To characterize the electrical response of a system as a function of frequency (from under 1 Hz up to the GHz range), a Vector network analyser or equivalent is typically used. The system will apply a known electrical stimuli of specific frequency and measure the reflected/absorbed voltages and current both in magnitude and phase. The resulting measurement presents a detailed electrical characterization of the system in terms of equivalent impedance of material under test as a function of frequency throughout the measured frequency range.
The technique is extremely powerful in the sense that it can provide complete electrical response information on any linear electrical system. Obtaining this information as a function of frequency enables detailed electrical description of complex electrical response in a simple and concise way.
It is commonly used to measure high frequency devices as well as measure the material properties in extended frequency ranges.