2024
Singhvi, A. (2024). Overcoming the sensitivity-bandwidth limit for next-generation air-coupled ultrasonic and thermoacoustic sensing. Stanford University.
(Note: The dissertation is embargoed to Stanford affiliates only until 2025, but I’m happy to share it if you shoot me an email).
Air-coupled ultrasound shows promise in several application spaces —— autonomous detection, 2-D and 3-D imaging, wireless communication and powering, among others. It is an increasingly lucrative sensing modality due to the ease of building scalable, cost, and power efficient ultrasonic systems that offer robust operation in challenging operational environments. Most current air-coupled ultrasonic systems either use off-the-shelf microphones that provide suboptimal performance or custom sensors that work well only for a specific application. Both these approaches are also limited by the fundamental sensitivity-bandwidth limit for ultrasound sensors, wherein one can achieve either long-range detection or fine image resolution, but not both simultaneously. Moreover, in most cases, due to its high reflectivity, air-coupled ultrasound cannot communicate with or sense targets that are embedded in a medium other than air.
In this work, we present techniques that allow for cross-medium acoustic sensing and overcome the inherent sensitivity-bandwidth tradeoff of resonant ultrasonic sensors. First, we demonstrate a novel multi-modal, non-contact thermoacoustic imaging system that enables imaging across boundaries using air-coupled ultrasonic sensors. Next, we introduce a coded-excitation scheme that allows us to improve and dynamically tune the sensitivity-bandwidth product of ultrasonic sensors. We then describe the design of a custom CMOS chip that facilitates multi-frequency ultrasonic operation for robust and programmable ultrasonic sensing with the capability to detect sub-100 µPa of pressure while achieving greater than 15% fractional bandwidth. Finally, we will present some application-oriented highlights and results made possible by the proposed techniques as it relates to remote underground and underwater sensing. The system and design approaches presented in this work demonstrate how to build a cost-effective, robust, and reconfigurable, next-generation air-coupled ultrasonic platform for ubiquitous ultrasonic sensing.
2023
A W-Band Transceiver Array with 2.4 GHz LO Synchronization Enabling Full Scalability for FMCW Radar
Zhang, Jingzhi,
Singhvi, Ajay,
Ahmed, Sherif S,
and Arbabian, Amin
In IEEE International Solid-State Circuits Conference (ISSCC)
2023
Closing the angular resolution gap between CMOS radar and optical imaging systems can enable an entirely new cost-effective radar-centric perception solution, but requires extremely large transceiver (TRX) arrays to achieve LiDAR-like angular resolution. Multi-chip cascading of mm-wave radars [1–3] has become the norm to enable these large TRX arrays, but the size of these arrays is still limited due to challenges in achieving low-cost signal distribution across a large aperture. Today, multi-chip radar cascading solutions use mm-wave LO frequencies (20GHz [1], [2], 40GHz [3]) along with on-chip frequency multipliers with modest multiplication factors (×4 [1], [2], ×2 [3]). However, operating at these frequencies is cost-prohibitive and severely limits the size of the array [1]. For example, for a 64-TX and 64-RX array, the calculated path loss on a Rogers 3003 substrate in an H-tree distribution network reaches 87dB at 40GHz, which requires more than 70 amplifiers with 15dBm output power and 25dB gain alongside the distribution network to compensate the loss. Moreover, maintaining phase coherency required for FMCW systems is also infeasible for such amplifier implementations. Thus, enabling truly scalable TRX arrays requires signal distribution at much lower LO frequencies, which introduces fundamental performance challenges.
Multi-modal sensor fusion towards three-dimensional airborne sonar imaging in hydrodynamic conditions
Fitzpatrick, Aidan,
Mathews, Roshan P,
Singhvi, Ajay,
and Arbabian, Amin
Nature Communications Engineering
2023
Analogous to how aerial imagery of above-ground environments transformed our understanding of the earth’s landscapes, remote underwater imaging systems could provide us with a dramatically expanded view of the ocean. However, maintaining high-fidelity imaging in the presence of ocean surface waves is a fundamental bottleneck in the real-world deployment of these airborne underwater imaging systems. In this work, we introduce a sensor fusion framework which couples multi-physics airborne sonar imaging with a water surface imager. Accurately mapping the water surface allows us to provide complementary multi-modal inputs to a custom image reconstruction algorithm, which counteracts the otherwise detrimental effects of a hydrodynamic water surface. Using this methodology, we experimentally demonstrate three-dimensional imaging of an underwater target in hydrodynamic conditions through a lab-based proof-of-concept, which marks an important milestone in the development of robust, remote underwater sensing systems.
Three-Dimensional Mapping of Water Surface Waves Using Air-Coupled Sonar
Fitzpatrick, Aidan,
Singhvi, Ajay,
Mukania, Jordy,
Ye, Brion,
Giebler, Eastan,
and Arbabian, Amin
In OCEANS 2023 - MTS/IEEE U.S. Gulf Coast
2023
More than 70 percent of the Earth’s surface is covered by the ocean and other bodies of water, making them abundant sources of valuable information. In particular, the water surface can be sensed and mapped to extract key parameters related to water dynamics. These parameters have far-reaching implications spanning industrial, environmental, energy, navigation, and various other applications. Existing approaches for measuring the water surface each have their own respective limitations, thus leaving high-resolution spatial and temporal water surface wave mapping an open challenge in the research community. This work proposes a non-contact, acoustic surface mapping system that uses air-coupled ultrasonic transducers to capture three-dimensional spatial maps of the water surface via active sonar imaging. Within, we provide a holistic overview of the system’s design parameters before narrowing in on details related to the hardware, ultrasonic transmit waveforms, and signal processing pipeline. Through verification in simulation, the proposed air-coupled sonar system demonstrates high-fidelity surface mapping with millimeter-scale accuracy and spatial resolution from standoffs up to several meters above the water.
2022
Adaptive Beamforming for Wireless Powering of a Network of Ultrasonic Implants
Wang*, Max L,
Singhvi*, Ajay,
Nyikayaramba*, Gift,
Murmann, Boris,
and Arbabian, Amin
In IEEE International Ultrasonics Symposium (IUS)
2022
Networks of implantable medical devices (IMDs) capable of operating deep in the body are crucial for many novel diagnostic and therapeutic applications. Ultrasound (US) power and data transfer has shown promise for minimally invasive, deeply implantable devices wherein an external portable base station is used to power, coordinate, and communicate with IMD networks. Efficient US power transfer requires focused radiation on the IMD receiver, but current US arrays may have unwanted side lobes due to large transducer pitch with focusing being especially challenging when the application requires simultaneous powering of only a subset of IMDs while avoiding other IMDs in the network. In this work, we devise an adaptive beamforming approach, implemented using a least mean squares (LMS) algorithm, that allows for precise control over both the peak and null regions in the transmitted US beam. We demonstrate successful beamforming with multiple peaks at desired angles and transmit frequencies. In conjunction, we also show significant nulling (>40 dB) in regions where implants should not be activated.
Laser Scanning for Single-Shot Frequency Diverse Photoacoustic Excitation
Meng, William L,
Fitzpatrick, Aidan,
Singhvi, Ajay,
and Arbabian, Amin
In IEEE International Ultrasonics Symposium (IUS)
2022
Photoacoustic and laser-induced ultrasound based imaging systems have been successfully deployed for a range of applications from biomedical imaging to non-destructive testing and remote sensing. Application-specific optimization of the spatial, spectral, and temporal properties of the generated photoacoustic waves requires careful consideration of the optical excitation sub-system. In this work, we study, both analytically and through simulations, the effects of a moving laser source on the generated photoacoustic waves and discuss the additional degrees-of-freedom provided by scanning the laser in a laser-induced ultrasound imaging system. By tuning laser parameters such as the laser scan velocity and modulation frequency, we explore the utility of a moving laser in implementing transmit beam-steering as well as in the generation of angle-dependent, multi-frequency acoustic waves. Finally, as an example application, we demonstrate single-shot, single-sensor imaging.
Dynamic Tuning of Sensitivity and Bandwidth of High-Q Transducers via Nested Phase Modulations
Fitzpatrick, Aidan,
Singhvi, Ajay,
and Arbabian, Amin
In IEEE International Symposium on Circuits and Systems (ISCAS)
2022
Non-contact and long-range acoustic or multi-modal sensing in air can be employed for a number of industrial, IoT, biomedical, and remote-sensing applications but requires the use of resonant, highly sensitive ultrasonic receivers to achieve sufficient SNR. Fabrication of such resonant MEMS sensors fundamentally trades off bandwidth for sensitivity, thus limiting resolution in these sensing systems. In this paper, we devise an all-electronic means of dynamically tuning the sensitivity and bandwidth of high quality factor sensors by using a nested phase modulation scheme that simultaneously achieves high bandwidth and high SNR. We demonstrate an > 8 × increase in bandwidth in ultrasonic ranging and non-contact photoacoustic measurements, while concurrently boosting the SNR by up to 13 × with potential for further enhancement.
A Thermoacoustic Imaging System for Non-Invasive and Non-Destructive Root Phenotyping
Singhvi, Ajay,
Fitzpatrick, Aidan,
Scharwies, Johannes Daniel,
Dinneny, José R,
and Arbabian, Amin
IEEE Transactions on Circuits and Systems II: Express Briefs
2022
Information about the root system architecture of plants is of great value in modern crop science. However, there is a dearth of tools that can provide field-scale measurements of below-ground parameters in a non-destructive and non-invasive fashion. In this paper, we propose a multi-modal, non-contact thermoacoustic sensing system to address this measurement gap and discuss various system design aspects in the context of below-ground sensing. We also demonstrate the first thermoacoustic images of plant material (potatoes) in a soil medium, with the use of highly sensitive capacitive micromachined ultrasound transducers enabling non-contact detection and cm-scale image resolution. Finally, we show high correlation (adj. R² = 0.95) between the measured biomass content and the reconstructed thermoacoustic images of the potato tubers.
Multi-Watt-Level 4.9-GHz Silicon Power Amplifier for Portable Thermoacoustic Imaging
Sutardja, Christopher,
Singhvi, Ajay,
Fitzpatrick, Aidan,
Cathelin, Andreia,
and Arbabian, Amin
IEEE Journal of Solid-State Circuits
2022
Microwave-induced thermoacoustic (TA) imaging, combining high microwave contrast with high ultrasonic resolution has the potential to revolutionize applications such as continuous healthcare monitoring, point-of-care imaging, and biometric authentication. However, the size, cost, and integration of a high-power microwave transmitter is a key bottleneck in making TA imaging truly portable, affordable, and ubiquitous. Toward that end, this work presents a compact 4.9-GHz pulsed power amplifier (PA) with a 4.87-mm² active area implemented in a 55-nm BiCMOS technology, operating in a duty-cycled mode and achieving 37.3-dBm peak output power–the highest demonstrated peak power in PAs fabricated on a silicon substrate with deep submicron CMOS integration. We also reconstruct the first known high-fidelity TA images of tissue phantoms using an integrated silicon PA.
An Electronically Tunable Multi-Frequency Air-Coupled CMUT Receiver Array with sub-100μPa Minimum Detectable Pressure Achieving a 28kb/s Wireless Uplink Across a Water-Air Interface
Singhvi, Ajay,
Fitzpatrick, Aidan,
and Arbabian, Amin
In IEEE International Solid-State Circuits Conference (ISSCC)
2022
We propose a non-contact approach for cross-medium acoustic communication from underwater or tissue-embedded nodes to air by using narrowband air-coupled capacitive micromachined ultrasound transducers (CMUT) with a sub-100 μPa minimum detectable pressure. By using two exclusively electronic frequency tuning knobs, we synthesize a wideband multi-frequency response across an 8-element array of identical CMUTs to demonstrate a 28 kb/s uplink from an underwater node with 3.3x10-5 BER at 10 dB SNR.
2021
Multi-Task Learning for Simultaneous Speed-of-Sound Mapping and Image Reconstruction Using Non-Contact Thermoacoustics
Singhvi*, Ajay,
Wang*, Max L.,
Fitzpatrick*, Aidan,
and Arbabian, Amin
In IEEE International Ultrasonics Symposium (IUS)
2021
Multi-modal imaging via thermoacoustic (TA) approaches provides contrast mechanisms differing from conventional ultrasound (US) imaging - opening up new applications like non-invasive, non-contact below-ground sensing. Due to the high correlation between soil moisture content and speed-of-sound (SoS), knowledge about the SoS in soil can be utilized to improve below-ground image reconstruction and soil moisture mapping at depth. In this work, we present multi-task deep learning networks to accurately predict arbitrarily varying SoS distributions in soil while concurrently reconstructing high-fidelity TA images of root structures. We deploy multi-task U-Net based fully convolutional neural networks trained using US data generated through TA simulations on a wheat root dataset. A multi-input, multi-output architecture performed best - achieving the highest root image contrast-to-noise ratio and lowest SoS mean absolute error.
2020
Spatial Reconstruction of Soil Moisture Content using Non-Contact Thermoacoustic Imaging
Fitzpatrick, Aidan,
Singhvi, Ajay,
and Arbabian, Amin
In IEEE Sensors
2020
Sensing of soil water content is useful in many precision agriculture and resource management applications, particularly if the sensing technique permits frequent field-scale measurements at depth. To date, soil moisture sensing technologies have a trade-off between point based measurements at depth or rapid, remote measurements of water content near the surface. In this paper, we propose a non-contact thermoacoustic soil moisture sensing modality which could permit high-resolution, high-throughput mapping of water content at depth. Within, we develop an algorithm for reconstructing the speed-of-sound in soil, which is known to be highly correlated with the soil moisture content. Through verification in simulation, our algorithm demonstrates high fidelity - reconstructing speed-of-sound profiles that match well with the ground-truth.
An Airborne Sonar System for Underwater Remote Sensing and Imaging
Fitzpatrick, Aidan,
Singhvi, Ajay,
and Arbabian, Amin
IEEE Access
2020
High-resolution imaging and mapping of the ocean and its floor has been limited to less than 5% of the global waters due to technological barriers. Whereas sonar is the primary contributor to existing underwater imagery, the water-based system is limited in spatial coverage due to its low imaging throughput. On the other hand, aerial synthetic aperture radar systems have provided high-resolution imaging of the entire earth’s landscapes but are incapable of deep penetration into water. In this work, we present a proof-of-concept system which bridges the gap between electromagnetic imaging in air and sonar imaging in water through the laser-induced photoacoustic effect and high-sensitivity airborne ultrasonic detection. Here, we use air-coupled capacitive micromachined ultrasonic transducers (CMUTs) which is a critical differentiator from previous works and has enabled the acquisition of an underwater image from a fully airborne acoustic imaging system - a task that has yet to be accomplished in the literature. With the entire imaging system located on an airborne platform, there is much promise for the scalability of our system to one which could perform high-throughput imaging of underwater in large-scale deployment. Non-contact acoustic-based imaging modalities are also of much interest to the medical imaging and non-destructive testing communities. Incorporating air-coupled transducers, for example CMUTs, or other resonant sensors in these applications could be aided by the analysis presented throughout this work.
Resolution Enhanced Non-Contact Thermoacoustic Imaging using Coded Pulse Excitation
Singhvi, Ajay,
Fitzpatrick, Aidan,
and Arbabian, Amin
In IEEE International Ultrasonics Symposium (IUS)
2020
Non-contact thermoacoustic and photoacoustic imaging systems which combine the high resolution of ultrasound with good dielectric/optical contrast show promise in many medical, remote sensing and non-destructive testing applications. One approach that enables meeting the challenging signal-to-noise constraints in non-contact imaging is the use of highly sensitive, resonant air-coupled capacitive micromachined ultrasound transducers (CMUT) as receivers. This sensitivity, however, is gained through a fundamental tradeoff with bandwidth and is at the cost of lower image quality due to slowly decaying residual oscillations of the underdamped, high quality factor CMUTs. In this paper, we propose a pulse-based, multi-cycle signal excitation scheme which optimally modulates the generated ultrasound pressure via the thermoacoustic effect so as to actively cancel the residual oscillations of the CMUT response. This improves achievable thermoacoustic image resolution and reduces multipath clutter. The proposed technique can also be used to improve resolution in contact-based imaging systems that use resonant sensors.
2019
Non-Contact Thermoacoustic Sensing and Characterization of Plant Root Traits
Singhvi, Ajay,
Ma, Bo,
Scharwies, Johannes Daniel,
Dinneny, José R,
Khuri-Yakub, Butrus T,
and Arbabian, Amin
In IEEE International Ultrasonics Symposium (IUS)
2019
Measuring below-ground plant and soil traits such as root biomass, water distribution, and soil compaction is of high interest to plant breeders and agronomists alike. However, there are limited technologies available that can sense these traits. Current methods for sensing roots are either invasive or not readily field deployable. Thus, we propose a novel non-contact thermoacoustic sensing system that can be used to characterize roots in a high-throughput, non-invasive and nondestructive fashion. Upon microwave excitation, the dielectric contrast between roots and soil generates an ultrasound signal via the thermoacoustic effect. Detection of the ultrasound signal in air is achieved using highly sensitive capacitive micromachined ultrasonic transducers with minimum detectable pressures as low as 278 μP a RMS , in order to overcome the large interface loss due to the impedance mismatch at the soil-air boundary. In this paper, we demonstrate a system that can detect agarose-based root phantoms in two different soil types. A linear-regression mapping of the received thermoacoustic data to properties like soil water content, root osmotic potential and root size shows excellent correlation, with R 2 greater than 0.9 in all cases.
A Microwave-Induced Thermoacoustic Imaging System with Non-Contact Ultrasound Detection
Singhvi, Ajay,
Boyle, Kevin C,
Fallahpour, Mojtaba,
Khuri-Yakub, Butrus T,
and Arbabian, Amin
IEEE Transactions on Ultrasonics, Ferroelectrics, and Frequency Control
2019
Portable and easy-to-use imaging systems are in high demand for medical, security screening, nondestructive testing, and sensing applications. We present a new microwave-induced thermoacoustic imaging system with non-contact, airborne ultrasound (US) detection. In this system, a 2.7 GHz microwave excitation causes differential heating at interfaces with dielectric contrast, and the resulting US signal via the thermoacoustic effect travels out of the sample to the detector in air at a standoff. The 65 dB interface loss due to the impedance mismatch at the air-sample boundary is overcome with high-sensitivity capacitive micromachined ultrasonic transducers with minimum detectable pressures (MDPs) as low as 278 μPa rms and we explore two different designs-one operating at a center frequency of 71 kHz and another at a center frequency of 910 kHz. We further demonstrate that the air-sample interface presents a tradeoff with the advantage of improved resolution, as the change in wave velocity at the interface creates a strong focusing effect alongside the attenuation, resulting in axial resolutions more than 10× smaller than that predicted by the traditional speed/bandwidth limit. A piecewise synthetic aperture radar (SAR) algorithm modified for US imaging and enhanced with signal processing techniques is used for image reconstruction, resulting in mm-scale lateral and axial image resolution. Finally, measurements are conducted to verify simulations and demonstrate successful system performance.
2016
A Fine-Grain, Uniform, Energy-Efficient Delay Element for 2-Phase Bundled-Data Circuits
Singhvi, Ajay,
Moreira, Matheus T,
Tadros, Ramy N,
Calazans, Ney LV,
and Beerel, Peter A
ACM Journal on Emerging Technologies in Computing Systems (JETC)
2016
Contemporary digitally controlled delay elements (DEs) trade off power overheads and delay quantization error (DQE). This article proposes a new programmable DE that provides a balanced design that yields low power with moderate DQE even under process, voltage, and temperature variations. The element employs and leverages the advantages offered by a 28nm fully depleted silicon on insulator technology, using back body biasing to add an extra dimension to its programmability. To do so, a novel generic delay shift block is proposed, which enables incorporating both fine and coarse delays in a single DE that can be easily integrated into digital systems, which is an advantage over hybrid DEs that rely on analog design.
2015
A Fine-Grained, Uniform, Energy-Efficient Delay Element for FD-SOI Technologies
Singhvi, Ajay,
Moreira, Matheus T,
Tadros, Ramy N,
Calazans, Ney LV,
and Beerel, Peter A
In IEEE Computer Society Annual Symposium on VLSI
2015
Contemporary digitally controlled delay elements trade off power overheads and delay quantization error. This paper proposes a new delay element that provides a balanced design that yields low power with low delay quantization error. The proposed element has a quasi linear delay characteristic, with uniform delay differences between adjacent code words. The element employs and leverages the advantages offered by a 28nm FD-SOI technology, using its back body biasing feature to add an extra dimension to its programmability. To do so, a novel generic delay shift block is proposed, which enables incorporating both fine and coarse delays in a single delay element that can be easily integrated into digital systems, an advantage over hybrid delay elements that rely on analog design.
Analysis and Optimization of Programmable Delay Elements for 2-phase Bundled-Data Circuits
Heck, Guilherme,
Heck, Leandro S,
Singhvi, Ajay,
Moreira, Matheus T,
Beerel, Peter A,
and Calazans, Ney LV
In 28th International Conference on VLSI Design
2015
We present the design and analysis of three commonly used types of programmable delay elements suitable for use in 2-phase bundled-data asynchronous circuits. Our objective is to minimize power consumption and delay margins needed for correct operation under voltage scaling. We propose both circuit design and transistor sizing strategies to optimize these elements and discuss the relative trade-offs observed in a 65 nm bulk CMOS technology.
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