| 摘要 |
A multiplierless single-bit dynamic focusing method is presented which can completely eliminate the demodulation error. In the proposed method, dynamic focusing is performed at the Nyquist rate, FN, for the received signal, meaning that the time interval between successive imaging points is the reciprocal of FN. For each imaging point, a single-bit SDM sample is selected from each active array channel, according to the corresponding focusing delay. Then, a block of data, centered at the selected SDM sample, is defined for each channel. Next, bit-wise block addition is performed to obtain the sum of all the blocks. Finally, the output of the block addition is fed to the demodulation filter. In consequence, if the block length is set equal to the demodulation filter length, the proposed method will produce the focused signal without any signal distortion. Since the demodulation filter length L is generally larger than the over sampling ratio M, however, the proposed method requires using K(=┌L/M┐) demodulation filters. In another approach, the demodulation filters are placed just behind the sigma-delta modulators. This modification does not change the final output, but requires using K filters per each channel. In each channel, the K filters produce successively the demodulated signals for K consecutive imaging points, which are stored in a two-port memory as they are generated. Then, by taking the sum of the samples retrieved from the same memory locations, dynamic focusing is achieved. Since each demodulation filter takes a single-bit SDM sequence as an input data, it can be implemented with a simple accumulator which is to calculate the sum of the filter coefficients that are to be multiplied by input samples. Consequently, a multiplierless SDM beamformer with dynamic focusing capability can be implemented. This new single-bit dynamic beamformer uses only K simple accumulators per each channel.
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