Conversion of a discrete-time quantized signal into a continuous-time, continuously variable signal
Provided are, among other things, systems, apparatuses, methods and techniques for converting a discrete-time quantized signal into a continuous-time, continuously variable signal. An exemplary converter preferably includes: (1) multiple oversampling converters, each processing a different frequency band, operated in parallel; (2) multirate (i.e., polyphase) delta-sigma modulators (preferably second-order or higher); (3) multi-bit quantizers; (4) multi-bit-to-variable-level signal converters, such as resistor ladder networks or current source networks; (5) adaptive nonlinear, bit-mapping to compensate for mismatches in the multi-bit-to-variable-level signal converters (e.g., by mimicking such mismatches and then shifting the resulting noise to a frequently range where it will be filtered out by a corresponding bandpass (reconstruction) filter); (6) multi-band (e.g., programmable noise-transfer-function response) bandpass delta-sigma modulators; and/or (7) a digital pre-distortion linearizer (DPL) for canceling noise and distortion introduced by an analog signal bandpass (reconstruction) filter bank.
1. An apparatus for converting a discrete-time quantized signal into a continuous-time, continuously variable signal, comprising:
an input line for accepting an input signal that is discrete in time and in value;
a digital pre-distortion linearizing filter having an input coupled to the input line and an output;
a plurality of processing branches having inputs coupled to the output of the digital pre-distortion linearizing filter, and each of said processing branches including: (a) a discrete-time noise-shaping/quantization circuit, (b) a multi-bit-to-variable-level signal converter coupled to an output of the discrete-time noise-shaping/quantization circuit, and (c) an analog bandpass filter coupled to an output of the multi-bit-to-variable-level signal converter; and
an adder coupled to an output of the analog bandpass filter in each of the processing branches,
wherein each processing branch converts a portion of the input signal spectrum,
wherein the discrete-time noise-shaping/quantization circuits in different ones of the plurality of processing branches have conversion-noise frequency-response minima at different frequencies,
wherein a frequency response of each of the discrete-time noise-shaping/quantization circuits has minimum conversion noise in a region that corresponds to a frequency band selected by the analog bandpass filter in the same processing branch, and
wherein the digital pre-distortion linearizing filter has a transfer function that compensates for at least one of a phase distortion or an amplitude distortion introduced by a filter bank formed by the analog bandpass filters.
2. An apparatus according to claim 1 , wherein the discrete-time noise-shaping/quantization circuit in each of a plurality of the processing branches comprises an assembly that includes:
an input;
a second adder having a first input coupled to the input of the discrete-time noise- shaping/quantization circuit, a second input and an output;
a quantization circuit that reduces a bit-width resolution of a signal, that has an input coupled to the output of the second adder, and that has an output coupled to the output of the discrete-time noise-shaping/quantization circuit; and
a feedback-loop filter having an input coupled to the output of the quantization circuit and an output coupled to the second input of the second adder.
3. An apparatus according to claim 2 , wherein the input of the quantization circuit also is coupled to the input of the feedback-loop filter so that both the input and the output of the quantization circuit are coupled to the second input of the adder through the feedback-loop filter.
4. An apparatus according to claim 2 , wherein the output of the quantization circuit comprises at least 4 bits.
5. An apparatus according to claim 2 , wherein for at least one of the processing branches, the discrete-time noise-shaping/quantization circuit includes a plurality of plural parallel paths and said assembly is provided in each of said plurality of plural parallel paths, with each said parallel path generating a different subsampling phase of a complete signal that is output by the discrete-time noise-shaping/quantization circuit.
6. An apparatus according to claim 2 , wherein the feedback-loop filter in each of a plurality of the processing branches has a conversion noise transfer function with a plurality of zeros, and wherein none of said zeros is equal to any other of said zeros.
7. An apparatus according to claim 6 , wherein each of said zeros is produced by a settable parameter of the feedback-loop filter that is represented by not more than 6 bits.
8. An apparatus according to claim 2 , wherein the feedback-loop filters have identical structures across the plurality of processing branches, but have different values for at least one settable parameter.
9. An apparatus according to claim 1 , wherein at least one of the processing branches includes non-linear bit mapping to compensate for imperfections in scaling parameters of the multi-bit-to-variable-level signal converter in said at least one of the processing branches.
10. An apparatus according to claim 9 , wherein the non-linear bit mapping is dynamically adjusted based on an error metric that minimizes output conversion noise.
11. An apparatus according to claim 1 , wherein the multi-bit-to-variable-level signal converter in each of a plurality of the processing branches comprises at least one of: a network of weighted resistors, a network of weighted voltage sources, or a network of weighted current sources.
12. An apparatus according to claim 1 , further comprising at least one additional digital pre-distortion linearizing filter having an input coupled to the input line and an output coupled to the processing branches, wherein a combined transfer function of the digital pre-distortion linearizing filters compensates for a least one of a phase distortion or an amplitude distortion introduced by a filter bank formed by the analog bandpass filters in the processing branches.
13. An apparatus according to claim 1 , wherein the digital pre-distortion linearizing filter is implemented as a polyphase structure and includes, for each subsampling phase, a substructure that operates at a clock rate that is a submultiple of a clock rate for the digital pre-distortion linearizing filter as a whole.
14. An apparatus according to claim 1 , wherein the digital pre-distortion linearizing filter includes both feedforward and feedback components.
15. An apparatus according to claim 1 , where the analog bandpass filters include at least one of a Butterworth, Bessel, and elliptic filter structure.
16. An apparatus according to claim 1 , wherein the discrete-time noise-shaping/quantization circuit in at least one of the plurality of the processing branches employs a parallel noise-shaping structure to produce a noise-shaped response which has an order greater than two.
17. An apparatus according to claim 1 , wherein the discrete-time noise-shaping/quantization circuit in each of a plurality of the processing branches is implemented as a polyphase structure and includes separate parallel substructures for generating different corresponding subsampling phases of a complete signal that is output by the discrete-time noise-shaping/quantization circuit as a whole.
18. An apparatus according to claim 17 , wherein outputs of the substructures are combined into a single composite output using at least one multiplexer.
19. An apparatus according to claim 17 , wherein outputs of the substructures are combined into a single composite output using at least one inverse moving-average filter in conjunction with at least one summing component.
20. An apparatus according to claim 17 , wherein outputs of the substructures are combined into a single composite output using a combination of at least one multiplexer and at least one inverse moving-average filter in conjunction with at least one summing component.