| 摘要 |
The filter is set up by firstly operating in oscillator mode, during which time the oscillator frequency is measured. The characteristics of the filter are adjusted in order to match the required cut-off frequency for the filter, as closely as possible, within a determined degree of tolerance. The filter control procedure includes a phase in which the filtering circuit (FF10) operates as an oscillator, and the frequency of operation of the oscillator is determined. The characteristics of the device are then corrected, taking account of the measured frequency of oscillation and a predetermined relationship between the frequency of oscillation and the theoretical cut-off frequency of the filter. This gives the circuit a cut-off frequency which is equal to the frequency of oscillation, within a close tolerance. After this adjustment phase, the filter moves into a working phase, in which the filter carries out its filtering function. When the filter has an order greater than three, the filter operates in oscillator mode by re-looping to itself with phase inversion. When the filter operates with order less than three, an additional series filter is provided to form the oscillator loop.
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