| 摘要 |
A phase-locked loop modulation circuit, such as may be used for a data communications purposes, in which undesired level variations are eliminated by employing d.c. coupling entirely. The circuit includes a level converter circuit which converts a logic signal into a signal having substantially equal levels above and below a predetermined voltage, a voltage-controlled oscillator, and a circuit for superposing the signal having substantially equal levels on a control voltage of the voltage-controlled oscillator so that the output of the voltage-controlled oscillator is subjected to angular modulation. In accordance with the invention, the level converter circuit includes a pair of voltage dividing resistors which divide the supply voltage, a pair of series circuit each composed of a resistor and a switch element connected in parallel with each of the series circuits being connected in parallel to a respective one of the pair of voltage dividing resistors, and a gating circuit for complementarily operating the switch elements on and off in accordance with the logic outputs signal.
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