A Self-calibrated Active-RC Complex Filter For Wireless Sensor Networks

Wireless Sensor Networks (WSN) are self-adaptive networks consisted of sensornodes. They are widely used in military and civilian applications, such as war control,environment supervision, and data collection. Recently, WSN have become more andmore popular. Because the WSN nodes have the features of adopting wirelesscommunication, low price, being battery powered, and applying in complexenvironments, there are several design difficulties in WSN receivers, such as highintegrity, low power, and high steadiness. In the mainstream receiver architectures,the low-IF receivers have been widely adopted for its high integrity and no DC driftproblem. Nevertheless, Image Rejection (IR) problem has become a key issue thataffects the performance of Low-IF receiver. An on-chip complex filter is a kind ofcircuit module that can effectively solve the IR problem. A complex filter with highintegration, high Image Rejection Ratio (IRR) and low power has become a designdifficulty. On the other hand, with the development of IC fabrication technology, theprocess induced variations of the on-chip filters’ time constants have become greaterand greater. This infection affects the performance of the filters badly. Theauto-tuning circuit can effectively calibrate such infection. In this paper, aself-calibrated complex filter is design and realized. Taking into account the workingfrequency and the modulation method, this work adopts the active-RC design methodand4th order Butterworth structure. The center frequency of the filter is200kHz andthe3dB bandwidth is140kHz. Post-layout simulation shows that the IRR is60dBand the power is only0.85mW. In this paper, we also introduce a newauto-calibration method applied in active-RC filters. The circuit level design andrealization of such method is also introduced. The auto-tuning circuit calibrates thefrequency transform feature of the filter. After calibration, the post-layout simulationshows that the relative error of center frequency and the3dB bandwidth is about3%.