| 摘要 |
In a phase-locked oscillating circuit comprising a voltage-controlled oscillator responsive to a control voltage for generating an output signal having an output frequency and a variable gain amplifier having a controllable gain and supplying the voltage-controlled oscillator with the control voltage, a frequency fluctuation detecting circuit detects fluctuation in the output frequency of the output signal to control the controllable gain on the basis of magnitude of the fluctuation in the output frequency of the output signal. The phase-locked oscillating circuit may comprise a reference voltage generating circuit for supplying the variable gain amplifier with a controllable reference voltage in response to the control voltage. The phase-locked oscillating circuit may comprise a selector for selecting one of original reference signals as a selected reference signal and an input frequency divider for frequency dividing the selected reference signal to produce an input divided signal which is supplied to a phase-frequency comparator. The input frequency divider may include a detection circuit for detecting absence of the selected reference signal to produce an input absence signal on absence of the selected reference signal. Responsive to the input absence signal, a digital signal processor holds a digital phase difference signal produced immediately before as a held digital phase difference signal.
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