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Cognitive radio has been put forward to make efficient use of scarce radio frequency spectrum. Spectrum sensing is the cornerstone of cognitive radio. As a part of the effort toward building a cognitive radio network testbed, we have demonstrated real-time spectrum sensing. However, current hardware platforms for cognitive radio introduce time delays that actually undermine the accuracy of spectrum sensing. The time delay, named response delay, incurred by hardware and software can be measured at the receiving antenna and the transmitting antenna co-located at a secondary user (SU). In this paper, minimum response delays are experimentally quantified and reported based on two hardware platforms, the universal software radio peripheral 2 (USRP2) and the small form factor software defined radio development platform (SFF SDR DP). In a subsequent paper, an approach for single-user prediction is proposed to combat with the response delays. This work is a part of the effort toward building a cognitive radio network testbed.