Due to efficiency and cost concerns, an increasing number of switch mode power supplies and inverter based sources are connected directly to the power grid. Switched components can change the impedance connected to the grid several times during a mains cycle, and often do so synchronously to the mains frequency. Measurements that average over one or more mains cycles and calculations of system behavior in the frequency domain only will fail in those cases. A measurement technique is presented to measure this time variable, periodic impedance behavior in the frequency domain over very short time slices, synchronized to the mains cycle and at high resolution in time and frequency. Additionally, the measurement results of a tablet charger are shown at frequencies up to 500 kHz. The dynamic in impedance is larger than 1:10000 which can easily be depicted by this measurement approach.