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A novel technique used in a DSP-based software phase-locked loop (SPLL) is developed, and named as the method of dynamical observation (MDOI). By dynamical adjusting the observation interval of the phase comparator in the SPLL, this new technique of MDOI conquers the contradiction of the different requirements to the observation interval between the frequency lock-in range and the loop stability, and then notably increases the frequency lock-in range of the SPLL in the low Eb/No signal condition. The proposed SPLL architecture contains a Viterbi & Viterbi digital phase estimator, an unwrapping module and a software number controlled oscillator (NCO). This SPLL gives a large lock-in range and hold range, and can rapidly get locked in the low Eb/No signal condition. The theoretical approach is given and some simulation results are presented.