The requirement for real-time processing indeed poses challenges on implementing spectrum sensing algorithms. Trade-off between the complexity and the effectiveness of spectrum sensing algorithms should be taken into consideration. In this paper, a fast Fourier transform based spectrum sensing algorithm, whose decision variable is independent of noise level, is introduced. A small form factor software defined radio development platform is employed to implement a spectrum sensing receiver with the proposed algorithm. To our best knowledge, it is the first time that real-time spectrum sensing on hardware platform with controllable primary user devices is demonstrated.