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We present the software-defined radio (SDR) implementation of a Radio Frequency IDentification (RFID) Listener, a passive receive-only device that decodes the signals exchanged between the RFID Reader and the interrogated Tag following the Electronic Product Code Class-1 Generation-2 standard. Our RFID Listener is based on the open-source project GNU Radio. It provides complete flexibility and full control over the entire protocol stack down to the physical layer. As such, it can be used as a powerful but inexpensive tool for testing and measuring the performance of RFID systems in real operating conditions. In this paper, we leverage the SDR Listener for a number of experiments that, taken collectively, are illustrative of the potential of such tool for RFID research. First, we use it to test comparatively different timing recovery schemes for the reception of the Tag signal in uplink, through the analysis of bit-error statistics measured experimentally. We find that the scheme proposed by Harris and Rice based on polyphase filter bank displays excellent performance in this context, very close to the theoretical bound and with a gain of 5 dB over the more common Mueller and Muller scheme. Second, we use the Listener to evaluate the impact of frequency nulls in the uplink RFID channel for different indoor scenarios. We find that frequency nulls are pronounced particularly in the presence of metal objects and obstructions of the Fresnel zone, but frequency hopping is effective in counteracting the problem. Our experiments show that the signal backscattered by a passive Tag can be correctly received up to a distance of 35 m, with low-cost equipment and without highly directional antennas.