| 摘要 |
PURPOSE:To obtain a filter circuit whose cut-off frequency is easily made variable and suitable for circuit integration by setting the cut-off frequency of the filter circuit by means of an error signal obtained digitally based on an oscillation frequency signal obtained by using the characteristic of a voltage- capacitance variable element formed by the same characteristic on a semiconductor substrate. CONSTITUTION:Plural voltage-capacitance variable elements C, C' are formed on the same semiconductor substrate by the same device process. A voltage controlled oscillator 1 is constituted by utilizing the voltage-capacitance variable element C and an oscillation signal is fed to a digital comparator 6. A preset digital signal Vp is fed to the digital comparator 6, where the signal is compared and counted with the oscillation signal, resulting that the obtained error signal corresponds to the count difference of the both. The error signal is fed to the voltage-capacitance variable element C applying the frequency control of the oscillator and fed to the other voltage-capacitance variable element C' constituting the filter circuit 2 so as to control the cut-off frequency of the filter circuit to a desired frequency attended with the frequency control of the oscillator 1.
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