PURPOSE: To maintain a high reliability of transmission even when a salt damage takes place to a distribution line and its earth impedance is fluctuated.
CONSTITUTION: A master station transmission/reception control circuit 16 fetches an output signal of a measurement circuit 15 of a low voltage control section 13 and measures a level of a noise component due to fluctuation of a zero phase voltage from an output of the measurement circuit 15 at non-transmission and calculates a level of the signal component at the transmission so as to obtain a preset S/N based on the level of the noise component. Then the circuit 16 controls switches SW0-SW3 so as to select an impedance element for generating the zero phase voltage of a zero phase voltage generating circuit 11 in response to the level of the signal component.
MIYAZAKI TERUNOBU
SAKO EIZABURO
NISHIJIMA KAZUO
