Abs+act― Two second-order bandpass delta-sigma A/D modulators have been implemented in a 0.8 μ BiCMOS process to demonstrate the feasibility of converting a 10.7 MHz radio IF signal to digital form. The circuits, based on switched-capacitor biquads, demonstrated 57 dB SNR in a 200 kHz bandwidth when clocked at 42.8 MHz, dissipating 60 mW from a 5 V supply. The two modulators use different clocking strategies to allow evaluation of a tradeoff between active and passive sensitivites.

Switched-capacitor bandpass delta-sigma A/D modulation at 10.7 MHz

Ultrasonic arrays are increasingly widely used in nondestructive evaluation (NDE) due to their greater flexibility and potentially superior performance compared to conventional monolithic probes. The characterization of small defects remains a challenge for NDE and is of great importance for determining the impact of a defect on the integrity of a structure. In this paper, a technique for characterizing reflectors with subwavelength dimensions is described. This is achieved by post-processing the complete data set of time traces obtained from an ultrasonic array using two algorithms. The first algorithm is used to obtain information about reflector orientation and the second algorithm is used to distinguish between point-like reflectors that reflect uniformly in all directions and specular reflectors that have distinct orientations. Experimental results are presented using a commercial 64-element, 5-MHZ array on two aluminum test specimens that contain a number of machined slots and side-drilled holes. The results show that the orientation of 1-mm-long slots can be determined to within a few degrees and that the signals from 1-mm-long slots can be distinguished from that from a 1-mm-diameter circular hole. Techniques for quantifying both the orientation and the specularity of measured signals are presented and the effect of processing parameters on the accuracy of results is discussed.

Advanced Reflector Characterization with Ultrasonic Phased Arrays in NDE Applications

This paper presents a pixel pitch-matched readout chip for 3-D photoacoustic (PA) imaging, featuring a dedicated signal conditioning and delta-sigma modulation integrated within a pixel area of 250 $\mu \text{m}$ by 250 $\mu \text{m}$ . The proof-of-concept receiver was implemented in an STMicroelectronics’s 28-nm Fully Depleted Silicon On Insulator technology, and interfaces to a $4 \times 4$ subarray of capacitive micromachined ultrasound transducers (CMUTs). The front-end signal conditioning in each pixel employs a coarse/fine gain tuning architecture to fulfill the 90-dB dynamic range requirement of the application. The employed delta-sigma beamforming architecture obviates the need for area-consuming Nyquist ADCs and thereby enables an efficient in-pixel A/D conversion. The per-pixel switched-capacitor $\Delta \Sigma $ modulator leverages slewing-dominated and area-optimized inverter-based amplifiers. It occupies only 1/4th of the pixel, and its area compares favorably with state-of-the-art designs that offer the same SNR and bandwidth. The modulator’s measured peak signal-to-noise-and-distortion ratio is 59.9 dB for a 10-MHz input bandwidth, and it consumes 6.65 mW from a 1-V supply. The overall subarray beamforming approach improves the area per channel by 7.4 times and the single-channel SNR by 8 dB compared to prior art with similar delay resolution and power dissipation. The functionality of the designed chip was evaluated within a PA imaging experiment, employing a flip-chip bonded 2-D CMUT array.

A Pixel Pitch-Matched Ultrasound Receiver for 3-D Photoacoustic Imaging With Integrated Delta-Sigma Beamformer in 28-nm UTBB FD-SOI

A wide variety of beamforming approaches are applied in modern ultrasound scanners, ranging from optimal time domain beamforming strategies at one end to rudimentary narrowband schemes at the other. Although significant research has been devoted to improving image quality, usually at the expense of beamformer complexity, we are interested in investigating strategies that sacrifice some image quality in exchange for reduced cost and ease in implementation. This paper describes the direct sampled in-phase/quadrature (DSIQ) beamformer, which is one such low-cost, extremely simple, and compact approach. DSIQ beamforming relies on phase rotation of I/Q data to implement focusing. The I/Q data are generated by directly sampling the received radio frequency (RF) signal, rather than through conventional demodulation. We describe an efficient hardware implementation of the beam-former, which results in significant reductions in beam-former size and cost. We present the results of simulations and experiments that compare the DSIQ beamformer to more conventional approaches, namely, time delay beamforming and traditional complex demodulated I/Q beam-forming. Results that show the effect of an error in the direct sampling process, as well as dependence on signal bandwidth and system f number (f#) are also presented. These results indicate that the image quality and robustness of the DSIQ beamformer are adequate for low end scanners. We also describe implementation of the DSIQ beamformer in an inexpensive hand-held ultrasound system being developed in our laboratory.

Direct sampled I/Q beamforming for compact and very low-cost ultrasound imaging

A real-time 3-D imaging system requires the development of a beamformer that can generate many beams simultaneously. In this paper, we discuss and evaluate a suitable synthetic aperture beamformer. The proposed beamformer is based on a pipelined network of high speed digital signal processors (DSP). By using simple interpolation-based beamforming, only a few calculations per pixel are required for each channel, and an entire 2-D synthetic aperture image can be formed in the time of one transmit event. The performance of this beamformer was explored using a computer simulation of the radiation pattern. The simulations were done for a full 64-element array and a sparse array with the same receive aperture but only five transmit elements. We assessed the effects of changing the sampling rate and amplitude quantization by comparing the relative levels of secondary lobes in the radiation patterns. The results show that the proposed beamformer produces a radiation pattern equivalent to a conventional beamformer using baseband demodulation, provided that the sampling rate is approximately 10 times the center frequency of the transducer (34% bandwidth pulse). The simulations also show that the sparse array is not significantly more sensitive to delay or amplitude quantization than the full array.

Theoretical assessment of a synthetic aperture beamformer for real-time 3-D imaging

The principles of oversampling are exploited in a simple beamforming architecture using a single bit delta-sigma (/spl Delta/C) analog to digital converter (A/D) on every channel. The high sampling rate required for the single bit A/D provides adequate delay accuracy for high quality beamforming using elementary sample manipulations. Images produced with this beamformer exhibit significant artifacts directly related to dynamic focusing. However, a simple digital recording technique following delays permits dynamically focused beamforming without degrading image quality. The simplicity of this beamformer compared to conventional methods may facilitate very large channel count or low power beamformers suitable for 1.5-D arrays or portable scanners.

Delta-sigma oversampled ultrasound beamformer with dynamic delays

Delta-sigma (/spl Delta//spl Sigma/) modulators can implement a simpler digital ultrasound beamformer than can traditional architectures based on multi-bit analog-to-digital converters (A/D). The signal-to-noise ratio (SNR) of the /spl Delta//spl Sigma/ modulators, however, suffers from limited oversampling ratios. To improve the SNR of each channel, a mixing signal heterodynes narrowband signals to lower frequencies where the baseband /spl Delta//spl Sigma/ modulator performs better. Noise figure analyses are presented that illustrate the effectiveness of this technique in improving noise performance. Also, spectral Doppler and color flow simulations are presented that realistically emulate a 32 channel oversampled beamformer and compare these results with traditional and ideal systems.

Heterodyning technique to improve performance of delta-sigma-based beamformers

Oversampled analog to digital converters (delta-sigma A/Ds) have been proposed to radically reduce the size, cost and power consumption of digital ultrasound beamformers. Simply replacing conventional A/Ds with single bit oversampled versions, however, produces significant image artifacts for a dynamic focus system. In addition, spectral Doppler measurements are severely compromised. Here, the authors disclose a complete beamformer exploiting oversampling principles on both transmit and receive that doesn't suffer from these problems. The architecture preserves the simplicity of an oversampled design, yet produces high quality B-scan and color flow images, as well as sensitive spectral Doppler measurements. Here, the authors present simulations and emulations using actual array data, to demonstrate the capabilities of the system.

An ultrasound beamformer using oversampling

High-performance and efficient beamforming circuitry is very important in large channel count clinical ultrasound systems. Current state-of-the-art digital systems using multi-bit analog to digital converters (A/Ds) have matured to provide exquisite image quality with moderate levels of integration. A simplified oversampling beamforming architecture has been proposed that may a low integration of delta-sigma A/Ds onto the same chip as digital delay and processing circuitry to form a monolithic ultrasound beamformer. Such a beamformer may enable low-power handheld scanners for high-end systems with very large channel count arrays. This paper presents an oversampling beamformer architecture that generates high-quality images using very simple; digitization, delay, and summing circuits. Additional performance may be obtained with this oversampled system for narrow bandwidth excitations by mixing the RF signal down in frequency to a range where the electronic signal to nose ratio of the delta-sigma A/D is optimized. An oversampled transmit beamformer uses the same delay circuits as receive and eliminates the need for separate transmit function generators.

Efficient high-performance ultrasound beamforming using oversampling

A 1536 element, 1.5D, curved linear array was designed to be incorporated into a probe containing integrated beamforming, front end and drive electronics as part of a DARPA initiative on portable battlefield medical diagnostic equipment. Phase aberration correction strategies employed by the ultrasound system require acoustic element performance provided by the more demanding phased array element type. Signal-to-noise (SNR) improvement strategies employed by the probe-mounted ultrasound system further complicate the probe construction. The selected array architecture, whereby the final array assembly is formed by assembling eight autonomous 192 element sub-array assemblies, is discussed. Heat transfer problems encountered when combining active electronics with an ultrasound array in a probe handle are quantified, and a design strategy to manage them is presented. An indium flex lead plating process is presented. Finally, a parasitic effect caused by wraparound electrodes, discussed previously, is presented briefly again because of its importance in large element count array construction.

R.C. Anderson

Papers

Delta-sigma oversampled ultrasound beamformer with dynamic delays

Heterodyning technique to improve performance of delta-sigma-based beamformers

An ultrasound beamformer using oversampling

Efficient high-performance ultrasound beamforming using oversampling

Development of a 1.5D, 1536 element ultrasonic array for use with integrated electronics