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Conventional MAC protocols for wireless sensor networks have adopted duty cycling to reduce idle listening, but duty cycling leads to energy-latency tradeoff, which is one of the major challenges for designing energy efficient and high performance MAC protocols for wireless sensor networks. To break through the energy-latency tradeoff caused by duty cycling in the existing wireless sensor network, we propose a new RF wakeup sensor, which is a dedicated small RF module to check potential communications by sensing the presence of a RF signal. With RF wakeup sensor each node no longer requires duty cycling, eliminating both sleep delay and idle listening. The distinctive feature of our RF design from the existing radio sensor studies is that the RF wakeup sensor can provide the same sensitivity as the underlying RF communication module with two orders of magnitude lower energy consumption. To equalize the RF sensing range to the communication range, the RF wakeup sensor uses a dedicated amplifier for input signal. The amplifier can selectively sense signals on the predefined frequency band by employing a frequency filter that requires neither mixer nor oscillator. We evaluate the circuital characteristics of our RF wakeup sensor design by using Advanced Design System 2009 simulator. The results show that RF wakeup sensor allows a sensor node to completely turn off their communication module by performing the around-the-clock carrier sensing while it consumes only 1% energy of an idle communication module.