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Advances in N-path Filtering for Broadband Tunable and Interference Robust Reception


This research aims at creating broadband tunable, fully integrated filters for the application of cognitive radio and signal classification receivers. The approach under study is the N-path filter technique which is capable of translating a baseband impedance to a reference frequency creating a tunable filter. The traditional N-path filter suffers from fundamental architectural limitations, namely : a trade-off between insertion loss and out-of-band rejection, reference clock feed-through, and jammer power handling limitations. In the first approach, the fundamental trade-off of the traditional N-path filter between insertion loss and out-of-band rejection is improved by a transmission line (T-line) N-path filter technique. The T-line N-path filter ideally absorbs the parasitic capacitance of the N-path filter into a synthetic transmission line, improving insertion loss. Moreover, the out-of-band rejection is improved by further low-pass filtering. A transmission line N-path filter was implemented in a 65 nm CMOS process that achieves a tunable band-pass filter with tunable pass-band range of 0.1-to-1.6 GHz, less than 5 dB insertion loss, 30 dB to 50 dB out-of-band rejection, in-band IIP3 of +29 dBm, and IP1dB out-of-band jammer tolerance of +11 dBm. In the second approach, a pseudorandom clocking scheme for an N- path bandpass filter is presented, which lowers the LO leakage to the filter's input and output. Measurements of a 65 nm CMOS prototype from 100 MHz to 1.4 GHz demonstrate 15 dB out-of-band rejection, P1dB of +0 dBm, in-band IIP3 of +22 dBm, out-of-band jammer tolerance of +11 dBm, and LO leakage improvement of 10 dB to 15 dB with magnitude ranging from -60 dBm to -80 dBm. Lastly, a GaN HEMT bandpass N-path filter is demonstrated for high jammer tolerance. Measurements from 50 MHz to 300 MHz of a series architecture implemented in hybrid form with Cree bare die in 400 nm technology demonstrate a IP1dB of +10 dBm, IIP3 of +24.6 dBm, and a IP1dB out-of-band jammer tolerance of +17 dBm. As an example application for the tunable front- end filter, a signal classification receiver (Cognitive radio Low-energy signal Analysis Senor IC - DARPA CLASIC program) topology is presented. The CLASIC receiver is a multi-antenna receiver that channelizes, separates, and then classifies signals within a band of interest. A key building block of the CLASIC receiver is the baseband channelizer that allows for parallel signal separation in the following stages in the receiver. Measurements were performed on a 1-to-16 BiCMOS channelizer to demonstrate feasibility. Current research avenues and potential future investigations are reviewed in the conclusion

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