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 Inductance, capacitance, resistance, and dissipation factor (DF) can also be measured by various bridge circuits. They involve adjusting variable calibrated elements until the signal at a detector becomes null, rather than measuring impedance and phase angle. Early commercial LCR bridges used a variety of techniques involving the matching or "nulling" of two signals derived from a single source. The first signal was generated by applying the test signal to the unknown and the second signal was generated by using a combination of known-value R and C standards. The signals were summed through a detector (normally a panel meter with or without some level of amplification). When zero current was noted by changing the value of the standards and looking for a "null" in the panel meter, it could be assumed that the current magnitude through the unknown was equal to that of the standard, and that the phase was exactly the reverse (180 degrees apart). The combination of standards selected could be arranged to read out C and DF directly which was the precise value of the unknown. An example of this type of measuring instrument is the GenRad/IET Labs Model 1620 and 1621 Capacitance Bridges. See also ESR meter Q meter Transformer ratio arm bridge References External links "LCR meter measurement principles", HIOKI E.E. CORPORATION. "LCR Primer", IET Labs Inc., April 2012. Measuring instruments Electronic test equipment Impedance measurements