A circuit includes a main connector and a single-pole double-throw switch. A common contact of the main connector is used as a port of a main receiver, and a normally-closed contact of the main connector is used as a port of a main antenna. Furthermore, a normally-open contact of the main connector is used as a port of an external antenna, and a common contact of the single-pole double-throw switch is used as a port of a diversity receiver. Additionally, a normally-closed contact of the single-pole double-throw switch is used as port of a diversity antenna, and a normally-open contact of the single-pole double-throw switch is connected to the normally-closed contact of the main connector.