Apparatuses and methods for sample-rate conversion
Provided are, among other things, systems, apparatuses methods and techniques for automatically adjusting the noise-transfer-function of a modulator which is designed to attenuate the level of unwanted noise and/or distortion in a particular frequency band, without similarly attenuating the level of a desired signal in the same frequency band. One such apparatus includes a processing block for generating and injecting an explicit reference signal, and a processing block for detecting the amplitude of that reference signal.
1. An apparatus for calibrating a noise-shaping modulator, said apparatus comprising:
an input line for receiving an input signal;
a combiner having a first input coupled to the input line, a second input coupled to a reference signal and an output coupled to a first output line;
a frequency downconverter having an input coupled to said first output line and also having a baseband output;
a summation circuit having an input coupled to the baseband output of said frequency downconverter and also having an output; and
an adaptive control unit having an input coupled to the output of said summation circuit and an output coupled to a second output line,
wherein the output of said summation circuit is proportional to a representative signal level at the baseband output of said frequency downconverter, and
wherein the output of said adaptive control unit is continually altered in an attempt to minimize a signal provided at the input of said adaptive control unit.
2. An apparatus according to claim 1 , wherein said combiner is coupled to said first output line via a sampling/quantization component.
3. An apparatus according to claim 1 , wherein said frequency downconverter uses sine and cosine sequences to produce quadrature and in-phase outputs at baseband, and each of said quadrature and in-phase outputs at baseband is coupled to said summation circuit.
4. An apparatus according to claim 1 , wherein said reference signal is a periodic binary pulse at a constant frequency, and wherein a frequency of a sinusoidal sequence used by said frequency downconverter is an integer multiple of said constant frequency.
5. An apparatus according to claim 1 , wherein said reference signal is a square wave at a constant frequency, and wherein a frequency of a sinusoidal sequence used by said frequency downconverter is an odd integer multiple of said constant frequency.
6. An apparatus according to claim 1 , wherein the function of said frequency downconverter is performed by a bandpass filter that employs bandpass moving-average filtering.
7. An apparatus according to claim 1 , wherein the output of said adaptive control unit is generated by said adaptive control unit using at least one of joint optimization, decision-directed feedback, gradient descent, or least squared-error (LSE) principles.
8. An apparatus according to claim 1 , wherein said reference signal includes a narrowband frequency component which corresponds to a frequency of a sinusoidal sequence used by said frequency downconverter.
9. An apparatus according to claim 1 , wherein said frequency downconverter uses at least one of a sine or a cosine sequence to generate said baseband output, and the frequency of said sine or cosine sequence corresponds to a center of an intended noise-transfer-function stopband response.
10. An apparatus according to claim 1 , further comprising a sampling/quantization device having an input coupled to the output of the combiner and an output coupled to the input of the frequency downconverter.
11. An apparatus according to claim 1 , further comprising a noise-shaping filter having an output coupled to the input line.