Tunable 0.7–2.8-GHz Reflection-Mode N-Path Filters in 45-nm SOI CMOS

This article presents the principles of operation of a reflection-mode N-path filter and then discusses both single-balanced and double-balanced implementations. The single-balanced approach can be used to realize tunable filters with extremely low insertion loss; however, the circuit also exhibits harmonic responses. The double-balanced reflector naturally rejects even-order harmonics and can be modified to cancel the third harmonics as well to provide a tunable N-path filter with the second through fourth harmonics rejected. This comes at the expense of increased insertion loss to support the required balun circuitry used to balance the mixers. Prototypes for both types of filters have been realized in the 45-nm SOI CMOS technology. Measurements on the single-balanced reflection-mode filter indicate 0.8-2.4-dB insertion loss across a 700-3000-MHz tuning range. The input-referred 1-dB compression point is 0 dBm and the input-referred third-order intercept point (IIP3) is +10-dBm in-band and +22-dBm out-of-band (OOB), and the circuit consumes 2.5-7.2 mW of power. Measurements on the double-balanced reflection-mode filter indicate 1.9-4.4-dB insertion loss across a 700-2800-MHz tuning range. The input-referred 1-dB compression point is +0.75 dBm, and IIP3 is >+16.3-dBm in-band and >+19.5-dBm OOB, and the circuit consumes a total of 4.0-15.6 mW of power.