Pulse or digital communications – Equalizers
Reexamination Certificate
1998-10-20
2001-07-03
Chin, Stephen (Department: 2734)
Pulse or digital communications
Equalizers
C708S323000
Reexamination Certificate
active
06256342
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Technical Field
The invention is based on a method and a regenerative filter for equalizing digitally transmitted signals in accordance with the independent claims.
2. Discussion of the Related Art
In addition to attenuation, the signal dispersion of optical signals is the main limiting criterion that influences transmission paths and bit rates in fiber optic systems. The effects of dispersion and their limitations can be compensated for by means of suitable signal processing of the electrical signals obtained. In practical application, it is necessary to embody the signal processing adaptively since the dispersion effects of the fiber change over time. As a result of the dispersion effects, for example caused by polarization mode dispersion (PMD), there are overlaps of signal components with different polarizations. The signals are chronologically blurred due to these dispersion effects and arrive in the optical receiver in a jumbled state. Nonlinear electronic filters are used for equalizing the signals in order to once more separate out the signals that arrive at the receiver overlapping one another due to dispersion effects. The published article “Adaptive Nonlinear Cancellation for High-speed Fiber Optic Systems” by Jack Winters and S. Kasturia,
Journal of Lightwave Technology
, Vol. 10, No. 9, July 1992, p. 971 ff. has disclosed a nonlinear electronic filter. In order to reduce the time problems with the analog regeneration in the nonlinear filter, two threshold decision elements with different thresholds are connected to each other in parallel. The results of the parallel connected threshold decision elements are combined by means of a controllable multiplexer. The embodiment represented in FIG. 7 on page 975 of the above-cited article uses two threshold decision elements, whose outputs are connected to a multiplexer. A D flip-flop and a feedback loop switch the multiplexer of the filter. Peripheral electronics determine the thresholds to be adjusted and store them by way of capacitors. The time constants of the threshold electronics are consequently fixed. With a nonlinear filter of this kind, signals can be equalized when the delays between the slow and rapid signal components vary within a time pulse.
In order to recover the signal clock pulse with which the threshold decision element is triggered, conventional clock circuits with phase-locked loops, so-called PLL circuits (phase-locked loop), can be used. However, with very intense distortions, which arise for example with a high PMD, the following problem occurs: the signal clock pulse, which is regenerated with conventional clock circuits, has an intense phase fluctuation whose magnitude is a function of the signal distortion. Therefore, with intense signal distortions, usually the clock circuit must be enlarged further by additional phase shifters, which are incorporated as adaptive regulators into the clock pulse path in order to compensate for phase fluctuations.
SUMMARY OF INVENTION
In contrast, the method according to the invention, as well as the regenerative filter according to the invention, has the advantage that echo delays between slow and fast signal components over a number of time pulses can be equalized. Furthermore, distortions from different sources, such as PMD (polarization mode dispersion) and chromatic dispersion, which occur at the same time, can likewise be reduced. To that end, at least four threshold decision elements are advantageously connected in parallel, wherein the outputs are united by way of at least one multiplexer and the switching of the multiplexers is carried out by way of at least one feedback loop. It is furthermore possible to adapt the thresholds to the changes of the dispersion behavior of the transmission fiber by way of an external unit, without the threshold adjustment having to follow a time-critical regeneration.
The object of recovering the signal clock pulse is attained by means of an equalizing circuit, which is equipped with a clock circuit according to the invention, which, by means of comparing the input signal to a predeterminable threshold, detects the rising or falling edge of the input signal and which regenerates the signal clock pulse by means of synchronization with this edge.
Advantageous improvements and updates of the method and the regenerative filter disclosed below are possible by means of the measures shown.
It is particularly advantageous that the regeneration of the output signals of the delay circuit is carried out with the data clock pulse c while the adjustment of the decision elements by way of a processor unit can occur more slowly.
It is furthermore advantageous that the slower adjustment of the thresholds takes place based on measurement values which are determined by an external detection of the output signal and are transmitted to a processor unit for evaluation.
It is furthermore advantageous that the delay unit is comprised of a master-slave D flip-flop and a subsequent latch flip-flop. As a result, a reliable detection of the signal state is achieved in the first feedback loop, while a rapid regeneration can take place in the second feedback loop.
The delay unit can be advantageously realized as a shift register with a logic unit.
A regenerative filter is advantageously used in a combination circuit, wherein the regenerative filter is connected in parallel either with single-mode, multiple linear filters or with other regenerative filters. It is advantageous that decision element thresholds are embodied as both constant and adjustable.
For a further equalization of the signals, series connecting an analog filter turns out to be an advantage, wherein this filter can be designed as an adaptive filter. For an optimal signal equalization, filter combinations can be used in which regenerative filters having two threshold decision elements cooperate with linear, adaptive, or non-adaptive filters, as well as with regenerative filters according to the invention having at least four threshold decision elements.
The signal clock pulse recovery in the equalizing circuit can be embodied in a particularly advantageous way by virtue of the fact that the clock circuit is provided with a first circuit that compares the input signal to the predeterminable threshold in order to limit the amplitude of the input signal to a value range predetermined by the threshold, and detects the rising or falling edge of the amplitude-limited input signal within a predeterminable time window in order to generate output pulses (FP) of uniform duration. The edges are detected, for example, by scanning at different times and by comparing these scanned values. The edge detection is particularly simple and reliable due to the amplitude limiting.
It is particularly advantageous if the clock circuit also includes a second circuit that follows the first circuit and regenerates the signal clock pulse from the output pulses by synchronizing with its edges. Therefore, a phase-locked loop (PLL) or a filter is additionally connected subsequently in order to further stabilize the edge detection.
In addition, it is advantageous if the first circuit is an edge-triggered mono-flop with a dead time element that delays the amplitude-limited input signal by the duration that determines the magnitude of the time window and is smaller than approximately half the bit duration of the input signal, with an inverter stage that inverts the time-delayed input signal, and with an AND gate that logically links the amplitude-limited input signal and the inverted, time-delayed input signal to each other in order to generate the output pulses. By means of these simple measures, a time window is set by way of the amplitude-limited signal in order to assure that even with particularly intense signal distortion, only the rising (positive) edge or only the falling (negative) edge is detected.
Another advantage is produced if the second circuit, which is preferably a phase-locked loop (PLL), synchronizes itself to the rising edge of the output pulses. As a result, a stab
Bulow Henning
Schlag Erwin
Alcatel
Chin Stephen
Jiang Lenny
LandOfFree
Method and regenerative filter for equalizing digitally... does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Method and regenerative filter for equalizing digitally..., we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Method and regenerative filter for equalizing digitally... will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-2449649