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N-path Filtering for Wideband Receivers


Growing demand for multi-band wireless systems has emphasized tunable highly linear filtering without sacrificing cost or form factor. The 3G/4G frequency-division duplex (FDD) cellular systems such as long-term evolution (LTE) are more prone to interference from strong blockers associated primarily with the co-located transmit signal. In an LTE receiver (RX), transmit signal introduces a strong out-of-band (OOB) blocker, transmit (TX) leakage, placing stringent linearity and power-handling capability requirements at the diversity RXs. In conventional multi-band systems, a large number of SAW filters are used and single-pole multi-throw switches select the appropriate SAW filter for the desired channel. The cascaded insertion loss of the filter and switch, impose a significant noise performance degradation. Therefore, filtering solutions that are tunable, blocker tolerant, and highly linear are critical to extending multi-band schemes. In this research work, tunable filters are proposed with offset band-reject filtering to address TX leakage.

First, a N-path filter is demonstrated in 45-nm CMOS SOI technology and consists of a combination of tunable band-pass and band-reject filtering. A tunable filter was presented with more than a decade tuning range, high linearity and blocker tolerance for a SAW-less diversity path receiver in FDD cellular systems. The 3-dB bandwidth (BW) of the pass band is greater than 80 MHz with an independently tunable reject band, providing the ultimate rejection is 33 dB to 41 dB while the pass band insertion loss is between 2.6 dB and 4.3 dB over the tuning range. The proposed filter offers 29-dBm OOB 3rd-order input-intercept power (IIP3) and 22-dBm in band (IB) IIP3 with a 10-dBm blocker 1-dB compression point (B1dB).

Then, A channel-selecting low-noise amplifier (CS-LNA) with blocker filtering is presented for a SAW-less diversity path receiver in frequency-division duplexing cellular systems. A hybrid -path bandpass filter/band-reject filter (BPF/BRF) feedback network is applied to the LNA to create close-in reject bands around the passband to suppress transmit leakage and improve the out-of-band (OOB) input-referred third-order intercept point (IIP3). Control of the frequency and depth of the reject bands is demonstrated with analysis and simulation. A cancellation linearization is applied to improve the in-band (IB) linearity. Adaptive LO swing and silicon-on-insulator (SOI) thick-oxide devices are demonstrated to improve the power-handling capability. The proposed CS-LNA is implemented in 32-nm CMOS SOI and operates from 0.4 to 6 GHz with the maximum rejection larger than 60 dB and power-handling capability up to 17 dBm. The prototype demonstrates an OOB IIP3 of 36 dBm and IB IIP3 of 10 dBm at 1 GHz. To the authors’ knowledge, this is the highest blocker rejection, power-handling capability, linearity, and the widest tuning range for an LNA with tunable filtering.

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