Arturas Aleksandrovas

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.

Investigation of the Time of Flight estimation errors induced by neighboring signals

Investigation of the errors of Time of Flight (ToF) estimate of ultrasonic signals in case of multiple reflections environment is presented. Multiple signals environment induces neighboring signal energy leak into analyzed signal. This in turn introduces some errors in ToF estimate. Narrow signals like single pulse, should posses lower influence that CW burst or chirp signals. Then high energy signals (burst or chirp) will always be jeopardized in case of multiple reflections. New class of SS signals, trains of pulses of arbitrary pulse width and position (APWP), is expected to have properties similar to chirp (wide, controllable bandwidth) and the single pulse (low correlation sidelobes) signals. Goal of the investigation was to compare the multi-reflections performance in Time of Flight estimation for classical signals (rectangular pulse, step and CW burst) and spread spectrum signals (chirp and APWP). Signals from fixed delay line were recorded, aligned in time and averaged to produce noise-free reference signal which was used to simulate the plate thickness measurement and liquid velocity estimation with variable spacing of signals in time. Simulation and experimental investigation results are presented and possible problem solutions suggested.

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.

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.

Epoxy cure monitoring using ultrasonic spread spectrum binary signals

Procedure for dual component epoxy cure monitoring is presented. Accurate time of flight estimation was used employing the correlation processing. Several spread spectrum signals have been put in comparison with conventional pulse and toneburst signals. Experimental results presented indicate that reliable separation of curing phases can be established using both ultrasound velocity and attenuation measurements. Lowest velocity variation was produced by 4 MHz toneburst signal.