Vytautas Dumbrava

LED Far Field Pattern Approximation Performance Study

Some factors influence on light emitting diode (LED) far-field pattern (FFP) approximation performance is analyzed. The mean square root intensity error, mis correlation factor and half power and peak angles variance have been suggested for performance evaluation. The polynomial order, range and the noise influence have been analyzed. The best function candidate has been named.

Optimization of the ultrasonic excitation stage

The ultrasonic transducer excitation circuits have been investigated. The system structure is presented where excitation circuit is planned to be used. The possible excitation solutions are presented and discussed. The result of impedance measurement of ultrasonic transducer has been used to derive the Butterworth-Van Dyke electrical model.Then this model was used for PSpice modeling of candidate topologies. The chosen circuit was simulated and experimentally investigated. In this article a structure of ultrasound generator is analyzed.

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.

Estimation of the louvers influence on LED video display image visibility

Sun and ambient light is a serious problem for outdoor LED video displays. Shading the front-face surface by louvers implementation is used in majority of LED displays, but louvers influence the LED directivity. The analytical method is developed for image visibility estimation. The louver geometry and position are taken into account. LED FFP goniometry is used for large LED batch and results later averaged. The averaged FFP is then approximated by Gaussian analytical function. Obtained function can be used to calculate the visibility of display image at various viewing distances.

EVALUATION OF THE ULTRASONIC PREAMPLIFIER NOISE VOLTAGE DENSITY

Analysis of the noise voltage density evaluation procedures of the ultrasonic preamplifier is presented. Ultrasonic testing is demanding low noise reception channel. Techniques and equipment for ultrasonic preamplifiers noise performance evaluation are suggested. Equipment used and measurement technologies applied are described. One of the techniques suggested allows estimating the preamplifier noise with only impedance measurement results available. Sine wave correlation technique was used in the impedance measurement procedure. Another technique, not demanding the spectrum analyzer, was suggested, which uses the analog-to-digital converter (ADC) sampling and the Fourier transform to obtain the noise spectral density. Experimental results for the measured complex gain and the equivalent input noise of the preamplifier are presented. Comparison with traditional noise estimation procedure, using the spectrum analyzer is given. Both direct measurement techniques (ADC record Fourier analysis based and spectrum analyzer) indicated good match. Noise whiteness was estimated: in a region of operation frequencies, 5–7 MHz, noise can be considered as white.

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).

Numerical comparison of LED directivity approximation functions for video displays

The approximation functions of the directivity of the light emitting diodes (LED) have been investigated. LEDs used in large scale LED video displays have been studied. LED directional properties among the other parameters define the video display image quality. The simplicity of an approximation function and ease of analytical handling have been targeted. These functions suppose to be used in display directivity engineering. Four candidate approximation functions were identified and their approximation performance analyzed. The evaluation is done on eight different type LEDs’ sample batches. These samples have been chosen to represent the variety of the main colors and the range of the most popular viewing angles used in large scale LED video displays design. The relative intensity approximation root mean square (RMS) error and approximation errors’ cross correlation have been used as performance estimation criteria. The radiometric intensity variation within a manufacturing lot was suggested as lower error bound. Approximation error variance was analyzed for various approximation ranges. Results of four candidate approximation functions’ performance on eight different type LEDs’ are presented.

Sampling parameters optimization of the data acquisition for ultrasonic time of flight measurements

An ultrasonic system using a pulse for time-of-flight (TOF) estimation from noisy echoes has been analyzed in paper. Simulation has been used as the sampling parameters influence evaluation tool. The precision of the time delay estimation using the direct correlation technique has been theoretically calculated and compared to the simulation results. The results indicate that sampling frequency selection is not important for TOF precision if noise spectral density is kept constant. ADC resolution influence on TOF estimation performance has also been analysed.