The Tracking Resonance Frequency Method for Photoacoustic Measurements Based on the Phase Response

Abstract

One of the major issues in the use of the resonant photoacoustic cell is the resonance frequency of the cell. The frequency is not stable, and its changes depend mostly on temperature and gas mixture. This paper presents a new method for tracking resonance frequency, where both the amplitude and phase are calculated from the input samples. The stimulating frequency can be adjusted to the resonance frequency of the cell based on the phase. This method was implemented using a digital measurement system with an analog to digital converter, field programmable gate array (FPGA) and a microcontroller. The resonance frequency was changed by the injection of carbon dioxide into the cell. A theoretical description and experimental results are also presented.

Non-dispersive sensing scheme based on mid-infrared LED and differential mode excitation photoacoustic spectroscopy

“…PA sensors measure directly the small fraction of light absorbed by the gas sample within the PA cell. A great variety of PA cell designs have been proposed and applied to trace gas monitoring: acoustically resonant PA cells [7] , [8] , [9] , eventually equipped with a resonance locking circuitry [10] or combined with a special resonance frequency tracking method [11] ; open cells [12] , [13] ; cells with special geometry [14] , [15] ; multipass cells [16] ; heatable cells [17] ; intracavity cells [18] ; parallel cells [19] and multifunctional, compact and miniaturized cells [20] , [21] , [22] . 3-D printing has also been applied to the fabrication of miniature PA cells [23] .…”

Novel approach for efficient resonance tracking in photoacoustic gas sensor systems based on a light-induced wall signal

“…A concept reported in literature, that works in this manner, suggests the consecutive monitoring of the phase between excitation and the microphone signal. If a phase deviation occurs, the modulation frequency is adapted accordingly [16, 17] . Although this concept works well for short and consecutive measurements, it has some drawbacks if the eigenfrequency changes significantly without intermediate measurements.…”