PURPOSE: To control an excitation signal source and automatically tune it to the center frequency of a narrow band-pass filter which consists of an element of relatively inexpensive constitution with a fixed characteristic value. CONSTITUTION: Instead of enabling a front-end section circuit 30 to be tuned, it is made possible to tune the excitation signal source 16. The digitizer control circuit 10 is equipped with a stylus 24 for detecting an excitation signal and a circuit which processes an information signal from the stylus 24. This circuit includes the narrow band-pass filter 48 which passes the excitation signal and cuts off an external noise. A controller 14 detects the position of the stylus 24 on the driving surface 22 of a digitizer and drives and controls the circuit. The excitation signal source 16 includes a tunable (n)-dividing circuit consisting of an 8-bit latch, an 8-bit counter, and a state device. A nonvolatile memory 34 stores the latest value of a divisor (n).
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