| 摘要 |
PURPOSE:To prevent the variantion of a local oscillation frequency due to noise by allowing a detected signal to pass an integral discharging filter and comparing absolute values of positive and negative peak-to-peak values of the output with each other and controlling the local oscillation frequency in accordance with the comparison output. CONSTITUTION:A received digital frequency modulated signal is detected by a frequency detecting circuit 2 and is inputted to an integral discharging filter 3. The filter 3 repeats integration and discharging at transition points of data and inputs the output and a signal, which is obtained by inverting the output in an inverting circuit 4, to holding circuits 8 and 6 through clippers 7 and 5, and absolute values of positive and negative peak-to-peak values are held. Output voltages 19 and 20 of circuits 8 and 6 are compared with each other by a comparator 9 and are inputted to differential amplifiers 10 and 11, and differential voltages 23 and 24 between reference voltages 21, which are equal to absolute values of output peak-to-peak values of the filter 3 for no frequency drift, from a reference voltage generating circuit 12 and voltages 19 and 20 are outputted. The circuit 9 instructs a switching circuit 13 to select the voltage 23 or the voltage 24 in accordance with (the voltage 19)<=(the voltage 20) or (the voltage 19)<(the voltage 20), and an output voltage 25 is fed back to a receiving circuit 1 to control the frequency of a local oscillator. |
