FLL discriminator for carrier tracking of extremely low-SNR high-dynamic signals

A novel design idea for frequency-locked-loop (FLL) discriminator is proposed, under the premise of changing rate of frequency-rate in returned signal is rather small, the signal after quadrature-demodulation can be regarded as a linear frequency-modulated (LFM) signal, by using estimations of its initial frequency and frequency rate as error control quantities of FLL, FLL's toleration ability to high dynamic signals is greatly enhanced. FFT discriminator and dot-product cross-product discriminator are exploited by FLL discriminator at different phase of frequency-tracking, thus wide discriminate range and high discriminate precision are acquired. An effective and simple abnormal value elimination method is presented especially for the abnormal discriminate value under extremely-low-SNR. Simulation results applying the discriminator to a FLL showed that FLL can tolerate rather high dynamics and tracking accuracy of frequency achieved 0.2 Hz even under a SNR as low as -40 dB, which verified the validity of above ideas.