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

  • Author(s): Thomas, Chris Michael
  • et al.

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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