Usually the
device under test (DUT) is subjected to an
AC voltage source. The meter measures the
voltage across and the
current through the DUT. From the ratio of these the meter can determine the magnitude of the impedance. The
phase angle between the voltage and current is also measured in more advanced instruments; in combination with the impedance, the equivalent capacitance or inductance, and resistance, of the DUT can be calculated and displayed. The meter must assume either a parallel or a series model for these two elements. An ideal capacitor has no characteristics other than capacitance, but there are no physical ideal capacitors. All real capacitors have a little inductance, a little resistance, and some defects causing inefficiency. These can be seen as inductance or resistance in series with the ideal capacitor or in parallel with it. And so likewise with inductors. Even resistors can have inductance (especially if they are wire wound types) and capacitance as a consequence of the way they are constructed. The most useful assumption, and the one usually adopted, is that LR measurements have the elements in series (as is necessarily the case in an inductor's coil) and that CR measurements have the elements in parallel (as is necessarily the case between a capacitor's 'plates'). Leakage is a special case in capacitors, as the leakage is necessarily across the capacitor plates, that is, in series. An LCR meter can also be used to measure the inductance variation with respect to the rotor position in permanent magnet machines. (However, care must be taken, as some LCR meters will be damaged by the generated EMF produced by turning the rotor of a permanent-magnet motor; in particular those intended for electronic component measurements.) Handheld LCR meters typically have selectable test frequencies of 100 Hz, 120 Hz, 1 kHz, 10 kHz, and 100 kHz for top end meters. The display resolution and measurement range capability will typically change with the applied test frequency since the circuitry is more sensitive or less for a given component (i.e., an inductor or capacitor) as the test frequency changes. Benchtop LCR meters sometimes have selectable test frequencies of more than 100 kHz, with the high end Keysight E4982A operating up to 3 GHz. They often include options to superimpose a DC voltage or current on the AC measuring signal. Lower end meters might offer the possibility to externally supply these DC voltages or currents while higher end devices can supply them internally. In addition benchtop meters typically allow the usage of special fixtures (i.e., Kelvin wiring, that is to say,
4-wire connections) to measure
SMD components, air-core coils or transformers. ==Bridge circuits==