which are that fast, but should follow transitions up 
to 100 KHz, the rate at which DC wander is observed to occur. This rate is 
built into peak detector 46. 
As will be understood by those of skill in the art, the present invention 
can be embodied in other specific forms without departing from the spirit 
or essential characteristics thereof. For example, the DC restoration 
circuit could be referenced to a high level, rather than ground, or to a 
negative voltage level. In addition, the peak detector could detect peaks 
at a higher rate, but the comparison circuit could respond more slowly so 
that the feedback does not occur for the slow data rates. Accordingly, the 
above descriptions are intended to be illustrative of the invention, but 
not limiting, and reference should be made to the following claims for 
setting forth the scope of the invention.