Signal transceiver with enhanced return loss in power-off state

1. A signal transceiver with enhanced return loss in a power-off state, comprising:

a connector, for receiving a signal;

a band-pass filter, coupled to the connector for filtering the signal;

a front-end module, for de-modulating the signal; and

an impedance transformation circuit, coupled between the band-pass filter and the front-end module for transforming an input impedance of the signal transceiver according to a power state of a power source, comprising:

an input terminal, coupled to the band-pass filter for receiving the signal;

an output terminal, coupled to the front-end module for outputting the signal to the front-end module;

an impedance transforming unit including a first node and a second node; and

a first power source input circuit, coupled to the first node of the impedance transforming unit for providing the power source;

wherein an impedance at the first node is equivalent to a short circuit and an impedance at the second node is equivalent to an open circuit when the power source is in a power-on state, and the impedance at the first node is equivalent to the open circuit and the impedance at the second node is equivalent to the short circuit when the power source is in a power-off state.

2. The signal transceiver of claim 1 , wherein the impedance transforming unit comprises:

a transmission line, wherein an end of the transmission line is coupled to the first power source input circuit via the first node, and another end of the transmission line is coupled to the second node; and

a first resistor, wherein an end of the first resistor is coupled to the input terminal, and another end of the first resistor is coupled to the second node.

3. The signal transceiver of claim 2 , wherein the length of the transmission line is substantially a quarter wavelength long of the signal.

4. The signal transceiver of claim 2 , wherein a resistor value of the first resistor is determined by a characteristic of an element connected to the connector.

5. The signal transceiver of claim 1 , wherein the first power source input circuit comprises:

a second resistor, coupled between a power supply and the impedance transforming unit.

6. The signal transceiver of claim 1 , wherein the impedance transformation circuit further comprises:

a second power source input circuit, wherein an end of the second power source input circuit is coupled to the input terminal for providing the power source.

7. The signal transceiver of claim 6 , wherein the second power source input circuit comprises:

a third resistor, coupled between a power supply to the input terminal.

8. The signal transceiver of claim 1 , wherein the impedance transformation circuit further comprises:

a bias circuit, coupled between the input terminal and the output terminal for changing a voltage value of the signal.

9. The signal transceiver of claim 8 , wherein the bias circuit comprises:

a first switch; and

a fourth resistor, wherein an end of the fourth resistor is coupled between the first switch and the output terminal.

10. The signal transceiver of claim 1 , wherein the impedance transformation circuit further comprises:

a second switch, wherein an end of the second switch is coupled between the first node of the impedance transforming unit and the first power source input circuit.