Apparatus and method thereof for compensating for gain changes of N-PAM modulate signals

1. An interface circuit for compensating for gain changes of an N-level pulse amplitude modulation (PAM-N) modulated signal, N being an integer, comprising:

a number of N−1 comparators for comparing the PAM-N modulated signal to N−1 configurable thresholds, wherein the PAM-N modulated signal was also equalized and the N−1 configurable thresholds are N−1 different voltage levels;

a compensation comparator for tracking gain changes in the input PAM-N modulated signal by comparing the input PAM-N modulated signal to a compensation threshold; and

a compensation circuit for changing at least one of the N−1 configurable thresholds of the N−1 comparators based on an output of the compensation comparator, thereby offsetting a crossing point of at least one comparator of the N−1 comparators respective of the at least one of the N−1 configurable thresholds to compensate for gain changes in the input PAM-N modulated signal, wherein the compensation circuit further comprises:

an accumulator for accumulating a number of crossings and non-crossings of the compensation threshold over time as output by the compensation comparator;

a controller for performing at least a process for changing the at least one of the N−1 configurable thresholds of the N−1 comparators based on the output of the compensation comparator; and

a voltage setting unit for setting the compensation comparator and the at least one of the N−1 configurable thresholds based on a control signal generated by the controller, wherein the controller, when performing the at least process, is configured to:

repetitively change the at least one of the N−1 configurable thresholds of the N−1 comparators based on crossings of a single PAM-N level of the compensation threshold and a fixed factor; and wherein the at least process further comprises:

set an initial compensation factor (ICF);

set the compensation threshold to an initial value;

detect, by the compensation comparator, crossings around a first PAM-N level out of N PAM-N modulation levels;

iteratively change a voltage level of the compensation threshold until an accumulated number of crossings and non-crossings of the compensation threshold respective of the first PAM-N level over time is equal;

save a voltage level of the compensation threshold that achieves an equal accumulated number of crossings and non-crossings as a current compensation threshold value; and

set the at least one of the N−1 configurable thresholds to a difference of the current compensation threshold and a previous value of the compensation threshold, multiplied by the ICF.

2. The interface circuit of claim 1 , wherein the initial compensation factor is set to a ratio of the at least one N−1 configurable threshold and the compensation factor and the compensation threshold initial value is set to a value respective of the first PAM-N level.

3. An interface circuit for compensating for gain changes of an N-level pulse amplitude modulation (PAM-N) modulated signal, N being an integer, comprising:

a number of N−1 comparators for comparing the PAM-N modulated signal to N−1 configurable thresholds, wherein the PAM-N modulated signal was also equalized and the N−1 configurable thresholds are N−1 different voltage levels;

a compensation comparator for tracking gain changes in the input PAM-N modulated signal by comparing the input PAM-N modulated signal to a compensation threshold; and

a compensation circuit for changing at least one of the N−1 configurable thresholds of the N−1 comparators based on an output of the compensation comparator, thereby offsetting a crossing point of at least one comparator of the N−1 comparators respective of the at least one of the N−1 configurable thresholds to compensate for gain changes in the input PAM-N modulated signal, wherein the compensation circuit further comprises:

an accumulator for accumulating a number of crossings and non-crossings of the compensation threshold over time as output by the compensation comparator;

a controller for performing at least a process for changing the at least one of the N−1 configurable thresholds of the N−1 comparators based on the output of the compensation comparator; and

a voltage setting unit for setting the compensation comparator and the at least one of the N−1 configurable thresholds based on a control signal generated by the controller, wherein the controller, when performing the at least process, is configured to:

repetitively change the at least one of the N−1 configurable thresholds of the N−1 comparators based on crossings of at least two PAM-N levels of the compensation threshold;

detect by the compensation comparator, crossings around a first PAM-N level out of N PAM-N modulation levels;

iteratively change a voltage level of the compensation threshold until an accumulated number of crossings and non-crossings of the compensation threshold respective of the first PAM-N level over time is equal;

save a voltage level of the compensation threshold that achieves an equal accumulated number of crossings and non-crossings as a high compensation threshold value;

detect by the compensation comparator, crossings around a second PAM-N level out of the N PAM-N modulation levels;

iteratively change a voltage level of the compensation threshold until an accumulated number of crossings and non-crossings of the compensation threshold respective of the second PAM-N level over time is equal;

save a voltage level of the compensation threshold that achieves an equal accumulated number of crossings and non-crossings as a low compensation threshold value; and

set the least one of the N−1 configurable thresholds to an average value of the high compensation threshold and the low compensation threshold.

4. The interface circuit of claim 3 , wherein the second PAM-N level is next to the first PAM-N level, wherein the least one of the N−1 configurable thresholds is used by at least one of the N−1 comparators being set to detect crossings between the second and the first PAM-N levels.

5. An interface circuit for compensating for gain changes of an N-level pulse amplitude modulation (PAM-N) modulated signal, N being an integer, comprising:

a number of N−1 comparators for comparing the PAM-N modulated signal to N−1 configurable thresholds, wherein the PAM-N modulated signal was also equalized and the N−1 configurable thresholds are N−1 different voltage levels;

a compensation comparator for tracking gain changes in the input PAM-N modulated signal by comparing the input PAM-N modulated signal to a compensation threshold; and

a compensation circuit for changing at least one of the N−1 configurable thresholds of the N−1 comparators based on an output of the compensation comparator, thereby offsetting a crossing point of at least one comparator of the N−1 comparators respective of the at least one of the N−1 configurable thresholds to compensate for gain changes in the input PAM-N modulated signal, wherein the compensation circuit further comprises:

an accumulator for accumulating a number of crossings and non-crossings of the compensation threshold over time as output by the compensation comparator;

a controller for performing at least a process for changing the at least one of the N−1 configurable thresholds of the N−1 comparators based on the output of the compensation comparator; and

a voltage setting unit for setting the compensation comparator and the at least one of the N−1 configurable thresholds based on a control signal generated by the controller, wherein the controller, when performing the at least process, is configured to:

repetitively change the at least one of the N−1 configurable thresholds of the N−1 comparators based on an eye diagram of a recorded and sampled input PAM-N modulated signal;

set the compensation threshold to an initial value, wherein the initial value is an upper limit of a maximum level of the N−1 configurable thresholds;

iteratively change a voltage level of the compensation threshold from the initial value to a lower limit of maximum level of the N−1 configurable thresholds and generating an eye diagram, at each iteration the compensation threshold is changed by a predefined voltage interval and a respective generated eye diagram is saved together with a current level of the compensation threshold;

select an eye diagram with a widest eye; and

set the at least one of the N−1 configurable thresholds to a voltage level of a compensation threshold associated with the eye diagram with the widest eye.

6. The interface circuit of claim 5 , wherein the at least one of the N−1 configurable thresholds includes at least a threshold value of a comparator that detects a +3 volts PAM-4 level crossing and at least a threshold value of a comparator that detects a +1 volt PAM-4 level crossing.

7. A method for compensating for gain changes of an N-level pulse amplitude modulation (PAM-N) modulated signal, N being an integer, comprising:

comparing the PAM-N modulated signal to N−1 configurable thresholds, wherein the PAM-N modulated signal was also equalized and the N−1 configurable thresholds are N−1 different voltage levels;

tracking gain changes in the PAM-N modulated signal by comparing the PAM-N modulated signal to a compensation threshold;

adjusting a level of the at least one of the N−1 configurable thresholds of N−1 comparators based on an output of a compensation comparator, thereby offsetting a crossing point of at least one comparator respective of the at least one of the N−1 configurable thresholds to compensate for gain changes in the PAM-N modulated signal;

accumulating a number of crossings and non-crossings of the compensation threshold over time as output by the compensation comparator;

performing at least a process for changing the at least one of the N−1 configurable thresholds of the N−1 comparators based on the output of the compensation comparator; and

setting the compensation comparator and the at least one of the N−1 configurable thresholds based on a control signal generated by the controller, wherein the at least process includes repetitively changing the at least one of the N−1 configurable thresholds of the N−1 comparators based on crossings of a single PAM-N level of the compensation threshold and a fixed factor, and wherein the at least process further comprises:

setting an initial compensation factor (ICF);

setting the compensation threshold to an initial value, wherein the initial value is set to a value respective of the first PAM-N level;

detecting, by the compensation comparator, crossings around a first PAM-N level out of N PAM-N modulation levels;

iteratively changing a voltage level of the compensation threshold until an accumulated number of crossings and non-crossings of the compensation threshold respective of the first PAM-N level over time is equal;

saving a voltage level of the compensation threshold that achieves an equal accumulated number of crossings and non-crossings as a current compensation threshold value; and

setting the least one of the N−1 configurable thresholds to a difference of the current compensation threshold and a previous first compensation threshold multiplied by the ICF.

8. A method for compensating for gain changes of an N-level pulse amplitude modulation (PAM-N) modulated signal, N being an integer, comprising:

comparing the PAM-N modulated signal to N−1 configurable thresholds, wherein the PAM-N modulated signal was also equalized and the N−1 configurable thresholds are N−1 different voltage levels;

tracking gain changes in the PAM-N modulated signal by comparing the PAM-N modulated signal to a compensation threshold;

adjusting a level of the at least one of the N−1 configurable thresholds of N−1 comparators based on an output of a compensation comparator, thereby offsetting a crossing point of at least one comparator respective of the at least one of the N−1 configurable thresholds to compensate for gain changes in the PAM-N modulated signal;

accumulating a number of crossings and non-crossings of the compensation threshold over time as output by the compensation comparator;

performing at least a process for changing the at least one of the N−1 configurable thresholds of the N−1 comparators based on the output of the compensation comparator; and

setting the compensation comparator and the at least one of the N−1 configurable thresholds based on a control signal generated by the controller, wherein the at least process further comprises:

detecting by the compensation comparator crossings around a first PAM-N level out of N PAM-N modulation levels;

iteratively changing a voltage level of the compensation threshold until an accumulated number of crossings and non-crossings of the compensation threshold respective of the first PAM-N level over time is equal;

saving a voltage level of the compensation threshold that achieves an equal accumulated number of crossings and non-crossings as a high compensation threshold value;

detecting by the compensation comparator, crossings around a second PAM-N level out of N PAM-N modulation levels, wherein the second PAM-N level is next to the first PAM-N level;

iteratively changing a voltage level of the compensation threshold until an accumulated number of crossings and non-crossings of the compensation threshold respective of the second PAM-N level over time is equal;

saving a voltage level of the compensation threshold that achieves an equal accumulated number of crossings and non-crossings as a low compensation threshold value; and

setting the least one of the N−1 configurable thresholds to an average value of the high compensation threshold and the low compensation threshold.

9. A method for compensating for gain changes of an N-level pulse amplitude modulation (PAM-N) modulated signal, N being an integer, comprising:

comparing the PAM-N modulated signal to N−1 configurable thresholds, wherein the PAM-N modulated signal was also equalized and the N−1 configurable thresholds are N−1 different voltage levels;

tracking gain changes in the PAM-N modulated signal by comparing the PAM-N modulated signal to a compensation threshold;

adjusting a level of the at least one of the N−1 configurable thresholds of N−1 comparators based on an output of a compensation comparator, thereby offsetting a crossing point of at least one comparator respective of the at least one of the N−1 configurable thresholds to compensate for gain changes in the PAM-N modulated signal;

accumulating a number of crossings and non-crossings of the compensation threshold over time as output by the compensation comparator;

performing at least a process for changing the at least one of the N−1 configurable thresholds of the N−1 comparators based on the output of the compensation comparator;

setting the compensation comparator and the at least one of the N−1 configurable thresholds based on a control signal generated by the controller, wherein the at least process includes repetitively changing the at least one of the N−1 configurable thresholds of the N−1 comparators based on an eye diagram of a recorded and sampled input PAM-N modulated signal, and wherein the at least process further comprises:

setting the compensation threshold to an initial value, wherein the initial value is an upper limit of a maximum level of the N−1 configurable thresholds;

iteratively changing a voltage level of the compensation threshold from the initial value to a lower limit of maximum level of the N−1 configurable thresholds and generating an eye diagram; at each iteration the compensation threshold is changed by a predefined voltage interval and a respective generated eye diagram is saved together with a current level of the compensation threshold;

selecting an eye diagram with a widest eye; and

setting the at least one of the N−1 configurable thresholds to a voltage level of a compensation threshold associated with the eye diagram with the widest eye.

10. The method of claim 7 , wherein the at least one of the N−1 configurable thresholds includes at least a threshold value of a comparator that detects a +3 volts PAM-4 level crossing and at least a threshold value of a comparator that detects a +1 volt PAM-4 level crossing.

11. A method for compensating for gain changes of a 4-level pulse amplitude modulation (PAM-4) modulated signal, N being an integer, comprising:

comparing the PAM-4 modulated signal to three configurable thresholds, wherein the PAM-4 modulated signal was also equalized and the three configurable thresholds are three different voltage levels;

tracking gain changes in the PAM-4 modulated signal by comparing the PAM-4 modulated signal to a compensation threshold; and

adjusting a level of at least one of the three configurable thresholds of three comparators based on an output of a compensation comparator, thereby offsetting a crossing point of at least one comparator of the three comparators respective of the at least one of the three configurable thresholds to compensate for gain changes in the PAM-4 modulated signal, wherein the at least process further comprises:

detecting by the compensation comparator crossings around a +3 volts PAM-4 level by a first comparator;

iteratively changing a voltage level of the compensation threshold until an accumulated number of crossings and non-crossings of the compensation threshold respective of the +3 volts PAM-4 level over time is equal;

saving a voltage level of the compensation threshold that achieves an equal accumulated number of crossings and non-crossings as a high compensation threshold value;

detecting by the compensation comparator, crossings around a +1 volt PAM-4 level by a second comparator;

iteratively changing a voltage level of the compensation threshold until an accumulated number of crossings and non-crossings of the compensation threshold respective of the +1 volt PAM-4 level over time is equal;

saving a voltage level of the compensation threshold that achieves an equal accumulated number of crossings and non-crossings as a low compensation threshold value; and

setting a reference level respective of configurable thresholds of the first comparator and the second comparator to an average value of the high compensation threshold and the low compensation threshold.