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Reliable spectrum sensing is the key requirement for secondary users to opportunistically utilize the spectrum in the absence of primary users. To increase the secondary user's throughput and efficiency, it has to be able to detect the needed white band out of all possible bands in the shortest possible time. To speed up the spectrum sensing process and to minimize the power consumption overhead, detectors have to sweep a wide band in a parallel fashion. In this paper, we analyze the effect of the I/Q mismatch on the performance of the wide-band energy detector as the simplest detection method. We demonstrate that the false alarm probability can go beyond 0.5 for 2° phase mismatch or 0.1dB gain mismatch between I and Q paths when there is 30dB near-far effect between the mirror primary users. Afterward, we suggest an adaptive threshold method that improves the performance knowing the I/Q imbalance parameters a priori.