A novel modular positive-sequence estimation algorithm for phasor measurement units (PMUs) is described in this paper with a focus on the restrictions imposed by the IEEE C37.118.1-2011 standard. The first stage consists in a three-phase demodulator which allows us to separate the positive-sequence from the negative-sequence signal in the frequency domain and to eliminate the zero-sequence signal. The second stage is a prefilter that mitigates noise and interference, thus relaxing the filtering requirements of the following stage. The suitability of a linear-phase FIR filter is shown and a comparison of single and multistage designs is presented. On the third stage, a digital state-space-based extension of a synchronous reference frame-phas...