A dual-use PLL frequency synthesizer for use in a transceiver is capable of operating as a local oscillation generator in a receiving mode and as a transmitter in a transmitting mode. The PLL frequency synthesizer includes a digital processor, a Digital-to-Analog Converter (DAC), a low pass filter and a phase locked loop. The digital processor generates a digital signal, in which the digital signal is a modulated digital signal in the transmitting mode, and the digital signal is a reference digital signal in the receiving mode. The DAC converts the digital signal to an analog signal, and the low pass filter filters the analog signal to produce a filtered analog signal. The phase locked loop up-converts the filtered analog signal to an RF signal. In the transmitting mode, the RF signal is a modulated RF signal, and in the receiving mode, the RF signal is a reference RF signal.