Previous studies of Super Dual Auroral Radar Network (SuperDARN) echoes from high latitudes have shown large spectral widths (≥200 m/s), which cannot be interpreted in terms of ionospheric drifts. A recent model of Andr¿ et al. <1999, 2000a, 2000b> invoked modulation of ionospheric plasma drift velocity by ultra low frequency (ULF) waves within the Pc1-2 band (0.1--5 Hz). Their simulated signal was processed in the same way as real radar echoes, and the output spectral width was much larger than the input signal bandwidth. This was attributed to signal undersampling. After re-examining their model we found that the spectral broadening was caused by the way the signal was simulated. This paper presents a revised model that gives spectral width estimates close to the input signal bandwidth. An alternative explanation of the experimental data involves spatio-temporal non-uniformity of the ionospheric irregularities due to particle precipitation.