Frequency-Selective Limiting Using Automatic Gain Control of Stagger-Tuned N-Path Filters

This paper presents a linear self-adaptive N-path filter that can autonomously adjust its gain according to the average rather than the instantaneous power level of the signals present in its passband. Instead of relying on a nonlinear limiting mechanism, an automatic gain control loop adjusts the gain of a 4th-order filter by modulating the frequency separation between its comprising 2nd-order filters to attenuate passband signals exceeding the loop programmable limiting threshold. The proposed self-adaptive filter offers better performance in several aspects over prior adaptive filters. Simulation results using 0.18-μm CMOS technology demonstrate that the self-adaptive filter can provide a programmable limiting threshold level down to −30 dBm that does not drift with the local oscillator frequency. Thanks to the piecewise constant gain, the proposed autonomous filter provides an in-band IIP3 that is 3.6 dB higher than prior adaptive filters.