Sampling/quantization converters
Provided are, among other things, systems, methods and techniques for converting a continuous-time, continuously variable signal into a sampled and quantized signal. According to one representative embodiment, an apparatus includes multiple continuous-time quantization-noise-shaping circuits, each in a separate processing branch and having an adder that includes multiple inputs and an output; an input signal is coupled to one of the inputs of the adder; the output of the adder is coupled to one of the inputs of the adder through a first filter; and the output of a sampling/quantization circuit in the same processing branch is coupled to one of the inputs of the adder through a second filter, with the second filter having a different transfer function than the first filter.
1. An apparatus for converting a continuous-time, continuously variable signal into a sampled and quantized signal, comprising:
an input line for accepting an input signal that is continuous in time and continuously variable; and
a processing branch coupled to the input line, said processing branch including: (a) a continuous-time quantization-noise-shaping circuit, (b) a sampling/quantization circuit coupled to an output of the continuous-time quantization-noise-shaping circuit, (c) a digital bandpass filter coupled to an output of the sampling/quantization circuit, and (d) a line coupling the output of the sampling/quantization circuit back into the continuous-time quantization-noise-shaping circuit;
wherein said continuous-time quantization-noise-shaping circuit comprises an adder that includes a plurality of inputs and an output, with the input signal being coupled to one of the inputs of the adder, the output of the adder being coupled to one of the inputs of the adder through a first filter having a first transfer function, and the output of the sampling/quantization circuit being coupled to one of the inputs of the adder through a second filter having a second transfer function,
wherein the continuous-time quantization-noise-shaping circuit has a noise transfer function with a minimum at a particular frequency,
wherein a minimum in the quantization-noise transfer function of the quantization-noise-shaping continuous-time circuit corresponds to a frequency band selected by the digital bandpass filter, and
wherein each of the first transfer function and the second transfer function is at least one of a weighted sum or difference of two filter responses that have group delay and insertion gain that: (i) are approximately constant over the frequency band selected by the digital bandpass filter, and (ii) approach zero at frequencies outside the frequency band selected by the digital bandpass filter.
2. An apparatus according to claim 1 , wherein the continuous-time quantization-noise-shaping circuit has a noise transfer function that shapes quantization noise in the frequency band of the digital bandpass filter with an equivalent of a second-order response.
3. An apparatus according to claim 1 , wherein the continuous-time quantization-noise-shaping circuit has a noise transfer function that shapes quantization noise in the frequency band of the digital bandpass filter with an equivalent of a fourth-order response.
4. An apparatus according to claim 1 , wherein the continuous-time quantization-noise-shaping circuit has a noise transfer function that shapes quantization noise in the frequency band of the digital bandpass filter with an equivalent of a sixth-order response.
5. An apparatus according to claim 1 , wherein the sampling/quantization circuit is a single-bit quantizer.
6. An apparatus according to claim 1 , wherein the sampling/quantization circuit is a multi-bit quantizer.
7. An apparatus according to claim 1 , further comprising an active calibration component that adjusts a parameter of at least one component of the continuous-time quantization-noise-shaping circuit.
8. An apparatus according to claim 1 , wherein each of the first transfer function and the second transfer function non-trivially modifies at least one of amplitude and phase.
9. An apparatus according to claim 8 , wherein the first transfer function delays by no less than ¼ of a sampling period used by the sampling/quantization circuit (T S ).
10. An apparatus according to claim 9 , wherein each of the first transfer function and the second transfer function comprises delay elements that are implemented using transmission lines.
11. An apparatus according to claim 8 , wherein each of the first transfer function and the second transfer function comprises low-pass filtering with a 3 dB corner frequency that is no more than 3·f S and no less than the bandwidth of the continuous-time, continuously variable input signal, where f S is the sample rate of the sampling/quantization circuit.
12. An apparatus according to claim 8 , wherein the continuous-time quantization-noise-shaping circuit shapes quantization noise with a second-order response.
13. An apparatus according to claim 8 , wherein the continuous-time quantization-noise-shaping circuit shapes quantization noise with a fourth-order response.
14. An apparatus according to claim 8 , wherein the continuous-time quantization-noise-shaping circuit shapes quantization noise with a sixth-order response.
15. An apparatus according to claim 1 , wherein each of the first transfer function and the second transfer function is one of the weighted sum or difference of two lowpass filter responses.
16. An apparatus according to claim 1 , wherein the first filter is implemented as a first component followed by a third component, and wherein the second filter is implemented as a second component followed by the third component.
17. An apparatus according to claim 1 , wherein at least a portion of the second transfer function is integrated into the sampling/quantization circuit.
18. An apparatus according to claim 1 , further comprising a configurable component that adjusts the response of the quantization-noise-shaping continuous-time circuit by varying a parameter of at least one component of the continuous-time quantization-noise-shaping circuit.
19. An apparatus according to claim 18 , wherein the configurable component dynamically adjusts the noise transfer function of the quantization-noise-shaping continuous-time circuit to minimize at least one of absolute value or power of residual quantization noise at an output of the digital bandpass filter.
20. An apparatus according to claim 18 , wherein the configurable component controls the location of the spectral minimum in the noise transfer function of the continuous-time quantization-noise-shaping continuous-time circuit, so that the spectral minimum can be placed at arbitrary locations.
21. An apparatus according to claim 1 , further comprising at least one additional processing branch coupled to the input line, and wherein each of said processing branches converts a different frequency band of the input signal.
22. An apparatus according to claim 21 , wherein there are a total of M said processing branches, wherein the sampling/quantization circuit in each said processing branch samples at a rate of 2·N·M·BW samples per second, with BW being the bandwidth of the frequency band for the corresponding processing branch, and wherein M×N is at least 16.
23. An apparatus according to claim 21 , wherein there are at least 16 said processing branches.
24. An apparatus according to claim 21 , wherein the minima of the noise transfer functions of the continuous-time quantization-noise-shaping circuits in said processing branches are spaced at equal frequency intervals.
25. An apparatus according to claim 21 , wherein the minima of the noise transfer functions of the continuous-time quantization-noise-shaping circuits in said processing branches are spaced at non-equal frequency intervals.
26. An apparatus according to claim 21 , wherein the digital bandpass filter in each of a plurality of said processing branches comprises a frequency downconverter, a low-pass filter, and a frequency upconverter.
27. An apparatus according to claim 26 , wherein at least one of the digital bandpass filters incorporates a recursive moving average structure.
28. An apparatus according to claim 26 , wherein at least one of the digital bandpass filters is comprised of cascaded moving average filter stages that collectively provide spectral minima at frequencies other than multiples of f S /(N·M), where f S is the sample rate of the sampling/quantization circuit, N is an excess-rate oversampling ratio, and M is the number of said processing branches.
29. An apparatus according to claim 26 , wherein at least one of the digital bandpass filters is implemented as a polyphase decomposition structure.
30. An apparatus according to claim 29 , wherein a polyphase decomposition factor, m, of the polyphase decomposition structure is a submultiple of the number of processing branches, M.
31. An apparatus according to claim 26 , wherein at least one of the digital bandpass filters is comprised of cascaded moving average filter stages, wherein each of the stages averages the same number of samples, and wherein the number of stages is other than one plus an effective noise-shaping order of the continuous-time quantization-noise-shaping circuit response.
32. An apparatus according to claim 31 , wherein the number of moving average stages is one of zero, two, three, or four plus an effective noise-shaping order of the continuous-time quantization-noise-shaping circuit response.
33. An apparatus according to claim 26 , wherein at least one of the digital bandpass filters is comprised of cascaded moving average filter stages, and wherein at least one of the stages averages a number of samples that is different by more than one from a number of samples averaged by the other stages.
34. An apparatus according to claim 33 , wherein the number of samples averaged by one of the stages is different from the other stages by more than ¼·M·N and less than 2·M·N, where N is an excess-rate oversampling ratio and M is the number of processing branches.
35. An apparatus according to claim 26 , wherein at least one of the digital bandpass filters is comprised of cascaded moving average filter stages having an impulse response length that is at least 2·N·M−5, where N is an excess-rate oversampling ratio and M is the number of processing branches.
36. An apparatus according to claim 26 , wherein at least one of the digital bandpass filters is comprised of cascaded moving average filter stages having an impulse response length that is at least 4·N·M−5, where N is an excess-rate oversampling ratio and M is the number of processing branches.
37. An apparatus according to claim 26 , wherein at least one of the digital bandpass filters is comprised of a recursive window filter having an impulse response length that is at least 2·N·M−2, where N is an excess-rate oversampling ratio and M is the number of processing branches.
38. An apparatus according to claim 26 , wherein at least one of the digital bandpass filters is comprised of a recursive window filter having an impulse response length that is at least 4·N·M−4, where N is an excess-rate oversampling ratio and M is the number of processing branches.
39. An apparatus according to claim 26 , wherein at least one of the digital bandpass filters further includes an equalizer.
40. An apparatus according to claim 39 , wherein the equalizer has a single complex tap.
41. An apparatus according to claim 39 , wherein the equalizer has a single real tap.
42. An apparatus according to claim 39 , wherein the equalizer has plural complex taps.
43. An apparatus according to claim 39 , wherein the equalizer has plural real taps.
44. An apparatus according to claim 26 , wherein sine and cosine sequences are used for quadrature downconversion and upconversion and are generated using a direct digital synthesis method based on digital accumulators and sinusoid lookup tables.
45. An apparatus according to claim 26 , wherein sine and cosine sequences are used for quadrature downconversion and upconversion and are generated using recursive structures.
46. An apparatus according to claim 21 , wherein the sampling/quantization circuit in each of a plurality of said processing branches samples at a frequency of at least 1 gigahertz (GHz).
47. An apparatus according to claim 21 , wherein the digital bandpass filters in different ones of said processing branches select different frequency bands that collectively cover a desired frequency range.
48. An apparatus according to claim 21 , wherein the digital bandpass filters in different ones of said processing branches have at least one of: (i) center frequencies that are spaced at equal intervals, and (ii) bandwidths that are equal to each other.
49. An apparatus according to claim 21 , wherein the digital bandpass filters in different ones of said processing branches have at least one of: (i) center frequencies that are spaced at non-equal intervals, and (ii) bandwidths that are not all equal to each other.
50. An apparatus according to claim 21 , wherein a configuration means is provided to enable plural modes of operation, including: (i) a mode for operation as a single, wideband converter with high output data rate, and (ii) a mode for operation as plural, independent converters having an arbitrary mix of narrowband and wideband outputs at various sample rates.
51. An apparatus according to claim 50 , wherein the configuration means comprises an add-multiplex array.
52. An apparatus according to claim 50 , wherein the number of processing branches M is a power of two.
53. An apparatus according to claim 21 , wherein there are at least 4 said processing branches, and in each of a plurality of said processing branches: the continuous-time quantization-noise-shaping circuit includes at least a second-order feedback filter, the sampling/quantization circuit samples at a frequency of at least 1 gigahertz (GHz), and the digital bandpass filter attenuates shaped quantization noise by at least 40 dB and introduces total aliasing distortion power limited to less than 40 dB below data converter signal level.
54. An apparatus according to claim 21 , wherein the continuous-time quantization-noise-shaping circuits in different ones of said processing branches have identical, or substantially identical, circuit configurations, and wherein different processing branches operate on different frequency bands by setting corresponding parameter values.
55. An apparatus according to claim 54 , wherein at least one of the lowpass filter responses is between second to fifth order, inclusively.
56. An apparatus according to claim 21 , further comprising an adder coupled to outputs of said processing branches.