Passive Gen2 RFID systems based on the wireless communication between readers and tags have become the enabling technology for Internet of Things (IoT) application. Such passive RFID tags are powered up by incident RF power and they respond back to the reader by modulating the backscattering power in terms of changing the tag IC impedance. In various environments this incident power can oscillate and have a certain influence on the power of tag signal. This may have a significant influence on the data available at the reader, especially when analyzing the multiple tag responses, which may happen at the same time (usually referred to as a collision of multiple tags). This paper associates the electromagnetic problem of chip's impedance matching with the variation of tag SNR at the reader. The simulation results depict the difference of tag's backscattering for some chip impedance states. By using fully configurable Software Defined Radio (SDR), the performances of the actual system and characterize the variation of SNR at the reader was retrieved.