A Tunable 0.2–1.2-GHz N-Path Bandpass Filter With Improved Harmonic and Out-of-Band Rejection

In this article, a novel cascaded N-Path filter architecture is proposed to enhance both out-of-band rejection and harmonic rejection (HR) of conventional N-Path filter. An active gain-boosted N-Path filter front end for signal amplification and noise suppression, and a passive N-Path filter with harmonic rejection switch backend provide high out-of-band and harmonic rejection and enhance linearity after cascade. The resulting filter is fabricated in a 180-nm process and operates under a six-phase nonoverlapping clock with a duty cycle of 16.7%. The measurement results show that in the tunable frequency range of 0.2–1.2 GHz, the filter achieves more than 32 dB of second and third harmonic rejections, a maximum of 55-dB out-of-band rejection, a bandwidth of 20–40 MHz, an in-band gain of 0–5 dB, and an in-band matching of $S 11\lt -15$ dB. The in-band noise figure (NF) is 4.5–5.2 dB. As for linearity, the in-band P1dB is −2.8 to −0.9 dBm, and the out-of-band input third-order intercept point (OOB-IIP3) is +10.9 and +24.6 dBm at $1\times $ BW and $5\times $ BW, respectively. The entire filter consumes 12.6–23.4 mW of power with an area of 0.45 mm².