| 摘要 |
A phase locked loop tuning system of a radio receiver includes circuitry for converting the setting of a potentiometer to a binary word for controlling the frequency of the local oscillator signal generated by the phase locked loop. The conversion circuitry includes a voltage comparator having a first input coupled to the wiper of the potentiometer and a second input coupled to a first capacitor. The first capacitor is charged during a first predetermined period. The time at which the voltage developed across the first capacitor exceeds the voltage at the wiper determines the binary control word. To self-stabilize the conversion circuitry, a second capacitor for developing the reference voltage for the potentiometer is charged during a second predetermined period in accordance with the level of the voltage developed across the first capacitor at the end of the first predetermined period.
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