Adaptive equalization and baseline wander correction circuit

The present relates to an adaptive equalization and baseline wander correction circuit using halfwave rectifiers and peak detectors with capacitors therein to store DC signals used in the baseline wander correction portion of the circuit and in the adaptive filter portion of the circuit. This sharing of capacitors minimizes the total number of circuit components required.

Adaptive equalization and baseline wander correction circuits are well known in the art. Referring to FIG. 1 there is shown generally an adaptive equalization and baseline wander correction circuit 8 of the prior art. The receiver circuit 8 receives an input signal which is supplied to a first summing circuit 18 a , which is one of three summing circuits 18 ( a-c ) shown in FIG. 1 . The first summing circuit 18 a also receives a signal from the output of a baseline wander correction circuit 20 or DC offset extractor circuit 20 , to be discussed later. The output of the first summing circuit 18 a is supplied to an adaptive filter 10 . The adaptive filter 10 filters the output of the first summing circuit 18 a based upon a control signal supplied thereto which is the output of a second summing circuit 18 b . The output of the adaptive filter 10 is a signal which is supplied to a third summing circuit 18 c . The third summing circuit 18 c also receives as an input thereof, the output of the baseline wander correction circuit 20 . The output of the third summing circuit 18 c is supplied as an input to the baseline wander correction circuit 20 and also as an input to a fullwave rectifier 12 .

The output of the fullwave rectifier 12 is supplied to a peak detector 14 and to a slicer 16 . The peak detector 14 , detects the peak of the rectified signal supplied from the fullwave rectifier 12 , and supplies the peak signal to the slicer circuit 16 . The output of the peak detector circuit 14 is also supplied to the second summing circuit 18 b . The second summing circuit 18 b also receives as an input thereof a reference signal. The output of the second summing circuit 18 b is supplied as the control signal to the adaptive filter 10 .

From the slicer circuit 16 , the output is supplied as another input to the baseline wander correction circuit 20 . As previously discussed, the baseline wander correction circuit or the DC offset extractor circuit 20 also receives as its input thereof the output of the third summing circuit 18 c . The output of the baseline wander correction circuit 20 is supplied as an input to the first and/or third summing circuit 18 a and/or 18 c , respectively. The output of the slicer circuit 16 is the output of the receiver circuit 8 of FIG. 1 .

Referring to FIGS. 2, 3 , and 4 , there is shown various embodiments of the portions of the receiver circuit 8 shown in FIG. 1 . In particular, FIG. 2 shows the circuit diagram of the peak detector circuit 14 . FIG. 3 is a circuit diagram of another embodiment of the peak detector 14 . FIG. 4 is an embodiment of a baseline wander correction circuit 20 .

Although baseline wander correction and adaptive filter circuits are well known in the art, the prior art circuits required many large capacitors and/or large number of digital counters to store extracted DC values for controlling the adaptive filter feedback loop and for correcting the baseline wander DC offset. Thus, the die size has been relatively large.

In the present invention, an improved adaptive equalization and baseline wander correction circuit comprises an adaptive filter means for receiving an input signal and filters the input signal to generate a first signal in response to a control signal. A first halfwave rectifier means receives the first signal and generates a first rectified signal in response thereto. A first peak detector means receives the first rectified signal and generates a first peak signal in response thereto. A signal level extractor means receives the first peak signal and generates a first level signal in response thereto. A low pass filter means receives the first level signal and generates a filtered signal in response thereto. A first summing amplifier means receives the filtered signal and a reference signal and generates a control signal in response thereto. The control signal is the difference of the filtered signal and the reference signal. A second summing amplifier means receives the first signal and a feedback signal and generates a second signal in response thereto. The second signal is the difference of the first signal and the feedback signal. A second halfwave rectifier means receives the second signal and generates a second rectified signal in response thereto. A limiter means receives the first and second rectified signals and generates a third signal in response to the reference signal. A second peak detector means receives the third signal and generates a second peak signal in response thereto. A signal extractor means receives the first and second peak signals and generates a feedback signal. The feedback signal is a DC offset signal of the first and second peak signals. Finally, a slicer means receives the second rectified signal and the filtered signal and generates an output signal in response thereto.

FIG. 1 is a schematic block level diagram of an adaptive equalization and baseline wander correction circuit of the prior art.

FIG. 2 is a first embodiment of a peak detector circuit used in the prior art circuit shown in FIG. 1 .

FIG. 3 is a second embodiment of a peak detector circuit used in the prior art circuit shown in FIG. 1 .

FIG. 4 is a circuit diagram of a baseline wander correction circuit used in the receiver circuit of the prior art shown in FIG. 1 .

FIG. 5 is a schematic block level diagram of an improved adaptive equalization and baseline wander correction circuit of the present invention.

FIG. 6 is a circuit diagram of the slicer portion of the improved adaptive equalization and baseline wander correction circuit of the present invention shown in FIG. 5 .

FIG. 7 is a circuit diagram of a DC offset extractor portion of the correction circuit of the present invention shown in FIG. 5 .

FIG. 8 is a circuit diagram of the DC signal level extractor portion of the wander correction circuit of the present invention shown in FIG. 5 .

FIG. 9 is a circuit diagram of a peak detector, one of a plurality used in the improved adaptive filter and baseline wander correction circuit of the present invention shown in FIG. 5 .

FIG. 10 is a circuit diagram of a limiter portion of the improved adaptive filter and baseline wander correction circuit of the present invention shown in FIG. 5 .

FIG. 11 is a circuit diagram of a halfwave rectifier, one of a plurality used in the improved adaptive equalization filter and baseline wander correction circuit of the present invention shown in FIG. 5 .

FIG. 12 is a circuit diagram of a summing amplifier circuit of a portion of the improved baseline wander correction circuit of the present invention shown in FIG. 5 .

Referring to FIG. 5, there is shown a schematic block level diagram of an improved adaptive filter and baseline wander correction circuit 50 of the present invention. The receiver circuit 50 comprises an adaptive filter 10 for receiving the input signal. The adaptive filter 10 can be the filter of the prior art. The adaptive filter 10 receives as its input thereof an input signal and based upon a control signal supplied from a first summing amplifier 18 d , generates an output signal.

The output of the adaptive filter circuit 10 is supplied to a first halfwave rectifier 52 a and a second halfwave rectifier 52 b , simultaneously. Each of the first and second halfwave rectifiers 52 a and 52 b is identical and is shown in FIG. 11 . However, the first halfwave rectifier 52 a receives the positive polarity component of the signal from the adaptive filter 10 , whereas the second halfwave rectifier 52 b receives the negative polarity component of the signal from the adaptive filter 10 . The first halfwave rectifier 52 a generates a first rectified signal which is supplied to a limiter circuit 54 and to a first peak detector 56 a . The second rectified signal, which is the output of the second halfwave rectifier 52 b , also is supplied to the limiter circuit 54 and to a second peak detector 56 b . The first peak detector 56 a and the second peak detector 56 b are identical and are shown in greater detail in FIG. 9 .

Each of the first and second peak detectors 56 a and 56 b generates a first and a second peak signal respectively. The first and second peak signals are supplied to a DC offset extractor circuit 58 , which is shown in greater detail in FIG. 7 . In addition, each of the first and second peak signals are supplied to a DC signal level extractor circuit 60 , shown in greater detail in FIG. 8 . The DC signal level extractor circuit 60 generates a first level signal in response thereto. The first level signal is supplied to a low pass filter circuit 62 , which is of conventional design. The low pass filter circuit 62 generates a filtered signal which is supplied to the first summing amplifier 18 d and to a slicer circuit 64 , which is shown in greater detail in FIG. 6 .

The first summing amplifier 18 d also receives as its input thereof a reference signal and together with the filtered signal generates a control signal which is supplied to the adaptive filter circuit 10 .

The foregoing circuit elements describes the adaptive filter loop portion of the receiver circuit 50 of the present invention.

The output of the adaptive filter circuit 10 is also supplied to a second summing circuit 18 e . The second summing circuit 18 e receives as its input thereof the output of the DC offset extractor circuit 58 . In response thereto, the second summing amplifier 18 e generates a second signal, which is supplied to a third halfwave rectifier 52 c and to a fourth halfwave rectifier 52 d , simultaneously. The third and fourth halfwave rectifier circuits 52 c and 52 d are similar to the first and second halfwave rectifier circuit 52 a and 52 b shown in FIG. 11 and described previously. The only difference is that the third halfwave rectifier 52 c receives as its input the negative polarity component of the second signal from the second summing amplifier circuit 18 e . The fourth halfwave rectifier circuit 52 d receives as its input a positive polarity component of the second signal from the second summing amplifier circuit 18 e . The output of the third halfwave rectifier 52 c is a third rectified signal which is supplied to the limiter circuit 54 . The output of the fourth halfwave rectifier 52 d is the fourth rectified signal which is also supplied to the limiting circuit 54 .

The limiting circuit 54 receives as its input the outputs of the first, second, third, and fourth halfwave rectifiers 52 ( a-d ) and the reference signal and based upon the reference signal generates a first and a second limited signal respectively. The first limited signal is supplied to a third peak detector 56 c . The second limited signal is supplied to a fourth peak detective signal 56 d . Each of the third and fourth peak detector circuits 56 c and 56 d is of the type shown and described in FIG. 9 .

The outputs of the third and fourth peak detectors 56 c and 56 d are also supplied to the DC offset extractor circuit 58 . The DC offset extractor circuit 58 receives as its input the first, second, third, and fourth peak signals 56 ( a-d ) and generates a DC offset signal, in response thereto. The DC offset signal is supplied as another input to the second summing amplifier circuit 18 e.

Finally, the output of the third and fourth halfwave rectifiers 56 c and 56 d are supplied to the slicer circuit 64 along with the filtered signal which is the output of the low pass filter 62 . The output of the slicer circuit 64 is the output of the receiver circuit 50 of the present invention.

The foregoing description of the circuit elements describes the baseline wander correction loop circuit portion of the receiver circuit of the present invention.

When an input signal is received by the receiver circuit 50 , the adaptive filter 10 aligns the phases and reshapes the magnitudes of the attenuated and distorted received signals. The output of the adaptive filter 10 is supplied to the first and second halfwave rectifiers 52 a and 52 b . Each of the halfwave rectifiers 52 a and 52 b clips off the receiving MLT-3 signals into two parts: a positive polarity component and a negative polarity component with each component signal (positive or negative) transformed into NRZ format. The outputs of the first and second halfwave rectifiers 52 a and 52 b are supplied to the first and second peak detectors 56 a and 5 G b . Each of the peak detectors 56 a and 5 G b samples and holds the DC values for the peaks of the incoming signals. Each peak detector 56 has a capacitor 70 to hold the DC value signal and to reduce unwanted ripples generated during the extraction process. Thus, for example, if the output signal of the adaptive filter 10 comprises a signal having a positive polarity component A and a negative polarity component B, the output of the first halfwave rectifier 52 a is the signal A and the output of the second halfwave rectifier 52 b is the signal B.

The peak signals A and B are supplied to the signal level extractor 60 which generates as its output thereof the signal (A+B)/2, or an average of the peak signals A and B. The capacitor 70 in each peak detector 56 holds the value of the signals received in the adaptive filter loop portion for use in the baseline wander loop portion of the receiver circuit 50 .

The DC level extractor 60 determines the average peak signal. In the case of incoming signals with baseline wander effect and/or system DC offset, the absolute AC magnitude level of the incoming signals is still the same but the relative peak levels to a reference are shifted. The low pass filter 62 removes the ripples riding on the DC signals extracted from the peak detectors 56 a and 56 b and averaged by the DC signal level extractor 60 . It addresses the stability of the adapting feedback loop.

The baseline wander correction portion of the circuit 50 operates as follows. The signal received by the adaptive filter 10 is passed through the second summing amplifier 18 e , which is supplied to a third and a fourth halfwave rectifiers 52 c and 52 d , respectively. The third and fourth halfwave rectifiers 52 c and 52 d are identical in function to the first and second halfwave rectifiers 52 a and 52 b . They each clip off the corrected MLT-3 signals, which are derived from the DC offset signals from 58 and the incoming MLT-3 signals from 10 , into two parts: a positive polarity component and a negative polarity component with each transformed into NRZ format. Thus, if the incoming signal has a positive polarity component A and a negative polarity component B, the third halfwave rectifier 52 c would generate the signal B. The fourth halfwave rectifier 52 d would generate the signal A.

The limiter circuit 54 receives the output of the first, second, third and fourth halfwave rectifiers 52 ( a-d ). In response to the reference signal, the limiter circuit 54 generates a first and a second limited signals. The limiter circuit 54 removes the variations and the amplitude of the incoming baseline wander signal and/or the DC offset signal by limiting each to a fixed level. Thus, the limiter circuit 54 receives the second rectified signal from the second halfwave rectifier 52 b and the third rectified signal from the third halfwave rectifier 52 c and ORs them together. The logical function to OR the signals from the second and third halfwave rectifiers 52 b and 52 c produces a rectified signal which has a larger in magnitude of the signals from the two halfwave rectifier circuits 52 b and 52 c , which rectify the negative polarity component of the incoming signal. Similarly, the output of the first and fourth halfwave rectifiers 52 a and 52 d are ORed together by the limiter circuit 54 . The logical function of ORing the output of the first and fourth rectified signals, outputs of the first and fourth halfwave rectifiers 52 a and 52 d , results in the selection of the rectified signal which is the larger in magnitude of the positive polarity component of the input signal. These are then the first and second limited signals which are supplied to the third and fourth peak detectors 56 c and 56 d respectively. Thus, if the input signal has a positive polarity component of A and a negative polarity component of B, the input to the peak detectors 56 c and 56 d will be {overscore (B)} and A respectively whereas {overscore (B)} and {overscore (A)} are the limited signals.

The result of the third and fourth peak detectors 56 c and 56 d , which are the third and fourth peak signals respectively, are supplied to the DC offset extractor circuit 58 .

As previously discussed, each of the peak detectors 56 ( a-d ) are identical and comprise a capacitor 70 to store the DC values of the peak of the incoming signals. The outputs of the first, second, third, and fourth peak detector circuits 56 ( a-d ) are supplied to the DC offset extractor circuit 58 . The DC offset extractor circuit 58 receives the output values of the first and second peak detectors 56 a and 56 b and performs a difference therebetween. In addition, it receives the output of the third and fourth peak detectors 56 c and 56 d and performs a difference therebetween. Finally, the two signals are then summed. Thus, if the output of the first, second, third and fourth peak detectors 56 ( a-d ) are A, B, {overscore (B)}, {overscore (A)}, respectively, then the result of the operation by the DC offset extractor circuit is the signal (A−B)+({overscore (B)}−{overscore (A)}). This is the same as (A−{overscore (A)})−(B−{overscore (B)}). The output of the DC offset extractor circuit 58 is then supplied as an input to the second summing amplifier 18 e as another input thereto, thereby forming the baseline wander feedback loop.

Finally, the output of the third and fourth halfwave rectifiers 52 c and 52 d , along with the output of low pass filter 62 , are supplied to a slicer circuit 64 . The slicer circuit 64 converts and slices the corrected MLT-3 signals into the digital NRZ output signals, which is the output of the receiver circuit 50 .

From the foregoing, it can be seen that the receiver circuit 50 of the present invention accomplishes baseline wander correction as well as adaptive equalization through the use of dual halfwave rectifiers in each of the feedback loops, instead of a single fullwave rectifier to extract the signal information. In addition, the capacitor 70 in each of the peak detectors 56 ( a-d ) serves to store the DC signal level of the incoming signal thereby permitting the signal to be shared between the adaptive filter block and the baseline wander correction block. This reduces the die size and the driving circuits. Finally, by separating the baseline wander DC offset value from the adaptive filter loop, this prevents the adaptive filter from falsely functioning out of the adaptive filter's DC controlled value. In addition, by implementing an OR slicer circuit, instead of another feedback path, this fixes the threshold reference within incoming signal swing.