Separating Bandwidth from Windowing in Spectral-Shift Evaluation of Power-Quality Waveforms

Harmonic distortion can more than halve a feeder's photovoltaic hosting capacity, and frozen time-series foundation models offer a label-efficient route to classifying those disturbances. However, their application rests on a degradation curve that bundles bandwidth removal with the evaluation's own windowing and reports only their sum. We contribute a protocol that separates the two, pairing a decimation ladder with a band-limiting ladder at fixed rate, both read through a window held fixed in seconds rather than samples, with a linear physical-feature baseline and randomly initialized twins as controls. Under that correction, all three models still fall steeply on both ladders, so the loss survives holding duration and visibility fixed. However, once duration is held fixed, the model-dependent sign reversal an uncorrected ladder comparison reports disappears, so how the window is specified mainly decides the disagreement. A label-free statistic computable from the sampling specification alone, harmonic order retained, ranks monitoring regimes out of sample here and on a second generator, but does not size the loss to the accuracy registered in advance. A spectral-shift evaluation therefore cannot be read as spectral, or used to tell a utility which monitoring rate a feeder needs, unless it reports window placement, window duration in seconds, and event-visibility fraction at every rung.