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A novel RF sensing circuit for a cognitive radio to sense spectral environment is proposed using injection locking and frequency demodulation techniques. The research starts with deriving a generalized locking equation for oscillators as a theoretical basis, and then develops the sensing principle and circuit architecture to a practical and useful level. To accurately evaluate the sensing performance, a discrete time approach is investigated to compute the sensed output signal. In the experiments, a prototype circuit is designed in the 2.4-GHz industrial-scientific-medical 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 agree quite well with the theoretical predictions, showing that the proposed RF sensing circuit can fast and reliably detect frequency and power for analog and digital modulation signals.