Receiver with distortion compensation circuit
A receiver containing analog circuitry that generates distortion, a distortion compensation circuit coupled to an output of the analog circuitry, and a slicer, operating as a signal peak detector, coupled to the distortion compensation circuitry. The distortion compensation circuit has a subtractor, a function generator, and a weighting circuit. The subtractor has a first input coupled to the output of the analog circuitry, a second input, and an output. The function generator has an input coupled to the first input of the subtractor. The weighting circuit, responsive to a weighting coefficient, is coupled between an output of the function circuit and the second input of the first subtractor. The function generator has a transfer function with a third-power term and the weighting coefficient is set to a value based on the level of the signal peaks that will least partially reduce distortion in signals on the output of the subtractor.
1. A method of reducing distortion in an amplifier, the method comprising the steps of:
applying an input signal, having a controllable amplitude, to the amplifier to produce an output signal;
measuring a peak amplitude in the output signal;
adjusting the amplitude of the input signal until a change in the measured peak amplitude is less than a threshold amount; and then
generating an intermediate signal that is a function of the output signal;
scaling an amplitude of the intermediate signal by a weighting factor;
subtracting the weighted intermediate signal from the output signal to produce a compensated signal; and
setting the weighting factor to a value related to the measured peak amplitude,
wherein measuring the peak amplitude in the output signal comprises:
slicing, using a threshold setting, the compensated signal to produce a binary output signal; and
adjusting the slicer threshold setting until the output of the slicer becomes substantially the same value for a period of time,
wherein the slice threshold setting is a saturation factor.
2. The method of claim 1 wherein the function is a cubic function.
3. The method of claim 1 wherein the amplifier is coupled to a continuous time linear equalizer in a receiver.
4. The method of claim 2 wherein the weighting factor is approximately 1/(3A 2 ), where A is the saturation factor related to the measured peak amplitude.
5. A receiver having:
a variable gain amplifier coupled to an input of the receiver;
a continuous time equalizer coupled between the variable gain amplifier and a node;
a distortion compensation circuit disposed in series with the continuous time equalizer and between the variable gain amplifier and the node;
a slicer having an input coupled to the node, an output, and a threshold setting input;
wherein the distortion compensation circuit comprises:
a first subtractor having a first input coupled to the variable gain amplifier, a second input, and an output coupled to the node;
a function generator circuit having an input coupled to the first input of the first subtractor, and having an output;
a weighting circuit, responsive to a weighting coefficient, coupled between an output of the function generator circuit and the second input of the first subtractor, wherein the function generator circuit is a cubic function and the weighting coefficient is approximately 1/(3A 2 ), where A is a saturation factor related to a peak amplitude of signals applied to the slicer;
wherein, the function generator circuit has a transfer function with a third-power term and the weighting coefficient is set to a value that will at least partially reduce distortion in signals on the output of the distortion compensation circuit.
6. The receiver of claim 5 further representing:
a controller responsive to the slicer output and coupled to the slicer threshold setting input;
wherein the controller is adapted to adjust the slicer threshold setting until the output of the slicer becomes substantially the same value for a period of time, and then the saturation factor is set to slicer threshold setting.
7. The receiver of claim 6 further comprising:
an additional distortion compensation circuit coupled between the variable gain amplifier and the equalizer; and
an additional slicer having an output, a threshold setting, and an input coupled to the additional distortion compensation circuit;
wherein the controller is further adapted to adjust the threshold setting of the additional distortion compensation circuit until the output of the slicer becomes substantially the same value for a period of time, and then the saturation factor is set to the slicer threshold setting of the additional slicer.
8. The receiver of claim 6 , wherein the receiver further comprises:
an error detector, coupled to the output of the slicer and the node, adapted to produce an error signal;
a decision feedback equalizer coupled to the output of the slicer, responsive to the error signal, and having an output; and
a second subtractor, disposed between the node and the slicer, and coupled to the output of the decision feedback equalizer;
wherein the decision feedback equalizer is disabled by the controller when the controller is adjusting the slicer threshold setting.
9. The receiver of claim 5 wherein the controller further controls the variable gain amplifier and the controller is further adapted to adjust the gain of the variable gain amplifier to establish, in conjunction with the adjustment of the slicer threshold setting, the saturation factor.
10. The receiver of claim 5 wherein the controller is further adapted to:
adjust the gain of the variable gain amplifier to establish, in conjunction with the adjustment of the slicer threshold setting, the saturation factor when a signal of a known amplitude is coupled to the input of the receiver.
11. The receiver of claim 5 wherein the first subtractor and the weighting circuit are implemented using transconductance amplifiers and a resistive load.
12. The receiver of claim 5 wherein the receiver is implemented using an integrated circuit.
13. The receiver of claim 5 , wherein the receiver further comprises:
an error detector.
14. The receiver of claim 5 , wherein the receiver further comprises:
a decision feedback equalizer.
15. A distortion compensation circuit having an input and an output, comprising:
a load coupled to the output of the distortion compensation circuit;
a first amplifier having a first gain, an input coupled to the input of the distortion compensation circuit, and an output coupled to the load;
a function generator having an input coupled to the input of the distortion compensation circuit, and an output; and
a second amplifier having a second gain, an input coupled to the output of the function generator, and an output coupled to the load;
an analog circuit having an output coupled to the input of the distortion compensations circuit;
wherein, the function generator has a transfer function with a third-power term, and a ratio of the gain of the second amplifier to the first amplifier is set to a value that will at least partially remove distortion from signals on the output of the distortion compensation circuit, wherein the ratio of the gain of the second amplifier to that of the first amplifier is approximately 1/(3A 2 ), where A is a saturation factor of the analog circuit.
16. The distortion compensation circuit of claim 15 wherein the first and second amplifiers are transconductance amplifiers and the load is a resistor.
17. The distortion compensation circuit of claim 15 wherein the transfer function is a cubic function.
18. The distortion compensation circuit of claim 15 wherein the distortion compensation circuit is formed in an integrated circuit.
19. The distortion compensation circuit of claim 15 , wherein the first and second amplifiers are transconductance amplifiers and the load is a resistor, wherein the transfer function is a cubic function.
20. The distortion compensation circuit of claim 17 wherein the function generator comprises:
a first multiplier having inputs coupled to the input of the distortion compensation circuit; and
a second multiplier having one input coupled to an output of the first multiplier, another input coupled to the input of the distortion compensation circuit, and an output coupled to the input of the second amplifier.