Resumen
Experimental confirmation of the possibility of real-time measurement of the internal (deep) temperature of a biological object by thermal acoustic radiation with an accuracy of at least 0.2 °C using an acoustic thermometer implementing the modified zero modulation method is obtained. The model of a single-channel passive noninvasive focussed acoustic thermometer based on a plate piezoceramic electro-acoustic transducer, acoustic elliptical lens and blocks of two serial voltmeters is developed. Electronic switching of the output signals of the noise simulator and the equivalent circuit of the focused piezoelectric transducer is proposed, when resistance generates thermal noise with an intensity equal to the sum of intensities of acoustic radiation of the biological object and natural noise of the piezoelectric transducer. This circuitry solution made it possible to exclude the mechanical modulator (shutter) unit used in analogs from the acoustic thermometer circuit. When developing the original switching and detection unit, it was proposed to use the key mode of n-channel field-effect transistors to perform modulation of the input noise signal. Calculation of equivalent circuits of the piezoelectric electroacoustic transducer and noise simulator, which are connected circuits with a bandwidth in which the average noise voltage level is determined by the noise voltage at the electric resonance frequency of the piezoelectric element, is performed. The possibility of using the model of the single-channel passive noninvasive focussed acoustic thermometer, constructed according to the circuit implementing the modified zero modulation method, for measuring noise voltages in the range from 5 to 30 µV is proved. Since the process of manufacturing or operating a piezoceramic electroacoustic transducer may lead to a deviation of its parameters from theoretically calculated, a method is developed to measure the frequency response of the active and reactive components of electrical impedance. Using the proposed method, the assembly quality of the focused piezoelectric electro-acoustic transducer is monitored and the temperature dependence of electric noise voltage at the electrodes of the focused piezoelectric receiver is obtained.