Karel Hajek

Low-distortion current-mode quadrature oscillator for low-voltage low-power applications with non-linear non-inertial automatic gain control

The paper introduces a novel sinusoidal quadrature oscillator (QO) with low harmonic distortion. The oscillator utilizes the non-linear non-inertial principle for amplitude stabilization and thus a fast transition to 0 steady state oscillations should be accomplished. This feature makes the QO suitable for operation at very low frequencies. The oscillator operates in the current-mode and is is intended for low-voltage low-power applications. It employs only three active elements, Z-Copy Current Inverter/Follower Transconductance Amplifiers (ZC-CITA/ CFTA), one of which provides automatic gain control (AGC). The proposed principle was experimentally verified.

New possibilities to increase sensitivity of the ultrasound non-linear modulation methods

This paper presents comparison of two basic methods of ultrasonic non-linear modulation spectroscopy and proposes new possibilities for rising of sensitivity. The rising of sensitivity by lowering of a dynamic range of the measured signals by using of analogue linear pre-filtration for suppressing of excitation signals is discussed in the first part. The second part discusses the possibility of using the burst exciting mode for ultrasonic non-linear modulation methods. The excitation by phase coupled burst signals is derived for obtaining of the resultant long time signal without the waste of its useful energy. The new variant of mixing method with burst signals is derived, where the linear analogue pre-filtration is used for the attenuation of exciting signals in sensing signal for the rising of signal processing dynamic range and the increasing of practical sensitivity.

Contact voltage of third harmonic distortion for contact reliability investigation

This paper shows a new possibility for quality and reliability testing of low-current contacts by nonlinearity measuring. A normal third harmonic distortion (THD) tester cannot be used for low-impedance elements. Therefore, a special variant of the THD tester is described for this aim. This method was proved at a set of relay contacts and the results of testing validate a propriety of proposed method.

Testing of low-current contacts quality and reliability by using third harmonic distortion

This paper shows a new possibility for quality and reliability testing of low-current contacts by their non-linearity measurement. A common THD tester cannot be used for low impedance elements. Therefore, a special variant of the THD tester was developed. It was used for testing a set of relay contacts. The results of the tests proved the applicability of the proposed method.

Cracks Detection in Mg Alloy by Electro-Ultrasonic Spectroscopy

New non-destructive testing (NDT) method is based on the effect of the ultrasonic vibrations on the electron transport in samples with macroscopic defects as cracks or defect centers affecting electrical conductivity. On the frequency given by the subtraction of exciting frequencies new intermodulation signal appears. Its value is given by electric resistance modulation by the defects and un-homogeneities in the sample structure. In our experiment we used the ultrasonic actuator with frequency fU when the period of wave is longer than the dielectric relaxation time in analyzed sample. In this case the effects of electron bunching by ultrasonic wave are negligible. The ultrasonic wave length is much larger than the electrons mean free path and the wave period is much longer than the mean free time among the electrons collision with scattering centers and defects. Then the electron transport is described by the quasi-steady state transport equation in one-electron approximation. Because of the requirement of charge neutrality, no net AC electric current with the ultrasonic wave frequency fU can be carried by the wave. Similar situation exists for samples excited by standing ultrasonic waves. The electrical conductivity varies with time due to that the cracks geometry is changed with frequency of the ultrasonic vibration. The sample conductivity is affected mainly by the presence of cracks and defects boundaries perpendicular to the electric current density vector. In our experiment the amplitude of ultrasonic vibration is so low that no new cracks are generated and then the proposed testing method belongs to NDT.

A Resonance Frequency Shift In Spectral Analysis Of The Impact Echo

The proposed improvement of impact echo signal analysis consists in dividing of the impact echo signal to more parts and in the spectral analysis of each one separately. In this case we can obtain more information about tested object. First of all, the frequency shifts of resonance frequencies are not averaged and it can be compared with resonance frequency for low excitation. Further advantage consists in the relatively comparison with the same tested body and not only with general standard value. By this way, this new method brings essential increasing of sensitivity to the body cracks in comparison with standard method.

General multiple LC prototype filter solutions and optimization

New possibilities of optimization of LC filter prototypes are presented in this paper. This optimization of LC filter prototypes is also useful for design of ARC and SC filters. The contribution describes various filter design procedures, which enables us to generate multiple filter solutions by means a traditional synthesis method. In comparison to other published methods, the presented procedures offer wider possibilities of multiple filter solutions using easy methods. A sensitivity comparison of generated multiple solutions is also shown. Some possibilities of filter element spread minimization as well as the filter tolerance design optimization are briefly documented in some examples of low-pass filter prototypes.

Efficiency of the class B-CE and class B-CC high-voltage wideband amplifiers with a capacitive load

The design of high-voltage wideband amplifiers for excitation of ultrasonic actuators and transmitters has problem with limited slew-rate for full range excitation. This problem is connected with value of total power dissipation of transistors, especially for the advantageous class B stage with common emitters. The power dissipation of transistor is substantially dependent on phase shift of capacitive load especially for high frequency range. This paper solves the dependency of the power dissipation and efficiency on phase shift of load and by this way it enables to solve optimum solution of problems connected with this effect. Especially, the maximum slew-rate can be attained by optimum choose of transistors.

Low-pass cascade filters with high attenuation rate in the stopband

In this paper, we present two biquadratic structures for cascade filter design, which are intended to provide high attenuation rate in the stopband area of frequency transfer characteristic. The filters optimized in this way can reach better electrical performances, such as higher maximal cut-off frequency or dynamic range, even with conventional (low power consumption) active devices. We present a modification of the well-known Sallen-Key filter and a biquadratic filter based on the second generation current conveyor CCII-. Presented approach relies on the elimination of parasitic transfer zeros in the real transfer function, which are caused by the non-ideal parameters of real active elements. Their impact is eliminated by unconventional definition of the filter output.

Reduction of the Voltage Noise of Preamplifiers by Chopping Synchronized with DSP

The voltage noise of preamplifiers limits a sensitivity of many systems in the case of the low value of an internal resistance of a signal source. The offset and 1/f noise as a special case of this noise can be reduced by some variant of chopping technique as it is known for DC amplifiers. This idea is used for new principle of reducing of the white voltage noise. In comparison to classical chopping technique the suitable synchronous DSP enables to differentiate chopped usable signal from voltage noise with frequencies around chopping frequency.