Modern grids are facing a massive integration of power electronics devices, usually associated to instability issues. In order to assess the likelihood and severity of harmonic instability in the high frequency region, this work develops a multi-variable input-admittance model that accurately reflects the following aspects: i) the discrete controller frequencies are defined inside a spectrum region limited by the Nyquist frequency; ii) the physical system aliases are transformed into lower frequency component inside the discrete controller. The proposed model shows that dynamic interactions are not theoretically band-limited; however, the control action tends to be strongly limited in a low frequency range, due to the natural low-pass filter behavior of acquisition and modulation blocks. This is reflected in a reduced resistive part (either positive or negative) of the input-admittance in the high frequency range. More specifically, considering the input-admittance passivity criterion, the excursions into the non-passive area are very smooth at high frequencies, where the input-admittance is well described by simply its inductive filter. Comprehensive experiments are conducted on a lab scale prototype, which includes measurements beyond the Nyquist frequency and alias identification. The experimental results well match the theoretical model.