PURPOSE:To attain detection of a switch input without BPF and cost-down by using a phase locked loop whose detection section has a lock frequency switching circuit and a phase discriminating detection circuit. CONSTITUTION:When an electromagnetic wave is generated from a coil 1 of a coil section 6 with a power switch 4 and an input switch 5 turned on, a maximum induced voltage is generated in X, Y coils closest to the coil 1 of a sense line 7 respectively. One of the coils is controlled by a CPU16 via a multiplexer 8 and the voltage connects to an amplifier circuit 9, and its output (a) is inputted to a detection circuit 10 and a phase locked loop circuit 11. The free running frequency of the circuit 11 is changed by using a control signal (c) from the CPU16. The circuit 10 uses an output signal (b) of the circuit 11 to discriminate the signal (a). The output (d) of the circuit 10 is inputted to the CPU16 via an integration circuit 13 and an A/D conversion circuit 14. The CPU16 via an integration circuit 13 and an A/D conversion circuit 14. The CPU16 recognizes a coil on the line 7 where the output of the circuit 14 is maximized to detect the position of a coil 6, and the position is converted into coordinate information and then outputted.
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