a full duplex data transceiver for transmitting and receiving frequency - modulated signals representing binary data includes separate transmit and receive antennas , and a single oscillator which serves as both the radio frequency signal source for the transmitter and the local oscillator signal source for the receiver . during signal transmission , the oscillator output is frequency - modulated to provide a fm transmit signal to the transmit antenna . the oscillator output is frequency - modulated with binary transmit data by modulating an error feedback signal which serves as the control voltage for a voltage - controlled oscillator in a phase - lock - loop , thereby producing the fm transmit signal . during signal reception , the oscillator output , in the form of the transmitted fm signal , is received via the receive antenna along with a fm receive signal for mixing therewith to down - convert the fm receive signal . as part of the demodulation of the down - converted fm receive signal , the binary transmit data is subtracted out .