A method for determining the extent to which power supply resonances alter measurements of onset voltage hash in MPD thrusters is presented. Such resonances can significantly distort the measured waveforms of voltage hash and can thus change the physical interpretations drawn from measurements. This artifice was likely present in many experimental onset studies. Lumped-parameter circuit analysis is used to model the resonance that arises in the transmission line connecting the power supply to the thruster. The resonance magnitude grows with increasing distance between the thruster and the voltage measurement. Experimental demonstration of the theoretically derived results is provided by a comparison of voltage hash measured near the thruster and at a point 1 m away, and by a measurement of the current using a novel Rogowski coil. It is shown that the power supply resonance produces a spurious dominant broadband frequency peak in the voltage hash power spectrum (in the present experiment, at 1.3 MHz), obscuring the physically meaningful hash waveform. The effect of the resonance on the current, however, is shown to be negligibly small, creating oscillations on the order of 1% of the quasisteady pulse value.