Automatic gain compression detection and gain control for analog front-end with nonlinear distortion

1. A method of controlling a gain of an analog front-end (AFE) circuit, the method comprising:

setting the gain to a baseline set gain;

determining an inner eye baseline amplitude of an inner eye of an eye pattern of a signal at the baseline set gain, and an outer eye baseline amplitude of an outer eye of the eye pattern of the signal at the baseline set gain;

setting the gain to a current set gain higher than the baseline set gain;

determining a gain compression ratio based on the inner eye baseline amplitude, the outer eye baseline amplitude, an inner eye current amplitude of the inner eye of the eye pattern at the current set gain, and an outer eye current amplitude of the outer eye of the eye pattern at the current set gain; and

increasing the current set gain until the gain compression ratio reaches a predefined limit.

2. The method according to claim 1 , wherein each one of the inner eye baseline amplitude, the outer eye baseline amplitude, the inner eye current amplitude, and the outer eye current amplitude is a different amplitude, and wherein determining each amplitude comprises:

determining each of a positive voltage and negative voltage of the amplitude by adjusting a corresponding threshold voltage until a proportion of samples of the signal in a sampling period exceeding the threshold voltage reaches a target proportion.

3. The method according to claim 2 , wherein the target proportion for the inner eye amplitude is greater than the target proportion for the outer eye amplitude.

4. The method according to claim 1 , wherein each amplitude is a median amplitude.

5. The method according to claim 1 , wherein the gain compression ratio is determined by:

determining a compression-free estimate of the outer eye current amplitude without gain compression; and

determining the gain compression ratio based on the compression-free estimate and the outer eye current amplitude.

6. The method according to claim 5 , wherein the gain compression ratio is a quotient of the compression-free estimate of the outer eye current amplitude without gain compression and the outer eye current amplitude.

7. The method according to claim 5 , wherein determining the compression-free estimate of the outer eye current amplitude comprises:

determining a compression-free gain response based on the inner eye current amplitude and the inner eye baseline amplitude; and

determining the compression-free estimate of the outer eye current amplitude based on the compression-free gain response and the outer eye baseline amplitude.

8. The method according to claim 7 , wherein the compression-free gain response is a quotient of the inner eye current amplitude and the inner eye baseline amplitude.

9. The method according to claim 7 , wherein the compression-free estimate of the outer eye current amplitude is a product of the compression-free gain response and the outer eye baseline amplitude.

10. The method according to claim 1 further comprising selecting the baseline gain to produce the inner eye baseline amplitude and the outer eye baseline amplitude below a nonlinear gain threshold of the AFE circuit.

11. A gain control circuit to control a gain of an analog front-end (AFE) circuit comprising at least one AFE stage to input an incoming analog signal and to generate a conditioned analog signal, the gain control circuit comprising:

a first circuit to control a set gain of the at least one AFE stage to a baseline set gain or a current set gain higher than the baseline set gain;

a second circuit to input the conditioned analog signal and to determine an inner eye amplitude of an inner eye of a signal eye of the conditioned analog signal and an outer eye amplitude of an outer eye of the signal eye; and

a third circuit connected to the second circuit to determine a current gain compression of the conditioned analog signal based on: the inner eye amplitude at the baseline set gain and the outer eye amplitude at the baseline set gain, and the inner eye amplitude at the current set gain and the outer eye amplitude at the current set gain.

12. The gain control circuit according to claim 11 , wherein the first circuit is responsive to the third circuit to increase the current set gain until the current gain compression reaches a predefined limit.

13. The gain control circuit according to claim 11 , wherein each of the inner eye amplitude and the outer eye amplitude comprises a positive and a negative voltage, and wherein the second circuit comprises:

a fourth circuit to sample the conditioned analog signal based on a threshold voltage, and, to determine each of the positive voltage and negative voltage by adjusting the threshold voltage until a proportion of samples of the conditioned analog signal in a sampling period exceeding the threshold voltage reaches a target proportion based on a target count.

14. The gain control circuit according to claim 13 , wherein the target proportion for the inner eye amplitude is greater than the target proportion for the outer eye amplitude.

15. The gain control circuit according to claim 11 , wherein each amplitude is a median amplitude.

16. The gain control circuit according to claim 11 , wherein the third circuit determines the gain compression ratio by:

determining a compression-free estimate of the outer eye amplitude at the current set gain without gain compression; and

determining the gain compression ratio based on the compression-free estimate and the outer eye amplitude at the current set gain.

17. The gain control circuit according to claim 16 , wherein the gain compression ratio is a quotient of the compression-free estimate of the outer eye amplitude at the current set gain without gain compression and the outer eye amplitude at the current set gain.

18. The gain control circuit according to claim 16 , wherein the third circuit determines the compression-free estimate of the outer eye amplitude at the current set gain by:

determining a compression-free gain response based on the inner eye amplitude at the current set gain and the inner eye amplitude at the baseline set gain; and

determining the compression-free estimate of the outer eye amplitude at the current set gain based on the compression-free gain response and the outer eye amplitude at the baseline set gain.

19. The gain control circuit according to claim 18 , wherein the compression-free gain response is a quotient of the inner eye amplitude at the current set gain and the inner eye amplitude at the baseline set gain.

20. The gain control circuit according to claim 18 , wherein the compression-free estimate of the outer eye amplitude at the current set gain is a product of the compression-free gain response and the outer eye amplitude at the baseline set gain.

21. The gain control circuit according to claim 11 , wherein the baseline set gain produces the inner eye amplitude baseline and the outer eye baseline amplitude below a nonlinear gain threshold of the AFE stages.

22. An analog front-end (AFE) circuit comprising:

at least one AFE stage to input an incoming analog signal and to generate a conditioned analog signal;

a gain control block to control a set gain of the at least one AFE stage;

a finite state machine (FSM) block to control the gain control block to set the set gain to a baseline set gain or a current set gain higher than the baseline set gain, and to set target counts for determining an inner eye amplitude and an outer eye amplitude of a signal eye of the conditioned analog signal;

a sampler block to sample the conditioned analog signal responsive to a threshold voltage;

a threshold control block responsive to the FSM block to adjust the threshold voltage until a proportion of samples of the conditioned analog signal in a sampling period exceeding the threshold voltage reaches a target proportion based on one of the target counts received from the FSM block;

a compression calculation block responsive to the threshold control block to determine a gain compression of the conditioned analog signal at the current signal gain based on threshold voltages received from the threshold control block,

wherein the FSM block increases the current set gain until the gain compression ratio reaches a predefined limit.