| 摘要 |
PROBLEM TO BE SOLVED: To provide a filter which has high reliability, has high quality, is hardly influenced by a fluctuation of a temperature, is low in cost, and is small in dimensions, and to provide its tuning method. SOLUTION: A filter 5 has at least one variable constant and a position of a pass band pc of at least one filter can be changed by the variable constant. At least one reference signal is inputted to a filter 5, and then a frequency of at least one reference signal is changed, and/or at least one time constant of the filter 5 is changed. The tuning method of the filter includes a step in which the intensity of the output signal of the filter 5 is measured, and a step in which the position of the pass band of the filter 5 is determined according to the measurement result of the intensity of the output signal of the filter 5.
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