arXiv:2611.06385v1 Announce Type: new Abstract: The rising penetration of inverter-based resources (IBRs) brings rich control dynamics that can amplify spatial differences in nodal frequency through frequency-voltage (F-V) coupling. Existing analyses often neglect voltage dynamics or only examine the aggregate frequency, leading to overoptimistic evaluation of frequency security. To address these drawbacks, this paper develops a unified model of synchronous generators (SGs), grid-following (GFL)-IBRs, and grid-forming (GFM)-IBRs based on the magnitude and phase dynamics of equivalent internal electromotive force (EMF). Integrating these source models with network equations yields a sparse implicit closed-loop model, which fully captures the device-network F-V coupling without transfer-matrix inversion. Then, the obtained nodal responses are decomposed into common and differential modes. Using this decomposition, coupling indices and pole-residue analysis reveal how F-V coupling can amplify spatial differences in nodal frequency and how source configuration and parameters modify this effect. Finally, case studies on the modified IEEE 39-bus and 118-bus test systems validate the proposed model and demonstrate its computational efficiency.