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 full-rate samples of an input signal that are discrete in time and in value, that are separated in time by a full-rate sampling period, and that represent a signal sampled at a full-rate sampling frequency corresponding to the full-rate sampling period;
a parallel signal processor having an input coupled to said input line and having a plurality of sub-rate outputs, each sub-rate output providing a different subsampling phase of a complete signal that is output by said apparatus;
a plurality of delay elements which are coupled to the plurality of sub-rate outputs provided by said parallel signal processor,
a plurality of multi-bit-to-variable-level signal converters which are coupled to said plurality of delay elements, and which operate at a sampling rate that is less than or equal to the full-rate sampling frequency of said input signal; and
a signal combiner coupled to an output of each of said first and second multi-bit-to-variable-level signal converters,
wherein each of said delay elements provides to one of said multi-bit-to-variable-level signal converters, an output which is offset in time by an increment equaling an integer multiple of the full-rate sampling period, and
wherein the outputs of said multi-bit-to-variable-level signal converters are combined by the signal combiner, as continuous-time signals at the sampling rate of said sub-rate outputs, using a summing operation.
2. The apparatus according to claim 1 , wherein there are a total of m delay elements, and values at the output of each of said delay elements reflect subsampling at a rate which is a factor of 1/m times the full-rate sampling frequency of said input signal.
3. The apparatus according to claim 1 , wherein said delay elements together with said signal combiner, produce a filter response having a lowpass cutoff frequency that is less than or equal to the full-rate sampling frequency of said input signal, and greater than or equal to a maximum frequency component of said input signal.
4. An apparatus according to claim 1 , wherein at least one of said multi-bit-to-variable-level signal converters comprises at least one of: a network of weighted resistors, a network of weighted voltage sources, or a network of weighted current sources.
5. An apparatus according to claim 1 , wherein said parallel signal processor has exactly m sub-rate outputs and the sampling rate of each of said sub-rate outputs is greater than 1/m times the full-rate sampling frequency of said input signal.
6. An apparatus according to claim 5 , wherein said parallel signal processor further comprises a plurality of noise-shaped quantization circuits, each of which accepts input samples and generates output samples, wherein said output samples of said noise-shaped quantization circuits have at least one of a lower resolution or a higher sampling rate than said input samples.
7. The apparatus according to claim 1 , wherein said delay elements together with said signal combiner produce a filter response having a lowpass cutoff frequency that is less than or equal to one-half of the full-rate sampling frequency of said input signal.
8. The apparatus according to claim 1 , wherein the maximum frequency component of said input signal is less than or equal to one-half of said full-rate sampling frequency.
9. An apparatus according to claim 1 , wherein said parallel signal processor has exactly m sub-rate outputs and the sampling rate of each of said sub-rate outputs is 1/m times the full-rate sampling frequency of said input signal.
10. The apparatus according to claim 9 , wherein a maximum frequency component of said input signal is approximately equal to
1
2
·
m
times the full-rate sampling frequency of said input signal.
11. An apparatus according to claim 1 , wherein at least one of said delay elements introduces an offset in time via digital resampling on different phases of a sub-rate clock.
12. The apparatus according to claim 1 , wherein at least one of said delay elements introduces an offset in time via signal propagation through a continuous-time delay line.
13. The apparatus according to claim 12 , wherein said delay line is entirely passive.
14. The apparatus according to claim 12 , wherein said delay line includes an active component.
15. The apparatus according to claim 1 , wherein all discrete-time components of said apparatus operate at a maximum switching rate which is less than the full-rate sampling frequency of said input signal.
16. The apparatus according to claim 1 , wherein said delay elements together with said signal combiner produce a filter response which is approximately a sinc function.
17. The apparatus according to claim 1 , wherein each of the outputs from said multi-bit-to-variable-level signal converters, which are coupled to said signal combiner, has been shaped according to a non-rectangular window to produce a filter response having an upper stopband with attenuation greater than a sinc function.
18. The apparatus according to claim 1 , wherein the sub-rate outputs of said parallel signal processor are coupled to inputs of said signal combiner and combined within said signal combiner without upsampling.
19. The apparatus according to claim 1 , wherein at least one of said delay elements and said summing operation together implement an operation that comprises a moving-average summation.
20. The apparatus according to claim 1 , wherein said parallel signal processor performs estimation and mitigation of at least one of clock jitter or sample-rate skew.
21. The apparatus according to claim 20 , wherein said parallel signal processor dynamically resamples a signal provided to the input of said parallel signal processor to reflect sampling skew that is approximately equal and opposite to sampling skew introduced by imperfections in at least one of said delay elements.
22. The apparatus according to claim 21 , wherein resampling is dynamically adjusted to minimize a level of at least one spurious signal present in the complete signal that is output by said apparatus.