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The new standard C37.118.1 lays down strict performance limits for phasor measurement units (PMUs) under steady-state and dynamic conditions. Reference algorithms are also presented for the P (performance) and M (measurement) class PMUs. In this paper, the performance of the Reference M class filter is analysed. Similarly to the Reference P class filter, the M class filter is found to have a relatively poor performance when the power system frequency is off-nominal. A different architecture for an M class Phasor Measurement Unit (PMU) algorithm is presented, and in particular a completely different design of M class filter. This is shown to have much improved rejection of unwanted harmonic and inter-harmonic components. This allows consistent accuracy to be maintained across a ±33% frequency range. ROCOF (Rate of Change of Frequency) errors can be reduced by factors of >;100.