An 4th-order N-path Bandpass Filter with a Tuning Range of 1-30 GHz and OOB Rejection > 30 dB in 28 nm CMOS

A 4th-order N-path bandpass filter technique with a tuning range of 1-30 GHz and a constant 3-dB bandwidth of 500 MHz is analyzed. To meet the demand for multi-mode and multi-band RF receiver chips, traditional off-chip and non-tunable filters such as SAW and BAW filters should be replaced by N-path filter due to its wide tuning range of center frequency, high Q and small noise figure. The proposed filter consists of four switches (mixers) to convert the frequency band of input signals and a sampling clock circuit which operates at 2-60 GHz. The tuning range of conventional N-path filters is limited by the bandwidth of its clock divider and the overlap of the four clocks with fixed phase difference. To overcome these shortages, dividers and NAND gates based on current mode logic(CML) structure are proposed. In this work, the out of band (OOB) rejection is greater than 30 dB and the simulated out-of-band IIP3 is 5.1 dBm at 25 GHz. To the best of the authors’ knowledge, I this is the first time for N-path filter to work at millimeter wave frequency band.