| 摘要 |
PURPOSE:To select plural frequencies with a simple and inexpensive constitution by using only one filter circuit, and switching the values of capacitors successively and varying the selected frequency. CONSTITUTION:A frequency selecting circuit for a receive total signal selects the frequency of SW pulses so as to obtain tN<=T, where T is the period of an observed (INT) pulse, (t) is the period of the a switched (SW) pulse, and N is the number of filters to be switched. A CPU70 switches capacitors of the filter circuit 10 every time an SW pulse is generated, recitifies 20 a receive signal, and input an output passed through a Schmitt circuit 30 to the input port of the CPU70 through a gate 60. The detection register in the CPU70 holds outputs corresponding to frequencies f1, fb-fn to be selected in the order of the SW pulses, and the contents of the detection register are monitored every time an INT pulse is inputted. |
