A Miller N-Path Bandpass Filter with Improved Second Harmonic Rejection

摘要

While a Miller N-path filter is capable of channel selection at RF, it cannot sufficiently reject harmonic responses before going through the baseband circuitry. To suppress the second harmonic at the LNA output, this paper adds a feedback path to the conventional Miller N-path filter. The added feedback path distinguishes between the first and the second harmonic and exhibits a relatively large loop gain for the latter. As a result, the design suppresses the second harmonic at RF nodes. This is achieved without losing the desired characteristics of a conventional Miller N-path at the fundamental harmonic. The design example is a 500-MHz four-path filter simulated with the 90 nm CMOS. It achieves 13 dB gain, 2.6 dB noise figure, and +9.6 dBm out-of-band IIP3 at 50 MHz offset from the center frequency and burns 16 mW of power.