Circuit to correct phase interpolator rollover integral non-linearity errors
A circuit for correcting phase interpolator rollover integral non-linearity errors includes a rollover detector circuit for detecting when a rollover event of a phase interpolator has occurred, and a correction circuit that adds a signed predistortion correction to a VCO clock cycle phase fraction value when the rollover detector circuit has detected the interpolator rollover event.
1. A circuit for correcting phase interpolator rollover integral non-linearity errors comprising:
a rollover detector circuit for detecting when a rollover event of a phase interpolator has occurred; and
a correction circuit that adds a signed predistortion correction to a voltage controlled oscillator (VCO) clock cycle phase fraction value when the rollover detector circuit has detected the rollover event of the phase interpolator.
2. The circuit of claim 1 further comprising an integral non-linearity distortion pre-correction circuit for pre-correcting the VCO clock cycle phase fraction value for integral non-linearity distortion.
3. The circuit of claim 1 wherein the rollover detector circuit comprises:
a forward rollover detector circuit that detects when a forward rollover event of the phase interpolator has occurred; and
a reverse rollover detector circuit that detects when a reverse rollover event of the phase interpolator has occurred.
4. The circuit of claim 3 wherein:
the forward rollover detector circuit comprises a circuit that compares an actual phase integer delta value with an expected phase integer delta value and detects that the forward rollover event of the phase interpolator has occurred when the actual phase integer delta value is equal to the expected phase integer delta value incremented by 1; and
the reverse rollover detector circuit comprises a circuit that compares the actual phase integer delta value with the expected phase integer delta value and detects that the reverse rollover event of the phase interpolator has occurred when the actual phase integer delta value is equal to the expected phase integer delta value decremented by 1.
5. The circuit of claim 4 wherein:
the circuit that compares the actual phase integer delta value with the expected phase integer delta value and detects that the forward rollover event of the phase interpolator has occurred when the actual phase integer delta value is equal to the expected phase integer delta value incremented by 1 comprises a data flip-flop having an input at which a current cumulative phase integer value appears and an output at which an immediately prior cumulative phase integer value appears, a subtractor that subtracts the current cumulative phase integer value from the immediately prior cumulative phase integer value to generate the actual phase integer delta value, and a first magnitude comparator that compares the output of the subtractor with the expected phase integer delta value incremented by 1; and
the circuit that compares the actual phase integer delta with the expected phase integer delta value and detects that the reverse rollover event of the phase interpolator has occurred when the actual phase integer delta value is equal to the expected phase integer delta value decremented by 1 comprises the data flip-flop having the input at which the current cumulative phase integer value appears and the output at which the immediately prior cumulative phase integer value appears, the subtractor that subtracts the current cumulative phase integer value from the immediately prior cumulative phase integer value to generate the actual phase integer delta value, and a second magnitude comparator that compares the output of the subtractor with the expected phase integer delta value decremented by 1.
6. The circuit of claim 3 wherein the correction circuit that adds the signed predistortion correction to the VCO clock cycle phase fraction value when the rollover event of the phase interpolator has been detected comprises:
a forward signed predistortion correction circuit that adds a first forward signed predistortion correction to the VCO clock cycle phase fraction value on a CARRY signal assertion following when the forward rollover event of the phase interpolator has been detected, and a reverse signed predistortion correction circuit that adds a first reverse signed predistortion correction to the VCO clock cycle phase fraction value on a CARRY signal assertion following when the reverse rollover event of the phase interpolator has been detected.
7. The circuit of claim 3 further comprising:
a circuit for setting a forward interpolator rollover event flag on a CARRY signal when a forward rollover event of the phase interpolator has been detected; and
a circuit for setting a reverse interpolator rollover event flag on a CARRY signal when a reverse rollover event of the phase interpolator has been detected.
8. The circuit of claim 7 further comprising:
a circuit for setting a second forward interpolator rollover event flag on a CARRY signal assertion following setting the forward interpolator rollover event flag; and
a circuit for setting a second reverse interpolator rollover event flag on a CARRY signal following setting the reverse interpolator rollover event flag.
9. The circuit of claim 8 further comprising:
a forward signed predistortion correction circuit for adding a forward signed predistortion correction to the VCO clock cycle phase fraction value on the CARRY signal following setting the forward interpolator rollover event flag; and
a reverse signed predistortion correction circuit for adding a reverse signed predistortion correction to the VCO clock cycle phase fraction value on the CARRY signal following setting the reverse interpolator rollover event flag.
10. The circuit of claim 8 further comprising:
a second forward signed predistortion correction circuit for adding a second forward signed predistortion correction to the VCO clock cycle phase fraction value on the CARRY signal assertion following setting the second forward interpolator rollover event flag; and
a second reverse signed predistortion correction circuit for adding a second reverse signed predistortion correction to the VCO clock cycle phase fraction value on the CARRY signal assertion following setting the second reverse interpolator rollover event flag.
11. A method for correcting phase interpolator rollover integral non-linearity errors comprising:
detecting when a rollover event of a phase interpolator has occurred; and
adding a signed predistortion correction to a voltage controlled oscillator (VCO) clock cycle phase fraction value of the phase interpolator when the rollover event of the phase interpolator has been detected.
12. The method of claim 11 wherein detecting when the rollover event of the phase interpolator has occurred comprises:
determining whether a forward rollover event of the phase interpolator has occurred; and
determining whether a reverse rollover event of the phase interpolator has occurred.
13. The method of claim 12 wherein:
determining whether the forward rollover event of the phase interpolator has occurred comprises comparing an actual phase integer delta value between a current cumulative phase integer value and an immediately prior cumulative phase integer value with an expected phase integer delta value and indicating that a forward rollover event of the phase interpolator has occurred when the actual phase integer delta value is equal to the expected phase integer delta value incremented by 1; and
determining whether the reverse rollover event of the phase interpolator has occurred comprises comparing the actual phase integer delta value between the current cumulative phase integer value and the immediately prior cumulative phase integer value with the expected phase integer delta value and indicating that a reverse rollover event of the phase interpolator has occurred when the actual phase integer delta value is equal to the expected phase integer delta value decremented by 1.
14. The method of claim 13 wherein:
comparing the actual phase integer delta value between the current cumulative phase integer value and the immediately prior cumulative phase integer value with the expected phase integer delta value and indicating that the forward rollover event of the phase interpolator has occurred when the actual phase integer delta value is equal to the expected phase integer delta value incremented by 1 comprises subtracting the current cumulative phase integer value from the immediately prior cumulative phase integer value, and comparing the difference with the expected phase integer delta value incremented by 1; and
comparing the actual phase integer delta value between the current cumulative phase integer value and the immediately prior cumulative phase integer value with the expected phase integer delta value and indicating that the reverse rollover event of the phase interpolator has occurred when the actual phase integer delta value is equal to the expected phase integer delta value decremented by 1 comprises subtracting the current cumulative phase integer value from the immediately prior cumulative phase integer value, and comparing the difference with the expected phase integer delta value decremented by 1.
15. The method of claim 13 wherein:
indicating that the forward rollover event of the phase interpolator has occurred when the actual phase integer delta value is equal to the expected phase integer delta value incremented by 1 comprises setting a forward interpolator rollover event flag; and
indicating that the reverse rollover event of the phase interpolator has occurred when the actual phase integer delta value is equal to the expected phase integer delta value decremented by 1 comprises setting a reverse interpolator rollover event flag.
16. The method of claim 11 wherein adding the signed predistortion correction to the VCO clock cycle phase fraction value when the rollover event of the phase interpolator has been detected comprises:
adding a first forward signed predistortion correction to the VCO clock cycle phase fraction value when a forward rollover event of the phase interpolator has been detected; and
adding a first reverse signed predistortion correction to the VCO clock cycle phase fraction value when a reverse rollover event of the phase interpolator has been detected.
17. The method of claim 16 further comprising:
adding a second forward signed predistortion correction to the VCO clock cycle phase fraction value after the first forward signed predistortion correction has been added to the VCO clock cycle phase fraction value; and
adding a second reverse signed predistortion correction to the VCO clock cycle phase fraction value after the first forward signed predistortion correction has been added to the VCO clock cycle phase fraction value.