| 摘要 |
A voltage-controlled oscillator and a control substitution circuit that interrupts the control loop in case a source of normal control voltage such as a referenced frequency fails, and allows the voltage-controlled oscillator to continue to operate with a nominal control voltage. To allow the oscillator of such a loop to continue to operate for weeks with a practically unchanged frequency under certain circumstances, after the normal control voltage source has failed, the nominal control voltage can be selected such that the oscillator produces an oscillation of a preset frequency. Simultaneously with change-over of the oscillator to the preset frequency, its voltage-to-frequency characteristic is changed by the control substitution circuit such that a change of the control voltage leads to a smaller change in the frequency than in normal operating conditions. To compensate the temperature dependency of the output frequency of the oscillator, the nominal control voltage that is applied to the oscillator input during control substitution is made temperature-dependent.
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