Decision feedback equalizer (DFE) for jitter reduction
View Patent ↗A decision feedback equalizer (DFE) for a receiver that can reduce jitter is disclosed. The DFE uses an equalizer structure that employs a symbol sampling operation at a decision device, also known as a slicer. In a receiver, the phase of a signal, such as an equalized signal, is typically estimated from zero crossings in the clock recovery operation. Fluctuations in these zero crossings makes phase of the reproduced clock unstable, which decreases error performance in an associated receiver. Embodiments advantageously align inter-symbol interference (ISI) canceling terms from a feedback filter (FBF) relatively well, and thereby provide equalization of a relatively large portion of a symbol period. This advantageously stabilizes the phase of an equalized signal and reduces jitter.
1. A receiver with a decision feedback equalizer (DFE), the receiver comprising:
a subtracting circuit configured to receive a received signal and a feedback signal as inputs, where the received signal has embedded therein symbols at a symbol rate with symbol intervals, where the subtracting circuit is configured to subtract the feedback signal from the received signal to generate an equalized signal for reduction of inter-symbol interference (ISI);
a slicer in communication with the subtracting circuit to receive the equalized signal as an input, where the slicer is configured to determine a logical state of the equalized signal and to provide an output signal; and
a feedback filter circuit in communication with the slicer and the subtracting circuit, where the feedback filter circuit is configured to receive the output signal from the slicer as an input and to provide the feedback signal to the subtracting circuit, where the feedback signal has canceling terms for cancellation of one or more post-cursor ISI terms by the subtracting circuit, where the feedback filter circuit includes a first delay tap for a feedback filter, where the first delay tap is configured to delay the feedback signal such that transitions of the feedback signal are substantially time aligned with symbol interval transitions of the received signal for the subtracting circuit.
2. The receiver as defined in claim 1 , further comprising a clock recovery circuit in communication with the slicer to provide a clock signal, wherein the clock recovery circuit is configured to generate the clock signal from the equalized signal at least during normal operation, wherein the slicer samples the equalized signal with an edge of the clock signal, where the edge of the clock signal is approximately in the middle of a symbol interval in the equalized signal.
3. The receiver as defined in claim 1 , wherein the delay of the first delay tap is about half of a symbol interval.
4. The receiver as defined in claim 1 , wherein the slicer further comprises a flip-flop such that a propagation delay of the slicer corresponds to a propagation delay for the flip flop, and where the delay of the first delay tap corresponds to about a difference between half of a symbol interval and the slicer propagation delay.
5. The receiver as defined in claim 1 , wherein the first delay tap is implemented with a transparent latch.
6. The receiver as defined in claim 1 , further comprising a low-pass filter for the feedback signal disposed in a communication path between the feedback filter circuit and the subtracting circuit.
7. The receiver as defined in claim 6 , wherein the low-pass filter is an RC filter.
8. The receiver as defined in claim 1 , further comprising an adaption engine in communication with the feedback filter circuit, where the adaption engine is configured to determine filter coefficients for the feedback filter circuit.
9. The receiver as defined in claim 2 , further comprising a switch in communication with the clock recovery circuit, where the switch is configured to provide the clock recovery circuit with the equalized signal during normal operation and to provide the clock recovery circuit with the received signal during determination of the filter coefficients by the adaption engine.
10. The receiver as defined in claim 1 , wherein the receiver is embodied in a serializer/deserializer (SerDes).
11. The receiver as defined in claim 1 , wherein the delay of the first delay tap is a fraction of a delay of other taps of the feedback filter.
12. The receiver as defined in claim 1 , wherein the feedback filter comprises a plurality of delay taps, wherein the first delay tap is triggered via both a rising and a falling clock edge, wherein other delay taps are triggered by a single clock edge.
13. A receiver with a decision feedback equalizer (DFE), the receiver comprising:
a subtracting circuit configured to receive a received signal and a feedback signal as inputs, where the received signal has embedded therein symbols at a symbol rate with symbol intervals, where the subtracting circuit is configured to subtract the feedback signal from the received signal to generate an equalized signal for reduction of inter-symbol interference (ISI);
a slicer in communication with the subtracting circuit to receive the equalized signal as an input, where the slicer is configured to determine the symbols in the equalized signal and to provide an output signal;
a feedback filter circuit in communication with the slicer and the subtracting circuit, where the feedback filter circuit is configured to receive the output signal from the slicer as an input and to provide the feedback signal to the subtracting circuit, where the feedback signal has canceling terms for cancellation of one or more post-cursor ISI terms by the subtracting circuit; and
means for timing the feedback signal such that a transition in the feedback signal is time aligned with a transition in the received signal.
14. A method of feedback equalization in a receiver, the method comprising:
receiving a received signal having embedded therein symbols at a symbol rate with symbol intervals;
subtracting a feedback signal from the received signal to generate an equalized signal;
determining symbols of the equalized signal for an output signal;
generating the feedback signal from the output signal, wherein the feedback signal includes cancellation terms for cancellation of inter-symbol interference (ISI); and
timing the feedback signal such that a transition in the feedback signal is time aligned with a transition in the received signal.
15. The method as defined in claim 14 , further comprising aligning the transition in the feedback signal with a transition of an immediately subsequent symbol in the received signal.
16. The method as defined in claim 14 , further comprising timing the feedback signal by delaying the output signal by about half a symbol interval.
17. The method as defined in claim 14 , wherein a transition for a symbol in the output signal is delayed from a corresponding symbol in the equalized signal by a propagation delay of a flip-flop associated with a slicer, further comprising timing the feedback signal by about half the symbol interval less the propagation delay of the slicer flip-flop.
18. The method as defined in claim 14 , further comprising low-pass filtering the feedback signal.
19. The method as defined in claim 14 , further comprising low-pass filtering the feedback signal with a low-pass resistor-capacitor (RC) filter.
20. The method as defined in claim 14 , further comprising adaptively determining filter coefficients for generation of the feedback signal.
21. The method as defined in claim 14 , further comprising:
generating timing signals from the received signal for determining the symbols of the equalized signal in an adaptive mode of operation;
adaptively determining filter coefficients for generation of the feedback signal during the adaptive mode of operation; and
generating timing signals from the equalized signal in a normal mode of operation.
22. The method as defined in claim 14 , wherein the method is embodied in a serializer/deserializer (SerDes).