A cable modem includes an amplifier, a power module, a switch circuit, a monitor signal, a duplexer and a RF matching circuit. The switch includes a PMOS transistor and a first electronic switch. The RF matching circuit included a second electronic switch and an electronic matching device. The monitoring signal source is configured for outputting a control signal. When the monitoring signal source stops outputting the control signal, both the first electronic switch and the PMOS transistor are turned off to cut off the power to the amplifier, the second electronic switch is turned on, and conducts the RF signal received by the signal receiving/transmitting circuit to the duplexer which further conducts the RF signal to the radio frequency matching circuit.