Patent ID: 6642780
Filing Date: 2003-11-04
Classification: H03H

Abstract:
A variable frequency filter circuit, comprising:a filter for adjusting a cut-off frequency; adjusting means for arbitrarily setting the cut-off frequency of the filter in accordance with a variable setting value so at to adjust the cut-off frequency; first recording means for storing therein a first compensating value for compensating the setting value, the compensating value being worked out in advance via simulation in order to compensate circuit characteristics that dynamically vary depending on usage; a reference signal source for generating a reference frequency signal; input switching means, provided in an input stage of the filter, for inputting, to the filter, the reference frequency signal instead of an input signal, during an adjusting mode period during which the adjusting means adjusts the cut-off frequency; control means for adjusting the setting value so that an output from the filter with respect to the reference frequency signal will have a predetermined value, and for outputting the setting value; and second recording means for storing the adjusted setting value as a second compensating value, wherein during a regular operation mode period the control means compensates a shift in the cut-off frequency due to a change in the circuit characteristics by compensating the setting value with the first compensating value stored in the first recording means, and compensates a shift in the cut-off frequency due to a circuit layout by compensating the setting value with the second compensating value stored in the second recording means.