We report the successful realization of quartz-enhanced photo-acoustic (QEPAS) sensors employing quartz tuning forks (QTFs) with novel geometrical parameters. We investigated the influence of QTF sizes on the main resonator parameters, in order to identify the best design parameters optimizing the QTF figures of merit for optoacoustic gas sensing. To evaluate the QTF acousto-electric energy conversion efficiency, we operated the QEPAS sensors in the near- IR and selected water vapor as the target gas. QTFs are forced to resonate at both the fundamental and the first overtone vibrational mode frequencies. Our results shows that two QTF designs exhibit an higher quality factor (and consequently an higher QEPAS signal) when operating on the first overtone mode with respect to the fundamental one.
The paper presents results of investigation into properties of digital filters implemented in compliance with ANSI S1.11
Standard: Octave-Band and Fractional-Octave Band Analog and Digital Filters. The discussed solutions are digital one-third-octave IIR filters with Direct Form I topology. Performed simulations showed that the filters exhibit strong
problems with stability, especially in the case of low center frequency filters. Another problem clearly visible in all
investigated structures was poor attenuation at the frequencies above the passband. Although the filters were
implemented according to the ANSI standard, none of them met all of the requirements of the standard.
Proc. SPIE. 6937, Photonics Applications in Astronomy, Communications, Industry, and High-Energy Physics Experiments 2007
KEYWORDS: Signal to noise ratio, Digital signal processing, Modulation, Error analysis, Fourier transforms, Interference (communication), Signal processing, Photoacoustic spectroscopy, Digital recording, Signal detection
Photoacoustic measurements are quite often performed by means of data acquisition systems that collect samples of
the photoacoustic signal, and use the samples for the purpose of calculation of the amplitude of its fundamental frequency.
The most common method of such calculations is the FFT, but the amplitude can be also obtained by means of the least
mean square (LMS) method. The paper compares properties of both methods from the point of view of their numerical
efficiency and errors vs. S/N ratio and number of samples per period of the investigated signal. The discussion shows that
in the case of an integer number of periods sampled, results of the FFT and LMS are identical, but due to higher numerical
efficiency the LMS method should be used.
Proc. SPIE. 6347, Photonics Applications in Astronomy, Communications, Industry, and High-Energy Physics Experiments 2006
KEYWORDS: Digital signal processing, Capacitors, Digital filtering, Interference (communication), Amplifiers, Microcontrollers, Photoacoustic spectroscopy, Resistors, Electronic filtering, Signal detection
The paper describes a design, test circuit and preliminary test results of a programmable filter dedicated to photoacoustic experiments. The filter was implemented as a state variable filter with independent digital control of top and bottom corner frequencies as well as quality factor. In order to perform frequency response measurements a dedicated miniature automatic measurement system was designed. The system was based on digital direct synthesis of the input sine wave and true-RMS detection of the output signal.
The paper describes a design of a very short rise/fall time programmable pulse generator. The circuit is based on a Si-Ge comparator, resulting in the rise/fall time of 50 ps. Low component count makes the design relatively simple. As a result, in many applications the presented generator can be a low-cost alternative to sophisticated test and measurements equipment. Due to use of in-system-programmable microcontroller and RS-232 interface, firmware upgrades of the device are very easy, and the generator can be controlled by a PC computer and used in automated measurement applications.
The paper describes a design of a +200 V polarization voltage supply for condenser microphones, dedicated for use in photoacoustic applications. Due to very low signal levels, typical for photoacoustic experiments, the design was optimized towards minimum electromagnetic and acoustic emission. In addition output voltage can be digitally adjusted in a relatively wide range.
The paper presents some improvements to the transmission line model of photacoustic Helmholtz cell. The improvements were aimed to obtain much better agreement between theoretical and practical values of Q-factors of the cells, and were based on measurements of number of cells and comparison of measurement results with theoretical frequency responses. The measured cells had dimensions typical for photoacoustic setups.
The paper describes a truly miniature system for data acquisition and control, dedicated for photoacoustic
measurements. Due to intensive use of electronics and digital signal processing most of expensive and/or inconvenient
elements of photoacoustic setups, like mechanical chopper or lock-in amplifier, were eliminated. The advantages of the
designed device are very low cost, small size, and high functional flexibility. Most of the measurement factors, e.g. analog
path gain, light source modulation frequency, number of samples a period, etc. are programmable and can be changed at
run-time. Implementation of real-time and stroboscope PAS signal sampling makes possible to acquire high resolution
samples while preserving wide input bandwidth. Due to in-circuit programmable components, upgrades of the device are
very easy. Small size and low power consumption make the device a very good choice for implementation of portable
Proc. SPIE. 6159, Photonics Applications in Astronomy, Communications, Industry, and High-Energy Physics Experiments IV
KEYWORDS: Signal to noise ratio, Digital signal processing, Modulation, Digital filtering, Interference (communication), Signal processing, Photoacoustic spectroscopy, Electronic filtering, Signal detection, Filtering (signal processing)
The paper presents some aspects of digital signal processing of sampled photoacoustic signal. In particular influence of DC drift and noise on photoacoustic signal amplitude detection errors is discussed. Some methods of dealing with these problems are described. Real-time sampling and stroboscope sampling are considered.
The paper discusses some problems concerned with instability of time delay in ultra fast sampling circuits, used in digital oscilloscopes of gigahertz bandwidth. Measurement of thermal change of delay time of step recovery diodes are presented. Suggestions how to reduce and/or compensate the effect are included.
The article describes implementation of Random Repetitive Sampling Oscilloscope based on an FPGA device. The main idea was to achieve high quality instrument at moderate costs. The oscilloscope consists of analogue and digital blocks and most of the digital circuits were implemented in a programmable device, which reduces production costs.
Time base and data acquisition circuits are discussed in details. Use of FPGA makes the design more flexible, as changes of the unctionality of the device will not affect hardware. Use of FPGA makes it easy to implement additional functions, e.g. IEEE 488 interface core, if the device is to be use in automatic measurement systems.
The paper presents results of comparative analysis of some basic properties of multicavity photoacoustic Helmholtz cells. The analysed cells were three-cavity Helmholtz resonators formed from a standard dual-cavity photoacoustic Helmholtz cell with an additional cavity connected by means of a capillary with the sample cavity of the cell at the other end. Influence of change of the volume of the additional cavity as well as diameter and length of the connecting duct on the microphone signal vs. frequency was studied. Analysis was based on computer simulations in which a modified transmission line model was used. The model was improved by introducing of additional damping factors which values were extracted from measurements of number of practically implemented dual-cavity photoacoustic Helmholtz cells. The analysis showed that additional cavity makes it possible to improve shape of the frequency response of the cell. In particular it is possible to extend range of frequencies in which resonance properties of the cell can be used for the purpose of photoacoustic signal amplification. It should be noticed that choice of the cell dimensions must be careful, as some combinations decrease resonance properties of the cell.
Some aspects of calibration of photoacoustic instruments are discussed in this paper. In most reports concerning photoacoustic experiments, measurement results are given in arbitrary units. Traditional calibration methods of photoacoustic cells are based on measurements of known concentration of a reference substance. However, from the point of view of such calibration methods, instruments of identical sensitivity but with different internal volumes will produce different responses if the same amount of investigated specimen is used in both cases. This may create problems in measurements when the amount of investigated specimen is limited. The paper presents a method that can be used to improve calibration of photoacoustic cells and associated electronics. The method uses acoustical pressure produced by a piezoelectric disc mounted in the cell and driven from an external oscillator circuit, so that it is possible to compare the amount of sound energy with the amplitude of the electric signal at the sound detector output. In such a way it is possible to evaluate energetic efficiency of the cell and the associated electronic circuit, to determine overall sensitivity of the setup, its signal to noise ratio, etc.
Proc. SPIE. 5775, Photonics Applications in Astronomy, Communications, Industry, and High-Energy Physics Experiments III
KEYWORDS: Signal attenuation, Computing systems, Field programmable gate arrays, Control systems, Linear filtering, Time metrology, Microcontrollers, Measurement devices, Oscilloscopes, Electronic circuits
This paper presents a concept of highly computerized students lab for teaching of electronic circuits. Wide use of modern technologies, like microcontrollers, FPGA chips and computer controlled measurement systems allows to free students from the tedious manual recording of measurements, and let them concentrate on the real investigation of the analyzed circuit. Additionally, due to the limited functionalities and lack of mechanical controls such setup may be a cheaper and more durable alternative for standard lab equipment.