We report a new mode of airborne operation for capacitive micromachined ultrasonic transducers (CMUT), in which the plate motion spans the entire gap without collapsing and the transducer is driven by a sinusoidal voltage without a dc bias. We present equivalent-circuit-based design fundamentals for an airborne CMUT cell and verify the design targets using fabricated CMUTs. The performance limits for silicon plates are derived. We experimentally obtain 78.9 dB//20 μPa@1 m source level at 73.7 kHz, with a CMUT cell of radius 2.05 mm driven by 71 V sinusoidal drive voltage at half the frequency. The measured quality factor is 120. We also study and discuss the interaction of the nonlinear transduction force and the nonlinearity of the plate compliance.

With the improvement in digital signal processor technology, the topic of Passive Coherent Location (PCL) radar systems that exploit 'illuminators of opportunity' became a popular area of research. Comparative studies of Ambiguity function properties of broadcast signals such as FM radio, analogue television, cellular phone signals and digital audio broadcast (DAB) revealed that FM radio broadcast signals, providing up to 30dB peak to side lobe levels in range, and up to 40dB peak to side lobe levels in Doppler, are well suited for passive radar purposes.However, due to low modulation bandwidth in FM, which never reaches the allowable channel bandwidth of 150 kHz, the range resolution is severely limited. While 150 kHz modulation bandwidth can provide lkm range resolution, at best, 30 kHz modulation bandwidth is achieved, which results in 5km range resolution. Furthermore, the modulation bandwidth is strongly dependent on the content that is being broadcasted.An improvement in range resolution is obtained by using multiple adjacent FM channels, emitted from co sited transmitters, which is often the case in large towns in countries, where the FM channel allocations are weakly regulated. A number of channels can be used independently or together. When channels are used together the range resolution is dominated by the total channel bandwidth. The auto ambiguity function of multiple channel FM signals shows high side lobes. Once the range is estimated using a single FM signal, it can be refined by using more FM channels subsequently. The number and locations of the FM channels that will be used in the PCL process can either be predetermined or chosen at each recording separately, taking into account the power and modulation bandwidth of each channel.A new scheme is proposed, which takes multiple FM radio channels emitted from co sited radio transmitters.Again two receiver antennas are used: One for direct signal reception and one for the target scattered signal reception. The direct signal is used in the computation of the autocorrelation function of the FM waveform. The signals from both antennas are used in the computation of the cross ambiguity function. The cross ambiguity function is compared to the autocorrelation function. The range information is obtained by the delay between the cross ambiguity function and the auto ambiguity function. Simulations show 24 that down to -30dB signal to noise ratio (SNR) the autocorrelation function of 7 adjacent FM channels with random content can be successfully extracted from the cross ambiguity function. Hence, the range resolution is significantly improved. Best results are achieved when all channels have equal power. For sufficiently large SNR, the cross ambiguity function for multi FM channel waveform has an envelope similar to the cross ambiguity function a singular FM channel waveform. Range resolution can be improved down to 180 meters when 7 adjacent FM channels are used together.

We introduce the large-signal and small-signal equivalent circuit models for a capacitive micromachined ultrasonic transducer (CMUT) cell, which has radiating plates on both sides. We present the diffraction coefficient of baffled and unbaffled CMUT cells. We show that the substrate can be modeled as a very thick radiating plate on one side, which can be readily incorporated in the introduced model. In the limiting case, the reactance of this backing impedance is entirely compliant for substrate materials with a Poisson's ratio less than 1/3. We assess the dependence of the radiation performance of the front plate on the thickness of the back plate by simulating an array of bilateral CMUT cells. We find that the small-signal linear model is sufficiently accurate for large-signal excitation, for the purpose of the determining the fundamental component. To determine harmonic distortion, the large-signal model must be used with harmonic balance analysis. Rayleigh-Bloch waves are excited at the front and back surfaces similar to conventional CMUT arrays.

An energy-consistent lumped-element equivalent circuit model for charged circular capacitive micromachined ultrasonic transducer (CMUT) cell is derived and presented. It is analytically shown and experimentally verified that a series dc voltage source at the electrical terminals is sufficient to model the charging in CMUT. A model-based method for determining this potential from impedance measurements at low bias voltages is presented. The model is validated experimentally using an airborne CMUT, which resonates at 103 kHz. Impedance measurements, reception measurements at resonance and off-resonance, and the transient response of the CMUT are compared with the model predictions.

We propose an airborne collapse capacitive micromachined ultrasonic transducer (CMUT) as a practical viable ultrasound transducer capable of providing a stable performance at the off-resonance frequencies. Traditional practice is to bias the CMUT plate close to collapse voltage to achieve high coupling coefficient and sense the incoming ultrasound as an open-circuit receive voltage signal of the transducer or shortcircuit receive current (SCRC). Maintaining CMUT plate in the vicinity of collapse threshold is rather difficult. In this paper, an analytic approach to design an airborne collapsedmode CMUT for maximum off-resonance sensitivity is presented. We use small-signal circuit model to evaluate the performance of a collapsed CMUT for varying operating conditions. CMUT operational parameters that yield the highest off-resonance SCRC are directly obtained from performance design curves. Collapsed CMUT plate is then biased in a critical biasing region that produces a stable and maximum off-resonance sensitivity. We experimentally verify and measure a stable sensitivity of a fabricated collapsed CMUT cell of −60 dB V/Pa at 100 kHz when biased between 50 to 65 V. We characterize our linear circuit model performance against the measured performance of collapsed CMUT in air within 4-dB tolerance. [2018-0058] Index Terms-CMUT, collapsed CMUT sensitivity, critical biasing region, off-resonance, small-signal circuit model. I. INTRODUCTION O WING to standard silicon integrated circuit (IC) fabrication technology and potential for integration with electronics, CMUTs have proved to be a viable technology in medical ultrasound. For example, the first pulse-echo 128-element, 1-D linear CMUT array was fabricated using simple photolithography process and was characterized with a wider bandwidth and higher sensitivity than piezoelectric ceramics in 2001 [1]. Commercial scanners using 1-D CMUT arrays are also reported to have produced clinical-quality