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A low-cost, high-speed, high-resolution, adaptively tunable microwave network for an SDR UHF RFID reader reflected power canceller

A key problem with custom built and software-defined RFID reader architectures is that the large continuous-wave (CW) signal that must be transmitted during RFID tag reading must be concurrently processed by the RFID receive (RX) chain. Methods for handling this signal that are easily implemented in custom RFID reader ASICs are not typically inherently available in low-cost commercial off the shelf components. In this paper, a subranging digitally-tunable capacitor approach to reflected power cancellation in a low-cost software-defined RFID reader to address this issue is described. The key components of the proposed architecture cost under $4 in bulk and together consume less than 1mA quiescent current. The subranging aspect of the tunable microwave network providing the variable reflection coefficient reduces the residual cancellation error of the network by 30dB over the non-subranging case. The resulting network improves the TX leakage signal at the SDR ASIC RX input by 50dB over the uncancelled case and can be driven by an adaptive cancellation algorithm that converges in a median time of less than 10ms.