A radio communication unit employs passive devices and a loop filter to receive a coded frequency modulated wakeup or synchronization signal that enables the receiving radio unit and initiates communications. The loop filter simultaneously tracks phase and frequency and provides smoothing to enable synchronization with weak signals (e.g., signals that typically do not enable a standard phase locked loop (PLL) to lock). In particular, when the communication unit is in a standby mode, the passive circuits of the present invention are receiving energy from a unit antenna and initially function as a phase locked loop (PLL) to lock onto an incoming signal. The wakeup or synchronization signal includes a series of tones at different frequencies, where the specific sequence is prearranged between the transmitting and receiving units. When each tone in the sequence has been detected by the receiving unit, the unit is enabled for communications.