Circuit and method for finding the sampling phase and canceling intersymbol interference in a decision feedback equalized receiver
A circuit comprises an analog to digital converter (ADC) that samples a received signal based on a sampling clock. A feed forward filter communicates with the ADC and does not remove first precursor intersymbol interference from the received signal. An adaptive timing loop circuit that adjusts a timing phase of the sampling clock of said ADC to remove the first precursor intersymbol interference from the received signal. The adaptive timing loop circuit adjusts the timing phase based on at least one of an estimate signal and a loop gain control constant.
1. A circuit, comprising:
an analog to digital converter (ADC) configured to sample a received signal based on a sampling clock;
an adaptive timing loop circuit configured to adjust a timing phase of the sampling clock of the ADC to remove first precursor intersymbol interference from the received signal;
a mapping circuit configured to generate, using a gain coefficient associated with the adaptive timing loop circuit, a filter coefficient; and
a feed forward filter having the filter coefficient that is generated using the gain coefficient, the feed forward filter configured to, subsequent to the first precursor intersymbol interference being removed from the received signal, use the filter coefficient, as adjusted using the gain coefficient, to remove remaining precursor intersymbol interference from the received signal,
wherein the adaptive timing loop circuit is configured to adjust the timing phase based on at least one of an estimate signal and a loop gain control constant.
2. The circuit of claim 1 , wherein the feed forward filter includes an open loop finite impulse response (FIR) filter.
3. The circuit of claim 2 , wherein the open loop FIR filter is configured to remove all precursor intersymbol interference from the received symbol except for the first precursor intersymbol interference.
4. The circuit of claim 3 , wherein the gain coefficient is based on a gain mapping condition measurement of the received signal.
5. The circuit of claim 1 , wherein the estimate includes a pulsed multilevel voltage signal estimate.
6. The circuit of claim 1 , wherein the adaptive timing loop circuit is configured to adjust the timing phase further based on an error signal.
7. The circuit of claim 6 , wherein the adaptive timing loop circuit includes a first delay element configured to receive the error signal.
8. A circuit, comprising:
an analog to digital converter (ADC) configured to sample a received signal based on a sampling clock;
an adaptive timing loop circuit configured to adjust a timing phase of the sampling clock of the ADC to remove first precursor intersymbol interference from the received signal; and
a feed forward filter having a filter coefficient that is adjusted using a gain coefficient, the feed forward filter configured to, subsequent to the first precursor intersymbol interference being removed from the received signal, use the filter coefficient, as adjusted using the gain coefficient, to remove remaining precursor intersymbol interference from the received signal, wherein the adaptive timing loop circuit is configured to adjust the timing phase based on at least one of an estimate signal and a loop gain control constant,
wherein the adaptive timing loop circuit is configured to adjust the timing phase further based on an error signal,
wherein the adaptive timing loop circuit includes a first delay element configured to receive the error signal, and
wherein the adaptive timing loop circuit includes i) a first multiplier circuit configured to receive the estimate signal and ii) a second multiplier circuit configured to receive the loop gain control constant.
9. The circuit of claim 8 , wherein the adaptive timing loop circuit includes a summing circuit in communication with i) the second multiplier circuit and ii) a second delay element.
10. The circuit of claim 1 , wherein the adaptive timing loop circuit is configured to adjust the timing phase according to the formula φ sampn+1 =φ sampn −μ t (â i e i−1 ), wherein:
φ sampn+1 is a timing phase adjustment for a next sample h n+1 from the ADC;
φ sampn is a timing phase adjustment for a current sample h n from the ADC;
μ t is the control constant; and
e i−1 is an error signal for a previous sample.
11. The circuit of claim 1 , wherein the feed forward filter is configured to adjust an output signal according to the formula f k =A v (h k +(h k+1 )*w −1 ), wherein:
f k is a digitized and sampled pulsed multilevel voltage signal;
A v is a gain constant of a gain block;
h k is a present sampled and digitized received pulsed multilevel voltage signal;
h k+1 is a next sampled and digitized received pulsed multilevel voltage signal; and
w −1 is a filter coefficient mapped from the gain constant A v .
12. A circuit, comprising:
an analog to digital converter (ADC) configured to sample a received signal based on a sampling clock;
an adaptive timing loop circuit configured to adjust a timing phase of the sampling clock of the ADC to remove first precursor intersymbol interference from the received signal; and
a feed forward filter having a filter coefficient that is adjusted using a gain coefficient, the feed forward filter configured to, subsequent to the first precursor intersymbol interference being removed from the received signal, use the filter coefficient, as adjusted using the gain coefficient, to remove remaining precursor intersymbol interference from the received signal,
wherein the adaptive timing loop circuit is configured to adjust the timing phase based on at least one of an estimate signal and a loop gain control constant, and
wherein the feed forward filter includes:
a first delay element responsive to the ADC;
a first multiplier circuit;
a first summing circuit responsive to the first delay element and the first multiplier circuit;
a second multiplier circuit;
a gain setting circuit responsive to the second multiplier circuit, wherein the second multiplier circuit is responsive to the first summing circuit and the gain setting circuit; and
a gain mapping circuit responsive to the gain setting circuit, wherein the first multiplier circuit is responsive to the gain mapping circuit and the ADC.
13. The circuit of claim 12 , wherein the gain setting circuit includes:
a third multiplier circuit responsive to an error signal, the second multiplier circuit, and a gain constant;
a second delay element; and
a second summing circuit responsive to the second delay element and the third multiplier circuit, wherein the second delay element is responsive to the second summing circuit.
14. A method, comprising:
sampling a received signal based on a sampling clock with an analog to digital converter (ADC);
receiving the received signal from the ADC at a feed forward filter that does not remove first precursor intersymbol interference from the received signal;
adjusting a timing phase of the sampling clock of the ADC to remove the first precursor intersymbol interference from the received signal based on at least one of an estimate signal and a loop gain control constant;
generating, using a gain coefficient associated with adjusting the timing phase, filter coefficient; and
subsequent to the first precursor intersymbol interference being removed from the received signal and using the feed forward filter, wherein the feed forward filter has the filter coefficient that is generated using the gain coefficient, removing remaining precursor intersymbol interference from the received signal using the filter coefficient as adjusted using the gain coefficient.
15. The method of claim 14 , wherein the feed forward filter includes an open loop finite impulse response (FIR) filter.
16. The method of claim 15 , further comprising using the open loop FIR filter to remove all precursor intersymbol interference from the received symbol except for the first precursor intersymbol interference.
17. The method of claim 16 , wherein the gain coefficient is based on a gain mapping condition measurement of the received signal.
18. The method of claim 14 , wherein the estimate includes a pulsed multilevel voltage signal estimate.
19. The method of claim 14 , further comprising adjusting the timing phase further based on an error signal.
20. The method of claim 19 , further comprising receiving the error signal at a first delay element.
21. A method, comprising:
sampling a received signal based on a sampling clock with an analog to digital converter (ADC);
receiving the received signal from the ADC at a feed forward filter that does not remove first precursor intersymbol interference from the received signal;
adjusting a timing phase of the sampling clock of the ADC to remove the first precursor intersymbol interference from the received signal based on at least one of an estimate signal and a loop gain control constant;
subsequent to the first precursor intersymbol interference being removed from the received signal and using the feed forward filter, wherein the feed forward filter has a filter coefficient that is adjusted using a gain coefficient, removing remaining precursor intersymbol interference from the received signal using the filter coefficient as adjusted using the gain coefficient;
adjusting the timing phase further based on an error signal;
receiving the error signal at a first delay element; and
receiving the estimate signal at a first multiplier circuit and receiving the loop gain control constant at a second multiplier circuit.
22. The method of claim 14 , further comprising adjusting the timing phase according to the formula φ sampn+1 =φ sampn −μ t (â i e i−1 ), wherein:
φ sampn+1 is the timing phase adjustment for a next sample h n+1 from the ADC;
φ sampn is a timing phase adjustment for a current sample h n from the ADC;
μ t is the control constant; and
e i−1 is an error signal for a previous sample.
23. The method of claim 14 , further comprising using the feed forward filter to generate an output signal according to the formula f k =A v (h k +(h k+1 )*w −1 ), wherein:
f k is a digitized and sampled pulsed multilevel voltage signal;
A v is a gain constant of a gain block;
h k is a present sampled and digitized received pulsed multilevel voltage signal;
h k+1 is a next sampled and digitized received pulsed multilevel voltage signal; and
w −1 is a filter coefficient mapped from the gain constant A v .
24. The method of claim 14 , wherein the feed forward filter includes:
a gain setting circuit; and
a gain mapping circuit responsive to the gain setting circuit.