| 摘要 |
system for measuring parameters, such as the density, of a liquid by an ultrasonic method, including a voltage controlled oscillator VCO (1) having a control input terminal and an output terminal; an ultrasonic resonator cell (2, 3, 4) including an electro-acoustical transmitting transducer (3), an electroacoustical receiving transducer (4) and a sample cavity (2) between the transducers, the transmitting transducer being coupled to the output terminal of the VCO; a phase comparator (7) having a first input terminal coupled to the receiving transducer (4), a second input terminal coupled to the output terminal of the VCO, and an output terminal, and a feedback path coupling the output terminal of the phase comparator to the control terminal of the VCO to form a phase locked loop (PLL), and means (10, 11) for disabling the phase locked loop, and means (13) for selecting the frequency of the signal applied to the transducer, and means (8, 9) for controlling and varying the phase of the signal.
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