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A novel RF sensing circuit for a cognitive radio to sense spectral environment is proposed based on injection locking and frequency demodulation techniques. In the experiments, a prototype circuit is designed in the 2.4 GHz ISM band to cover a sensing frequency range from 2.4 to 2.484 GHz. The spectrum scanning over the 84-MHz bandwidth is accomplished in less than 1 ms. The experimental results show that the RF sensing circuit can sense the frequency and the power of frequency-modulated signals as well as digitally modulated signals with a constant envelope.