Abstract:
A signal compensation circuit and associated method dynamically compensate for signal baseline wandering in a transmission line. The compensation circuit has a detection circuit and a correction circuit. The detection circuit first compares a transmission signal with a reference level and generates a comparison result. The correction circuit then corrects the transmission signal according to the comparison result. The compensation circuit can adjust its compensation over time based on the quality of the transmission signal.

Description:
BACKGROUND OF INVENTION 
   1. Field of the Invention 
   The present invention relates to a circuit for receiving a transmission signal and associated method, and more particularly, to a circuit for dynamically compensating a baseline wander of the transmission signal and associated method. 
   2. Description of the Prior Art 
   Please refer to  FIG. 1 , which is a schematic diagram of typical user terminals  10 A and  10 B connected to each other by a network transmission line  18 . The user terminals  10 A and  10 B can be network servers, routers, clients, or other kinds of network terminals. The transmission line  18  is a network transmission line, such as an Unshielded Twisted Pair Category 5 (UTP-Cat.5) Ethernet cable. The user terminal  10 A and the user terminal  10 B utilize a signal circuit  12 A and a signal circuit  12 B respectively to transmit signals via the transmission line  18 . The signal circuits  12 A,  12 B include transformers  16 A,  16 B and resistors R 0   a , R 0   b  respectively that match the impedance of the transmission line  18 . A transmitter  14 A of the signal circuit  12 A utilizes a differential transmission signal. That is, the transmission signal includes both positive and negative transmission signals transmitted by a pair of conductive lines. 
   The differential transmission signal is sent through two output terminals of the transmitter  14 A to nodes P 0 A, P 1 A, and is then transformed onto the transmission line  18  by the transformer  16 A. The differential transmission signal is received by the signal circuit  12 B of the user terminal  10 B after the differential transmission signal travels through two wires of the transmission line  18 . The differential transmission signal is transformed by the transformer  16 B and is sent to nodes P 0 B, P 1 B of the signal circuit  12 B. The differential transmission signal is then sent to two differential input terminals of a receiver  14 B, so that the user terminal  10 B receives the signal from the user terminal  10 A via the transmission line  18 . 
   Several problems can occur during the signal transmission process. For instance, each signal circuit must match the electrical characteristic of the transmission line  18  in order for signal transmission to be effective. However, a high pass characteristic of the transformer in the signal circuit undesirably affects the signal level of the transmission signal. 
   The Ethernet network system suffers from this high pass problem. In Ethernet, an MLT-3 coded transmission signal is sent from the transmitter, transformer, to the transmission line. The coded transmission signal has a fixed baseline that is a longtime average of a level-changeable digital signal. The electrical characteristic of this signal is that the baseline of the signal relates to a low frequency (LF) component of the signal, which relates to the transmission data. When the coded transmission signal passes through the transformer and onto the transmission line, the direct current (DC) component of the LF is filtered out due to the high pass characteristic of the transformer. After the transmission signal passes through the transmission line to the signal circuit of another user terminal, the original baseline of the signal is degraded and baseline wander occurs. 
   For the purposes of explaining the baseline wandering phenomenon, please refer to  FIG. 2  showing a prior art waveform-time diagram of the transmission signal during the signal transmission process in the network system shown in  FIG. 1 . The horizontal axis in  FIG. 2  is time, and the vertical axis is signal amplitude. In  FIG. 2 , because the transmission signal is a differential signal, a waveform of a positive transmission signal shown is representative. At the user terminal  10 A in  FIG. 1 , the transmission signal is generated as illustrated by a waveform  20 . The waveform  20  is composed of three different waveform levels: a high level, a zero level, and a low level representing the different digital signals according to the MLT-3 code. A dotted line  20 A and a dotted line  20 B perfectly represent the overall signal profile of the waveform  20 . 
   After the transmission signal passes through the transformer  16 A, the transmission line  18  and the transformer  16 B of the user terminal  10 B, the LF component is filtered out due to the high pass characteristic of the transformer. The transmission line affects the original baseline and baseline wander occurs. A waveform  22  of the transmission signal affected by baseline wander is received at the node P 0 B of the user terminal  10 B. The level of the waveform  22  has different deviations at different times, so that enveloping signal levels  22 A and  22 B of the waveform  22  appear to wander between a high level and low level. Therefore, the waveform  22  does not represent the original digital signal of the waveform  20  correctly because of the baseline wandering phenomenon. 
   Signal transmission errors caused by the baseline wandering phenomenon will now be briefly explained with reference to  FIG. 1  and  FIG. 2 . A signal with a level higher than a fixed high reference level  24 A is determined as a high level digital signal. A signal with a level lower than a fixed low reference level  24 B is determined as a low level digital signal. The waveform  20  is originally generated by the transmitter  14 A with respect to the high reference level  24 A and low reference level  24 B. The waveform  20  is then affected by the baseline wandering phenomenon during its transmission as previously described, and is finally received at the receiver  14 B. The receiver  14 B then interprets the waveform  22  with reference to the high reference level  24 A and low reference level  24 B and generates a received signal waveform  24 . 
   The waveform  24  contains errors in time periods T 1  and T 2 . In time period T 1  the waveform  22  wanders low enough so that the original high signal in period T 1  falls below the high reference level  24 A. The original high signal during period T 1  is interpreted as zero level as shown by waveform  24 , incorrectly. A plurality of low level pulses error in a similar way during time period T 2 . The baseline wandering phenomenon thus causes the digital signal of the network transmission to not be received and decoded correctly. 
   In the prior art signal circuit, a fixed DC bias is added to the received transmission signal at the receiving terminal to compensate for baseline wandering. This solution is not adequate as the deviation of the baseline changes over time, and a fixed compensation cannot eliminate the signal deviation phenomenon entirely. 
   SUMMARY OF INVENTION 
   It is therefore a primary objective of the claimed invention to provide a signal compensation circuit and associated method for compensating signals by adjusting signal levels dynamically to solve the above-mentioned problem, and to ensure the quality of network signal transmissions. 
   According to the claimed invention, the signal compensation circuit comprises a detection circuit for detecting an enveloping signal level of a transmission signal transmitted via a transmission line, and a correction circuit for dynamically compensating the transmission signal according to a comparison result generated by comparing the transmission signal with a reference level. 
   It is an advantage that the claimed invention uses a dynamic detection and compensation method to adjust the signal levels so as to correct the deviations of the signal levels caused by the baseline wandering phenomenon. This ensures that the receiver receives the digital signal correctly, thus improving the signal receiving quality of network communication. 
   These and other objectives of the claimed invention will no doubt become obvious to those of ordinary skill in the art after reading the following detailed description of the preferred embodiment that is illustrated in the various figures and drawings. 

   
     BRIEF DESCRIPTION OF DRAWINGS 
       FIG. 1  is a schematic diagram of a prior art computer network. 
       FIG. 2  shows waveforms at different nodes in the computer network shown in  FIG. 1 . 
       FIG. 3  is a schematic diagram of a signal circuit according to the present invention. 
       FIG. 4  shows waveforms by the signal circuit shown in  FIG. 3 . 
   

   DETAILED DESCRIPTION 
   Please refer to  FIG. 3 , which is a schematic diagram of a signal circuit  30  used in a network system according to the present invention. A user terminal  10 A and a user terminal  50  are connected with each other via a network transmission line  18 . The user terminals  10 A and  50  can be network servers, routers, clients, or other network terminals. The transmission line  18  can be an Ethernet Unshielded Twisted Pair Category 5 (UTP-Cat.5) cable, and a transmission signal can be an MLT-3 coded, or 100Base-T coded signal. The user terminal  10 A and the user terminal  50  use a signal circuit  12 A and a signal circuit  30  respectively to process transmission signals. The signal circuit  12 A includes a transformer  16 A and a resistor R 0   a  that is matched to the impedance of the transformer  16 A. The signal circuit  30  includes a transformer  32  and a resistor R 0  that is matched to the impedance of the transformer  32 . A transmitter  14 A of the signal circuit  12 A generates a transmission signal. A receiver  36  of the signal circuit  30  receives the transmission signal via the transmission line  18 . 
   A detection circuit  38  and a correction circuit  40  are provided in the signal circuit  30  for the purpose of compensating baseline wander of the transmission signal to be received by the receiver  36 . The detection circuit  38  detects a signal level of the transmission signal at a node N 3  and a node N 4 , and compares the signal level with a reference level. If the signal level is larger than the reference level, a corresponding first control signal  38 A and a corresponding second control signal  38 B are generated by the detection circuit  38 . The correction circuit  40  compensates for baseline wander of the transmission signal according to the results generated by comparing the transmission signal with the reference level by the detection circuit  38 . After the differential transmission signal passes through the transformer  32  of the signal circuit  30 , the positive and negative transmission signals are sent to a node N 1  and a node N 2  respectively. A second resistor R 2  and a current source  34 A are disposed on a signal transmitting path from the node N 1  to the node N 3  in the correction circuit  40 . A second resistor R 2  and a current source  34 B are disposed on another signal transmitting path from the node N 2  to the node N 4  in the correction circuit  40 . On the two signal paths for transmitting the differential signal, the node N 3  and the node N 4  are connected to a common-mode power supply by two first resistors R 1 . The common-mode power supply is used for providing the differential signal with a common-mode voltage V CM  at a node N 5 . The common-mode voltage V CM  for an MLT-3 coded, or 100 Base-T coded, signal is 1.8 volts. 
   The current sources  34 A and  34 B of the correction circuit  40  include controllable current sources  46 A,  48 A and  46 B,  48 B respectively. Bias circuits  41 A,  42 A and  41 B,  42 B provide appropriate bias voltages to the current sources  34 A and  34 B so that the current sources  34 A and  34 B can operate normally. The current intensities of the controllable current sources  48 A and  48 B are controlled by the first control signal  38 A generated by the detection circuit  38 . The current intensities of the controllable current sources  46 A and  46 B are controlled by the second control signal  38 B also generated by the detection circuit  38 . The controllable current sources  46 A and  48 A of the current source  34 A together provide a compensation current Ic 1  at the node N 3 . The controllable current sources  46 B and  48 B of the current source  34 B together provide a compensation current Ic 2  at the node N 4 . A compensation voltage Vc 1  is generated across the resistor R 1 , between nodes N 3  and N 5 , by the compensation current Ic 1 . Likewise, a compensation voltage Vc 2  is generated across the resistor R 1 , between nodes N 4  and NS, by the compensation current Ic 2 . After the differential transmission signal passes through the node N 1  and the node N 2 , the positive and negative transmission signals pass through the resistors R 2  to the node N 3  and the node N 4 , respectively. The positive and negative transmission signals refer to the common-mode voltage V CM  as a center voltage level and utilize the resistors R 2  as a loading. The compensation voltages Vc 1  and Vc 2  are added to the positive and negative transmission signals, respectively. In the preferred embodiment of the present invention, the intensities of the compensation currents Ic 1  and Ic 2  are substantially equal. The two first resistors R 1 , having the same resistance, are provided so that the compensation voltages Vc 1  and Vc 2  are also substantially equal. After compensation, the transmission signal is received by the receiver  36  at the node N 3  and the node N 4  in a differential manner so that the transmission signal is transferred from the user terminal  10 A to the user terminal  50 . 
   The operation for correcting baseline wander according to this invention is described as follows. Please refer to  FIG. 4 , which is a waveform of related signals during baseline wander correction of the transmission signal. The horizontal axis in  FIG. 4  represents time, and the vertical axis represents signal magnitude. A waveform  54  shown in  FIG. 4  is the waveform of the positive component of the differential transmission signal, at the node N 3 . An enveloping signal level  56  represents the positive signal profile of the waveform  54 . As mentioned previously, the enveloping signal level  56  of the waveform  54  deviates with time due to baseline wander, so that the enveloping signal level  56  of the waveform  54  of the transmission signal does not form an ideal straight horizontal line. The detection circuit  38  in the present invention detects a difference between the enveloping signal level  56  and a fixed reference level  52  at a time t1. As illustrated in  FIG. 4 , the enveloping signal level  56  is larger than the reference level  52 . Accordingly, the detection circuit  38  generates the first control signal  38 A and the second control signal  38 B to control the current source  34 A and the current source  34 B respectively to produce the negative compensation current Ic 1  and the negative compensation current Ic 2  (the flowing directions of the compensation currents Ic 1  and Ic 2  can be seen in  FIG. 3).To  accomplish this, the second control signal  38 B turns off the controllable current sources  46 A and  46 B, and the first control signal  38 A turns on the controllable current sources  48 A and  48 B. The negative compensation currents Ic 1  and Ic 2  cause the compensation voltages Vc 1  and Vc 2  to be negative as well (the polarization directions of the compensation voltages Vc 1  and Vc 2  are shown in  FIG. 3 ). The resulting negative compensation voltage Vc 1  is added to the positive transmission signal at the node N 3 , and the enveloping signal level  56 , which is larger than the reference level  52 , is reduced and thus corrected. 
   After the positive transmission signal at the node N 3  has been corrected, the detection circuit  38  detects the enveloping signal level  56  and the reference level  52  every predetermined period. At a time t2, the detection circuit  38  detects that the enveloping signal level  56  has been corrected but is still larger than the reference level  52 . The detection circuit  38  generates the first control signal  38 A and second control signal  38 B to control the current source  34 A and the current source  34 B to further negatively compensate the enveloping signal level  56  via the compensation voltage Vc 1 . Finally, after this correction procedure is repeated several times, shown as times t2 through t6 in  FIG. 4 , the enveloping signal level  56  will be eventually corrected. 
   The signal level may also wander lower than a predetermined level. Suppose that the detection circuit  38  detects an enveloping signal level  56  that is lower than the reference level  52 . The detection circuit  38  controls the current sources  34 A and  34 B to generate a positive compensation current Ic 1  resulting in a positive compensation voltage Vc 1 . The positive compensation voltage Vc 1  then adjusts the enveloping signal level  56  positively. 
   Each control current source can be implemented by a plurality of unit current sources, with each unit current source providing a fixed current. For example, one or more unit current sources can be activated in the controllable current source  46 A to increase the positive compensation current Ic 1 . Similarly, one or more unit current sources can be activated in the controllable current source  48 A to generate the negative compensation current Ic 1 . 
   In summary, the above-mentioned process of comparing the enveloping signal level  56  with the reference level  52  using the detection circuit  38 , and then generating the compensation current from the current source, and finally compensating the transmission signal will be repeated to dynamically compensate the signal level deviations due to baseline wander. Although the above description uses the positive transmission signal at the node N 3  as an example, the negative transmission signal at the node N 4  is compensated for at the same time. The node N 3  and the node N 4  are symmetrically provided with respect to the common-mode power supply of the node N 5 . Consequently, the controllable current sources  48 A and  48 B controlled by the first control signal  38 A and the controllable current sources  46 A and  46 B controlled by the second control signal  36 A cause the compensation current Ic 1  to be substantially equal to the compensation current Ic 2  and the compensation voltages Vc 1  and Vc 2  to be substantially thus equal. The polarizations of the compensation voltages correspond with the positive and negative signals of the differential transmission signal to respectively compensate the positive and negative signals. 
   The symmetric operation of the correction circuit  40  will now be explained. A waveform  54   n  shown in  FIG. 4  is a waveform of the negative transmission signal of the differential transmission signal at the node N 4 . A negative signal envelope  56   n  represents the extent of the waveform  54   n . A horizontal signal level  60  represents a stable DC level provided by the common-mode voltage V CM  at the node N 5 . The waveform  54   n  of the negative transmission signal is thus the negative mirror image of the waveform  54  of the positive transmission signal, with reference to the common-mode voltage V CM  acting as a voltage reference. If the enveloping signal level  56   n  of the negative transmission signal deviates from a reference level  52   n  because of the baseline wandering phenomenon, the detection circuit  38  shown in  FIG. 3  detects the difference between the enveloping signal level  56   n  and the fixed reference level  52   n . In fact, the nature of the differential signal prescribes mirrored deviations of the positive and negative transmission signals. From the circuit diagram shown in  FIG. 3  of the present invention, the voltage at the node N 3  is equal to V CM +Ic 1 *R 1 , and the voltage at the node N 4  is equal to V CM −Ic 2 *R 1 . Due to the symmetrical nature of the positive and negative differential signals and the construction of the correction circuit  40 , the compensation current Ic 1  is equal to the compensation current Ic 2 . The mirror image property of the correction circuit  40  compensates the positive and negative transmission signals equally in magnitude but opposite in direction. As shown in  FIG. 4 , the waveform  54  of the positive transmission signal at time t2 is corrected downward as the waveform  54   n  of the negative transmission signal is corrected upward. Therefore, the positive and negative components of the differential transmission signal are corrected simultaneously. 
   Compared with the prior art, which uses a fixed DC bias voltage, the present invention dynamically detects and compensates the deviations of the signal level caused by baseline wandering. The present invention thus ensures that the receiver receives the digital signal correctly, and significantly improves the receiving quality of network communication. 
   Those skilled in the art will readily observe that numerous modifications and alterations may be made while retaining the teachings of the invention. Accordingly, the above disclosure should be construed as limited only by the metes and bounds of the appended claims.