| 摘要 |
A gain-controlled amplifier (24) in a sonic flow meter accepts an output signal from each transducer of a plurality of pairs of transducers (1a, 1b; 2a, 2b; 3a, 3b; and 4a, 4b) in sequence. A ramp generator (62) generates a time-varying-gain output, which increases between the transmission of a pulse by one transducer and the reception thereof by another. This time-varying-gain signal is one component of the amplifier's gain-control signal, which includes as another component an AGC value fetched from a memory (50) and rendered into analog form by a digital-to-analog converter (42). The value in the memory (50) is the difference between the time-varying-gain value and the value that other circuitry (30, 36) has determined to be necessary by monitoring the amplifier output produced during a previous operation of the same transducer pair. A third component of the gain-control signal is provided by further circuitry (40, 72, 76, 102), which acts to reduce the gain-controlled value if it determines that noise is present. It concludes that noise is present if the receiving transducer produces signal magnitudes that exceed a predetermined threshold before sound is expected from a transmitting transducer.
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