Miscellaneous active electrical nonlinear devices – circuits – and – Signal converting – shaping – or generating – Amplitude control
Reexamination Certificate
1999-11-16
2001-08-14
Lam, Tuan T. (Department: 2816)
Miscellaneous active electrical nonlinear devices, circuits, and
Signal converting, shaping, or generating
Amplitude control
C327S307000
Reexamination Certificate
active
06275087
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention is related to the field of communication systems and more particularly to a method and circuit for mitigating static and/or dynamic DC offset or baseline wander at the transmitter, receiver, and/or in the channel.
2. History of the Related Art
Baseline wander is a well-known phenomenon observed in a wide variety of applications. Such applications may include, but are not limited to, any system that contains low frequency, random data signals, or uses transformer coupling, AC coupling, DC notching, and all systems suffering from DC drift and baseline wander at the transmitter, receiver, or in the communication channel itself. Applications that may experience such baseline wander may include wire line modem systems, digital subscriber line (DSI) and cable communication systems, Ethernet, radar, and television receivers. Typically, the principle methods of trying to compensate or eliminate baseline drift in a communication system include methods in which the received signal is observed over time to obtain an estimate of the DC offset or the DC droop by integrating the signal over time. This approach typically requires long term signal averaging that slows the response time and makes this type of approach difficult to implement in a system where the baseline wander changes rapidly. Only if the DC droop is significantly larger than the signal and is slowly changing over time would such an approach be appropriate. Otherwise, the droop estimate will be masked by the running sum of the inherently locally imbalanced signal samples. In other words, by integrating the signal over time, instantaneous signal information is lost and the sensitivity of the system to address rapidly changing baseline wander is limited. In another approach, an inverse filter is designed and used to directly cancel the effect of the channel and/or receiver component that causes the DC error. Unfortunately, this method requires knowledge of the drift source, its characteristics, and its invertability. Unfortunately, this information is not always readily available. For example, AC coupling transformers that are used in twisted pair applications typically have a zero at s
z
=0 and hence their inversion requires a poll at s
p
=0, which results in an unstable system. A third approach to addressing DC wander involves replicating the DC wander at the receiver by passing the detected data symbols through a filter whose frequency response is similar to that of the channel/receiver causing the DC error. This approach requires accurate knowledge of the drift causing channel and/or receiver component as well as reliable detection of transmitted symbols. Therefore, it would be highly desirable to implement a communication system incorporating the ability to cancel time varying DC offset that overcomes the described limitations of conventional approaches.
SUMMARY OF THE INVENTION
The problems identified above are in large part addressed by a DC drift canceller circuit with an input for receiving an input signal. The circuit includes a decision device including an input and an output and a first adder configured to receive the input and the output of the decision device. The first adder produces an error signal indicative of the difference between the input and output of the decision device. A noise filter of the circuit is configured to receive the error signal. A second adder of the circuit includes a first input coupled to the input of the canceller circuit and a second input coupled to the output of the noise filter such that the output of the second adder is substantially free of any DC drift component of the input signal. The output of the second adder is coupled to the input of the decision device. In one embodiment, the decision device comprises a slicer. In one embodiment, the noise filter comprises a low pass filter. The low pass filter may includes a first multiplier with an input configured to receive the error signal, a third adder circuit with a first input connected to the output of the first multiplier and a second input connected to the output of a delay circuit, and a second multiplier with an input connected to the output of the third adder and an output connected to the delay circuit. In another embodiment, the noise filter may comprise an integrator that includes a multiplier with an input configured to receive the error signal and a third adder circuit with a first input connected to the output of the multiplier and a second input connected to a delay circuit. The input of the delay circuit is connected to the output of the adder circuit and the output of the noise filter. The circuit may further include an analog to digital converter coupled between the input of the canceller circuit and the second adder or a digital to analog converter with an input connected to the output of the noise filter. In this embodiment, an output of the digital to analog converter is compared with the received signal at an input of the analog to digital converter.
The invention further contemplates a method of canceling DC drift in a received signal by generating a quantized signal responsive to the received signal. An error signal indicative of the difference between the received signal and the quantized signal is then generated. Noise from the error signal is then filtered to produce a filtered error signal that is used to modify the received signal. In one embodiment, the noise is filtered by integrating the error signal. Filtering noise from the error signal comprises filtering the error signal with a low pass filter. The method may further include converting the received signal from an analog signal to a digital signal prior to generating the quantized signal and converting the filtered error signal from a digital signal to an analog signal prior to using the filtered error signal to modify the received signal.
REFERENCES:
patent: 4360929 (1982-11-01), Isobe
patent: 4607236 (1986-08-01), LeQueau
patent: 6038262 (2000-03-01), Mestdagh
Lam Tuan T.
LSI Logic Corporation
Nguyen Hiep
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