| 摘要 |
PURPOSE:To adjust automatically the center frequency of a filter to an optional frequency, by controlling the switching duty ratio of a switching element, which is connected in series to a resistance provided in the filter, to vary the resistance value. CONSTITUTION:Changeover switches 5 and 6 are switched to input a reference signal of the center frequency to be adjusted by a reference signal generating circuit 7, and simultaneously, sweep scanning is so performed that the duty ratio of the switching signal to a switching element S in a switching resistance circuit 2 of a filter 1 becomes 0-100%. The resistance value of the switching resistance circuit 2 is changed in inverse proportion to the increment or the reduction of the switching-on time by the sweep scanning of the duty ratio, and the center frequency of the filter 1 is varied. When the center frequency is so adjusted that it coincides with the frequency of the reference signal, the DC voltage outputted from an integrating network 8 becomes maximum, and scanning of the duty ratio is stopped, and the device is fixed to the switching control state. |
