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This paper presents improvements to frequency-tracking P-class and M-class phasor measurement unit (PMU) algorithms. The performance of these algorithms is tested in simulation against the reference (basic) algorithms from the C37.118 standard, using the formal test procedures specified in that standard. The measurements of total vector error (TVE), frequency and rate-of-change-of-frequency (ROCOF) are all investigated for compliance. Generally, TVE is found to be compliant for all algorithms. By contrast there are significant excursions for frequency and ROCOF measurements. More extreme nonstandard tests are also applied, involving multiple simultaneous interferences. The proposed algorithms exhibit improvements in frequency and ROCOF accuracy, with errors reduced by factors of up to 150 compared to the basic algorithms. A Hybrid P/M-class PMU is proposed and demonstrated, offering P-class response to dynamic steps but M-class steady-state accuracy. However, setting usable trigger thresholds for this device requires a thorough investigation of interharmonic effects on P-class PMUs. This investigation poses more questions than it answers, leading to a questioning of the validity of any frequency or ROCOF measurement from any P-class PMU.