Beamformed ultrasonic imager with delta-sigma feedback control
A method and apparatus are provided for reducing distortion in a dynamically delayed digital sample stream of an imaging system. The method includes the steps of delta-sigma modulating an input analog signal of the imaging system at a frequency above the Nyquist frequency of the input analog signal to generate a digital sample stream and changing a length of the sample stream to delay a portion of the sample stream while maintaining synchronism between a delta-sigma modulator and a demodulator of the system.
1. A method of improving system noise performance of a delta-sigma based dynamically delayed beamformer receiving a plurality of analog signal streams from a plurality of transducers of a transducer array and providing an amplitude modulated output signal corresponding to a signal intensity as a function of range from the transducer array, such method comprising the steps of:
downconverting the plurality of analog signal streams on a plurality of processing channels by mixing each input signal stream of the plurality of analog input signal streams with a periodic signal having a fundamental frequency greater than zero Hertz but less than twice a carrier center frequency of the received analog sigal streams;
digitizing each mixed signal stream within a delta-sigma modulator;
dynamically delaying a corresponding set of samples among the digitized signal streams within independent delay lines to compensate each sample of the corresponding set of samples for a sample source's geometric origin relative to a desired dynamic receive focus;
dynamically adjusting the phase of the periodic signal on each channel of the plurality of processing channels based upon a total delay applied to the sample stream of that channel;
summing the corresponding set of delayed samples; and
basebanding and low pass filtering the stream of summed samples to provide an output signal whose amplitude corresponds to the signal intensity of the formed beam as a function of range.
2. The method as in claim 1 further comprising the step of time gain compensating each analog signal of each analog signal stream of the plurality of analog signal streams based upon a distance of the signal from the transducer.
3. The method as in claim 1 wherein the step of digitzing each mixed signal stream within a delta-sigma modulator further comprising sampling the mixed signal stream above the Nyquist frequency for the signal steam.