Patent ID: 7155005

Claim:
An apparatus of ring-back constriction, coupled to a transmission line, for constricting a ring-back effect, the apparatus comprising: a comparator, coupled to the transmission line, for comparing a line signal on the transmission line with a reference voltage, and accordingly outputting a comparison signal; a termination controller, coupled to the comparator, for outputting a termination control signal according to the comparison signal; a termination variable resistor, coupled to a termination voltage and the transmission line, the resistance of the termination variable resistor being adjusted according to the termination control signal for providing a voltage to the transmission line; a constriction controller, coupled to the comparator, for outputting a constriction signal; and a transistor, having a gate and a source, the gate receiving the constriction signal, the transistor being coupled to a constriction voltage and the transmission line, the resistance of the transistor being adjusted according to the voltage difference between the gate and the source; wherein when the level of the line signal changes from a high voltage level to a low voltage level, the level of the constriction signal successively changes from a flint voltage level to a second voltage level to turn on the transistor, maintains the second voltage level for a delay period, and returns to the first voltage level; wherein after the level of the line signal changes from the high voltage level to the low voltage level, the transistor is turned on to provide the constriction voltage to the transmission line, so that the level of the transmission line is pulled up to reduce the undershot and thus the ring-back effect is constricted; wherein the constriction voltage is larger than the low voltage level.