| 摘要 |
A two-wire transmission system for a vortex-type flowmeter whose sensor generates a low-level a-c signal having a frequency that depends on flow rate, the signal being fed to the input of a pre-amplifier adjacent the meter. The two-wire system serves to supply d-c power to the pre-amplifier from a remote d-c source and to conduct the a-c output of the pre-amplifier to a remote output point. The system includes a DC-to-DC converter constituted by a high-frequency oscillator whose circuit incorporates the primary of an isolation transformer, the secondary of which is coupled to a rectifier supplying d-c to the power terminals of the pre-amplifier. Connected across the pre-amplifier output is a load resistor which draws a sufficiently heavy current to modulate the d-c power consumed by the pre-amplifier in accordance with the amplified a-c signal. These modulations are reflected in the demand imposed by the converter on the d-c source through the two-wire line, the fluctuating demand being detected at the remote output point to extract the a-c signal from the line.
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