Complex bandpass modulator and method for analog-to-digital converters

• US 6,225,928 B1
• Filed: 03/10/1999
• Issued: 05/01/2001
• Est. Priority Date: 03/10/1999
• Status: Expired due to Term

• Current Assignee: Cirrus Logic Incorporated
• Original Assignee: Cirrus Logic Incorporated
• Inventors: Green, Brian D.
• Primary Examiner(s): Williams, Howard L.
• Assistant Examiner(s): Jeanglaude, Jean Bruner
• Application Number:
  US09/265,758
• Time in Patent Office: 783 Days
• Field of Search: 341/143, 341/144, 341/155, 375/345, 375/176, 708/319, 333/176
• US Class Current: 341/143
• CPC Class Codes: H03M 3/40 Arrangements for handling quadrature signals, e.g. complex modulators
• CPC Class Codes: H03M 3/43 the quantiser being a single bit one
• CPC Class Codes: H03M 3/452 with weighted feedforward summation, i.e. with feedforward paths from more than one filter stage to the quantiser input

Abstract

A discrete-time strongly cross-coupled complex bandpass modulator is disclosed that achieves the full potential of bandpass delta-sigma conversion by providing a strongly cross-coupled discrete-time complex loop filter structure with very low sensitivity to mismatches and by providing a simple scheme for correcting the effects of modulator mismatches. The complex bandpass modulator includes a plurality of non-linear resonators connected together and acting as a linear complex operator. Each resonator will act as a linear complex operator when an imaginary input signal is delayed by half a sample interval and an imaginary output signal is advanced by half a sample interval. In addition, degradation effects due to modulator mismatches are eliminated by digitally adjusting the relative gain of the real and imaginary paths following the output of the analog-to-digital converter and by adjusting the relative gain of the real and imaginary input signals.

Claims

1. An analog-to-digital converter, comprising:

real and imaginary input signals;

a real path quantizer having a real digital output signal;

an imaginary path quantizer having an imaginary digital output signal; and

a complex loop filter coupled to receive the real and imaginary input signals and to provide output signals to the real and imaginary path quantizers, said complex loop filter including a plurality of non-linear resonators connected together and acting as a linear complex operator the plurality of non-linear resonators being connected in series so that the output of a previous resonator is coupled to the input of the next resonator;

wherein a plurality of outputs from the non-linear resonators are combined to provide an output to the real path quantizer; and

wherein a plurality of outputs from the non-linear resonators are combined to provide an output to the imaginary path quantizer.

2. The analog-to-digital converter of claim 1, further comprising a real path adder coupled to subtract a real path feedback signal from the real input signal and an imaginary path adder coupled to subtract an imaginary path feedback signal from the imaginary input signal.

3. The analog-to-digital converter of claim 2, wherein the real feedback signal comprises the real digital output signal passed through a real path digital-to-analog converter and the imaginary feedback signal comprises the imaginary digital output signal passed through an imaginary digital-to-analog converter.

4. The analog-to-digital converter of claim 2, further comprising an input delay block coupled to delay the imaginary input signal by half a sample interval relative to the real input signal before the imaginary input signal is received by the complex loop filter and an output advance block coupled to advance the imaginary digital output signal by half a sample interval relative to the real digital output signal.

5. A complex loop filter for an analog-to-digital converter, comprising:

a plurality of non-linear resonators coupled to a real input signal and an imaginary input signal to provide an unquantized real output signal and an unquantized imaginary output signal;

wherein the plurality of non-linear resonators are connected together to act as a linear complex operator, the plurality of non-linear resonators being connected in series so that the output of a previous resonator is coupled to the input of the next resonator;

wherein a plurality of outputs from the non-linear resonators are combined to provide the unquantized real output signal to be digitized; and

wherein a plurality of outputs from the non-linear resonators are combined to provide the unquantized imaginary output signal to be digitized.

6. The complex loop filter of claim 5, further comprising a real path adder and an imaginary path adder, the real path adder receiving a first output signal from each of the non-linear resonators and the imaginary path adder receiving a second output signal from each of the non-linear resonators.

7. The complex loop filter of claim 6, wherein the first and second output signals from each resonator are weighted by an equivalent coefficient.

8. The complex loop filter of claim 5, wherein a noise shaping profile achieved is symmetrical around a half sampling frequency such that there is one noise zero located at a desired center frequency below the half sampling frequency.

9. The complex loop filter of claim 6, wherein the real path adder further receives a third output signal from each of the non-linear resonators and the imaginary path adder receiving a fourth output signal from each of the non-linear resonators.

10. The complex loop filter of claim 9, wherein the first and second output signals from each resonator are weighted by an equivalent coefficient and the third and fourth output signals from each resonator are weighted by an equivalent coefficient.

11. A resonator for a modulator within an analog-to-digital converter, comprising:

a real signal path including a first input and first output signals; and

an imaginary signal path including a second input and second output signals;

wherein said resonator acts as a linear complex operator when the second input signal is delayed by half a sample interval and the second output signal is advanced by half a sample interval; and

wherein the resonators are configured to be connected in series so that the output of a previous resonator is coupled to the input of the next resonator.

12. The resonator of claim 11, wherein the imaginary signal path further comprises a third output signal.

13. The resonator of claim 11, further comprising a first integrator coupled between the first input signal and the second output signal and a second integrator coupled between the second input signal and the first output signal.

14. The resonator of claim 11, wherein a relationship between the first input signal (I₁), the second input signal (I₂), the first output signal (O₁) and the second output signal (O₂) is represented by:

$\left(\begin{array}{c}{O}_1\\ O_2\end{array}\right)=\frac{1}{z^2+z<br><br>\left(a<br><br>\text{\hspace{1em}<br><br>}<br><br>b-2\right)+1}<br><br>\left(\begin{array}{cc}-z<br><br>\text{\hspace{1em}<br><br>}<br><br>a<br><br>\text{\hspace{1em}<br><br>}<br><br>b& z<br><br>\text{\hspace{1em}<br><br>}<br><br>b<br><br>\left(1-z\right)\\ -a<br><br>\left(1-z\right)& -z<br><br>\text{\hspace{1em}<br><br>}<br><br>a<br><br>\text{\hspace{1em}<br><br>}<br><br>b\end{array}\right)<br><br>\left(\begin{array}{c}{I}_1\\ I_2\end{array}\right)$

wherein a and b represent coefficients from coefficient blocks within the resonator.

15. The resonator of claim 11, wherein the resonator is a switched-capacitor implementation.

16. An analog-to-digital converter system, comprising:

an analog-to-digital converter coupled to receive real and imaginary input signals and to provide real and imaginary digital output signals;

variable gain circuitry coupled to the analog-to-digital converter the variable gain circuitry comprising input Rain circuitry coupled to the input of the analog-to-digital converter and output gain circuitry coupled to the output of the analog-to-digital converter, and

control circuitry having control signals as an outputs, the control signals being coupled to the variable gain circuitry and acting to adjust the input gain circuitry and the output gain circuitry;

wherein the control circuitry is operable to adjust the control signals to compensate for undesired interference in the output signals from the analog-to-digital converter.

17. The analog-to-digital converter system of claim 16, further comprising input selection circuitry coupled to provide the real and imaginary input signals to the analog-to-digital converter, the input selection circuitry receiving and selecting between multiple sets of signals, and wherein the control circuitry has a selection control signal that is coupled to the input selection circuitry to determine which of the multiple sets of signals are selected by the input selection circuitry.

18. The analog-to-digital converter system of claim 17, wherein the variable gain circuitry comprises output gain circuitry coupled to the imaginary digital output signal to provide a gain adjusted imaginary digital output signal from the analog-to-digital converter and wherein the variable gain circuitry comprises input gain circuitry coupled to the imaginary input signal to provide a Rain adjusted imaginary input signal to the analog-to-digital converter.

19. The analog-to-digital converter system of claim 18, wherein the multiple sets of signals comprises a first set of signals including a real signal and an imaginary signal and a second set of signals including a zero level real signal and a zero level imaginary signal.

20. The analog-to-digital converter of claim 19, wherein the control signal is adjustable to reduce a level of interference in the output signals from the analog-to-digital converter while the selection control signal is asserted to select the zero level real and imaginary signals.

21. The analog-to-digital converter of claim 20, wherein the interference is caused by mismatches within the analog-to-digital converter that folded quantization noise into the desired signal frequency range.

22. The analog-to-digital converter system of claim 16, wherein the analog-to-digital converter comprises:

a real path quantizer having a real digital output signal;

an imaginary path quantizer having an imaginary digital output signal; and

a complex loop filter coupled to receive the real and imaginary input signals and to provide output signals to the real and imaginary path quantizers, said complex loop filter including a plurality of non-linear resonators connected together and acting as a linear complex operator, the plurality of non-linear resonators being connected in series so that the output of a previous resonator is coupled to the input of the next resonator;

wherein a plurality of outputs from the non-linear resonators are combined to provide an output to the real path quantizer; and

wherein a plurality of outputs form the non-linear resonators are combined to provide an output to the imaginary path quantizer.

23. The analog-to-digital converter system of claim 22, further comprising an input delay block coupled to delay the imaginary input signal by half a sample interval relative to the real input signal before the imaginary input signal is received by the complex loop filter and an output advance block coupled to advance the imaginary digital output signal by half a sample interval relative to the real digital output signal.

24. An analog-to-digital converter system, comprising:

an analog-to-digital converter coupled to receive real and imaginary input signals and to provide real and imaginary digital output signals;

variable gain circuitry coupled to the analog-to-digital converter;

control circuitry having a control signal as an output the control signal being coupled to the variable gain circuitry; and

input selection circuitry coupled to provide the real and imaginary input signals to the analog-to-digital converter, the input selection circuitry receiving and selecting between multiple sets of signals, and wherein the control circuitry has a selection control signal that is coupled to the input selection circuitry to determine which of the multiple sets of signals are selected by the input selection circuitry;

wherein the control circuitry is operable to adjust the control signal to compensate for undesired interference in the output signals from the analog-to-digital converter.

25. The analog-to-digital converter system of claim 24, further comprising power estimation circuitry coupled to the output signals from the analog-to-digital converter and having a power estimation signal as an output, the power estimation signal being provided to the control circuitry and the control signal being dependent upon the power estimation signal.

26. The analog-to-digital converter system of claim 24, wherein the variable gain circuitry comprises circuitry coupled to the imaginary input signal to provide a gain adjusted imaginary input signal to the analog-to-digital converter.

27. The analog-to-digital converter system of claim 26, further comprising second variable gain circuitry coupled to the imaginary digital output signal to provide a gain adjusted imaginary digital output signal from the analog-to-digital converter, and wherein the control circuitry has a second control signal as an output that is coupled to the second variable gain circuitry, the control circuitry being also operable to adjust the second control signal to compensate for undesired interference in the output signals from the analog-to-digital converter.

28. The analog-to-digital converter system of claim 24, wherein the multiple sets of signals comprises a first set of signals including a real signal and an imaginary signal and a second set of signals including a real image signal and an imaginary image signal.

29. The analog-to-digital converter of claim 28, wherein the control signal is adjustable to reduce a level of interference in the output signals from the analog-to-digital converter while the selection control is asserted to select the real and imaginary image signals.

30. The analog-to-digital converter of claim 29, wherein the interference is caused by mismatches within the analog-to-digital converter that have folded the image signals into the desired signal frequency range.

31. A method for modulating real and imaginary input signals within an analog-to-digital converter, comprising:

connecting together a plurality of non-linear resonators to act as a linear complex operator for a complex bandpass loop filter for an analog-to-digital converter;

the plurality of non-linear resonators being connected in series so that the output of a previous resonator is coupled to the input of the next resonator;

utilizing a plurality of outputs from the non-linear resonators to provide an output to a real path quantizer and a plurality of outputs from the non-linear resonators to provide an output to an imaginary path quantizer; and

filtering real and imaginary input signals with the complex bandpass loop filter.

32. The method of claim 31, further comprising quantizing the filtered real and imaginary input signals with a real path quantizer and an imaginary path quantizer to produce real and imaginary digital output signals.

33. The method of claim 32, further comprising generating a real path feedback signal by passing real digital output signal through a real path digital-to-analog converter and generating an imaginary feedback signal by passing the imaginary digital output signal through an imaginary digital-to-analog converter.

34. The method of claim 33, farther comprising utilizing a real path adder to subtract the real path feedback signal from the real input signal and utilizing an imaginary path adder to subtract the imaginary feedback signal from the imaginary input signal.

35. The method of claim 32, wherein the providing step further comprises delaying the imaginary input signal by half a sample interval before the imaginary input signal is received by the complex bandpass modulator and advancing the imaginary digital output signal by half a sample interval.

36. The method of claim 31, wherein the producing step further comprises adding together a first output signal from each of the non-linear resonators to obtain the modulated real output signal and adding together a second output signal from each of the non-linear resonators to obtain the modulated imaginary output signal.

37. The method of claim 36, wherein the producing step further comprises weighting the first and second output signals from each resonator by an equivalent coefficient.

38. A method for modulating real and imaginary input signals within an analog-to-digital converter system, comprising:

receiving a real input signal and an imaginary input signal during a normal mode of operation;

receiving a zero level real input signal and a zero level imaginary input signal during an interference compensation mode of operation;

converting the input signals into a real digital output signal and an imaginary digital output signal with an analog-to-digital converter in both the normal and compensation modes of operation;

selecting to receive the real and imaginary input signals when in the normal mode of operation and the zero level real and imaginary input signals when in the interference compensation mode of operation; and

during the compensation mode of operation, adjusting the gain of the imaginary digital output signal to compensate for undesired interference in the output signals from the analog-to-digital converter.

39. The method of claim 38, wherein the interference is caused by mismatches within the analog-to-digital converter that folded quantization noise into the desired signal frequency range.

40. The method of claim 38, further comprising receiving a real input image signal and an imaginary input image signal and adjusting the gain of the imaginary input signal to compensate for undesired interference in the output signals from the analog-to-digital converter during the compensation mode of operation.

41. The method of claim 40, wherein the zero level real and imaginary input signals are received and the imaginary digital output signal is adjusted before the real and imaginary input image signals are received and the imaginary input signal is adjusted.

42. The method of claim 38, wherein the converting step comprises connecting together a plurality of non-linear resonators to act as a linear complex operator for a complex bandpass loop filter for an analog-to-digital converter the resonators being connected in series so that the output of a previous resonator is coupled to the input of the next resonator.

43. A method for modulating real and imaginary input signals within an analog-to-digital converter system, comprising:

receiving a real input signal and an imaginary input signal during a normal mode of operation;

receiving a real input image signal and an imaginary input image signal during a compensation mode of operation;

converting the input signals into a real digital output signal and an imaginary digital output signal with an analog-to-digital converter in both the normal and compensation modes of operation;

selecting to receive the real and imaginary input signals when in the normal mode of operation and the real and imaginary input image signals when in the interference compensation mode of operation; and

during the compensation mode of operation, adjusting the gain of the imaginary input signal to compensate for undesired interference in the output signals from the analog-to-digital converter.

44. The method of claim 43, wherein the interference is caused by mismatches within the analog-to-digital converter that have folded the image signals into the desired signal frequency range.

45. The method of claim 44, wherein the converting step comprises connecting together a plurality of non-linear resonators to act as a linear complex operator for a complex bandpass loop filter for an analog-to-digital converter, the resonators being connected in series so that the output of a previous resonator is coupled to the input of the next resonator.