Nonlinear filtering of narrowband interference in a wireless synchronizer

A narrowband interference rejection technique based on non-linear digital-signal-processing is demonstrated. It is implemented in a 180nm CMOS receiver IC for radio-controlled-clocks (RCC) operating in accordance with the US government´s atomic-clock BPSK-enhanced broadcast signal from station WWVB. The proposed technique combats narrowband interferers that are close in frequency to the broadcast signal and originate either from electromagnetic interference (EMI) sources outside of the RCC, or from circuitry within the RCC, such as the clock´s display. The proposed technique combines the use of the Fast-Fourier-Transform (FFT), performing a frequency-domain search, with a correlation operation in the time-domain, and makes use of non-linear mechanisms to distinguish between the desired signal and the artifacts of interfering signals. It demonstrates over 10dB of interference rejection for interferers as close as 4 Hz to the received signal, without degrading the receiver sensitivity.