Andrius Chaziachmetovas

Half bridge topology 500 V pulser for ultrasonic transducer excitation.

Application of half bridge topology for ultrasonic transducer excitation using long pulse trains is presented. The novelty of the approach is the high speed solution for a high side drive. A commercially available high speed digital isolator and a high speed MOSFET driver were combined to give the possibility to deliver fast driving signals to a high side N-channel MOSFET. The experimental investigation indicates that the output amplitude of the fundamental harmonic can reach 624 Vp-p for light loads and 552 Vp-p when driving 50 Ω loads. The operation frequency at such voltages can reach 10 MHz for unloaded or 50 Ω load condition and 6 MHz when driving capacitive 3000 pF loads. The output impedance is 13 Ω for voltages below 500 Vp-p and 16-26 Ω for voltages 500 Vp-p and above.

Pulser for arbitrary width and position square pulse trains generation

Special kind of Spread Spectrum excitation is analyzed: trains of arbitrary position and width pulses (APWP). Such signals are easy to generate and to handle in digital domain. They offer an advantage over a single pulse: spectral response has no dips and posses higher that single pulse energy, yet being capable of short duration after matched filer compression. Pulser design for such signals generation is described. In order to have the electrical power consumption economy, both high and low - pulling elements are active. Such decision would allow to save the energy when pulser is loaded with capacitive (piezoceramic transducers, capacitive micromachined ultrasonic transducers, CMUT) or inductive load (electromagnetic acoustic transducer, EMAT). In order to evaluate the amount of energy consumed theoretically and real generator performance, energy per pulse was suggested as comparison parameter. Reasoning for output topology, element parameters is given. Pulser structure is described. Investigation of prototype performance is given. Output signal waveforms, energy per pulse performance is presented.

Universal acquisition system for frequency domain parameters measurement

The universal acquisition system is presented. System is dedicated for multiple frequency domain parameters measurement: transfer, insertion loss, AC response, harmonic distortion, noise and electrical impedance. The primary application was the ultrasonic equipment AC parameters evaluation. Operation range is 20 kHz to 30 MHz. System is targeted for parameters comparison and estimation, but it is universal: all the measurements are accomplished using just one excitation and two analog reception channels. System structure and operation are presented. Modular PC104 form-factor construction allows for both local and remote PC connectivity. Setup for most common tasks and example measurement results with system is presented.

Comparison of spread spectrum and pulse signal excitation for split spectrum techniques composite imaging

Ultrasonic imaging of composites was investigated. Glass and carbon fiber reinforced plastic produced by resin transfer molding and prepreg forming were analyzed. In some of the samples air bubbles were trapped during RTM (resin transfer molding) process and interlayer gaps were present in prepreg technology samples. One of the most expected techniques to apply in such case is the Split Spectrum processing. On the other hand such signals require specific processing to reliably reconstruct the temporal position of the defect reflection. Correlation processing can be used for signal compression or Wiener filtering can be applied for spectral content equalisation. Pulse signals are simple to generate, but lack the possibility to alter the signals spectrum shape. Spread spectrum signals offer a powerful tool for signal energy over frequency band increase and resolution enhancement. CW (continuous wave) burst has high energy but lacks the bandwidth needed for SSP (spread spectrum processing). The aim of the investigation was to compare the performance of the above signals in case of composite imaging, when various Split Spectrum Processing techniques are used with preceding Wiener processing for spectral content compensation. Resulting composite signals and images obtained are presented. Structural noise removal performance was evaluated as Receiver Operating Characteristics (ROC).

Advanced hardware for fine-tuning and optimization of sonoporation efficiency in vitro

To our knowledge current achievements in tuning the sonoporations bio-efficiency is limited by the inflexibility of the ultrasound excitation hardware. Advanced instrumentation for fine-tuning and optimization of sonoporation efficiency in-vitro has been developed. The advantages of developed hardware are: the fine tuning of center frequency of the burst; the fine tuning of amplitude of the burs from 50 V to 500 V; the programmable number of burst periods, duty cycle of series of bursts and repetition frequency, and total time of excitation. Our hardware together with wideband focused transducer has specified parameters in space, and ability to control the acoustic pressure amplitude with digital precision. Operation range is 500 kHz to 5 MHz; length of burst variable up to 100 periods; pulse repetition frequency up to 5 kHz. The linear RF power amplifier with matched output impedance was replaced by the low output impedance square wave generator. Such hardware solution allows having better output amplitude stability and digital programmability, yet at increased economy: both power consumption and cost of equipment. Instrumentation structure and operation principles are presented.1

Investigation of the Half Bridge and Transformer Push–Pull Pulser Topologies for Ultrasonic Transducer Excitation

Comparison of two high power pulser topologies is presented. Pulser design was aimed for piezoelectric transducer excitation, yet it can also be used for electromagnetic acoustic transducer (EMAT) or capacitive micromachined ultrasonic transducers (CMUTs) excitation. Pulser can produce both single rectangular pulse and trains of rectangular arbitrary duration pulses. In order to achieve the economy of the electrical power consumption and speed both high-pulling and low-pulling elements are active switches. Energy per pulse was used to evaluate the amount of energy consumed. Two topologies were selected for evaluation: transformer output push–pull topology and half bridge output. Experimental investigation results are presented.

Split Spectrum processing applications for new composite materials imaging

In this paper the performance of Split-Spectrum Processing algorithms combined with Spread Spectrum excitation for ultrasonic imaging of new composites were analyzed. Glass and carbon fiber reinforced plastic produced by Resin Transfer Molding (RTM) were the analyzed specimens. These materials are strongly affected by structural noise since air bubbles were trapped during RTM process and interlayer gaps were resent in prepreg technology. For such sort of materials, classic criterions for the parameters of Split-Spectrum algorithms are not feasible and should be revised. Furthermore, as attenuation is highly frequency dependent for these materials, Spread Spectrum and Frequency Selective Loss Compensation techniques were used in order to increase the energy and equalize the received signal. Flaw-to-Clutter Ratio Gain, Signal-to-Noise Ratio Gain and Receiver Operating Characteristics were selected as the figures of merit to make the comparisons, and resulting 3D images of the composites are given.

Application of binary excitation spread spectrum signals for spectral compensation

Ultrasonic probing usually uses pulse signals. Resolution of the ultrasonic imaging depends on the bandwidth of the received signal. It is also important while measuring thin materials. Wide bandwidth and bandwidth flatness is important in ultrasound spectroscopy 1. Bandwidth of the received signal is mainly limited by the ultrasonic transducer. Both bandwidth and SNR improvement can be accomplished by using spread spectrum signals that have been compensated for spectral losses. Yet, such signals require arbitrary waveform generation using digital-to-analog converter and linear power amplifier. Equipment size, cost and efficiency of excitation are an issue in such case. Frequency modulation spread spectrum signals (both linear frequency modulation and nonlinear frequency modulation) explore the amplifier capabilities better, but still suffer for size and efficiency. Essential, that derivation of such signals is an approximate procedure. Rectangular pulses are relatively easy to generate offer small equipment size and low cost. Spread spectrum signals can be generated using arbitrary position and width pulses (APWP) sequences. Unfortunately, linear and nonlinear frequency modulation (chirp) signals have large spectral ripple. Direct derivation of APWP sequences is a lengthy process. Novelty of the approach presented in this paper is that nonlinear frequency modulated and amplitude modulated spread spectrum signal is converted into bipolar APWP sequence and this sequence is used for further optimization to improve the spectral flatness of the received signal.

Comparison of direct and transformer drive high voltage ultrasonic pulser topologies

Ferroelectret films are a relatively new material. Highly anisotropic polymers can be polarized resulting in a piezoelectric response. Advances in manufacturing techniques and polarization have made this technology a viable option in various applications: pressure sensors, air transducers (0.05 MHz–0.6 MHz) and in configuration of patch transducers as pulse sensors and for Lamb waves. Previous experience with this material indicates that this type of material also possess the electrostatic traction effect in ultrasound generation. Yet, excitation with voltages exceeding 1000 V is needed. Probing using rectangular pulse is expected. Production of high voltage pulses beyond 1000 V is problematic, because corresponding semiconductor devices are available only up to 1700 V. Accounting that ferroelectret films have high input impedance and capacitance is low it is reasonable to apply a transformer in order to provide both impedance matching and voltage step-up. Though, higher transformer turns ratio can increase the leakage inductance and driving impedance limiting the attainable bandwidth in such way. Investigation presented was aimed to compare the direct drive and transformer drive excitation topologies when load is the ferroelectret film ultrasonic transducer. Dedicated ferroelectret transducer was used to study two excitation topologies.

Technique for the performance evaluation of the ultrasonic preamplifier input protection circuits

Usually, imaging and measurement using ultrasound employs the application of single pulse signals for probing. Despite low correlation sidelobes and wide bandwidth which contribute to the good temporal resolution such single pulses have low energy. If material attenuation and scattering are high, or application is air-coupled, signal losses are high and signal-to-noise ratio is low. Spread spectrum signals are used to spread the energy over time, producing long signals which can be compressed by matched filter producing the processing gain, which in turn is used for signal-to-noise ratio improvement. Long duration of the SS signals imposes specific requirement. Existing standards are dedicated for single pulse signals and low impedance equipment and are not exhaustive in full performance evaluation. New technique, aimed at SS signals application is presented. Aim of the work was to develop the technique for ultrasonic preamplifier performance under high energy probing pulses investigation. Recovery time, gain and input impedance frequency response, clamping energy consumption performance evaluation parameters and measurement procedures have been developed and presented. Preliminary investigation results are presented.