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What Is an LCR Meter
An LCR meter is an electrical test instrument used to measure inductance (L), capacitance (C), and resistance (R) of components and circuits at various frequencies. Besides displaying these electrical characteristics, LCR meters can also typically display impedance (Z), phase angle (θ), dissipation factor (D), quality factor (Q), and equivalent series resistance (ESR). Some LCR meters display admittance and its components, conductance and susceptance.
Benefits of an LCR Meter
It is likely that once you become comfortable making measurements with an LCR meter, you will find many applications for it. An LCR meter is a powerful adjunct to a digital multimeter. The following are some benefits of an LCR meter:
● Measure the impedance of a circuit and help you troubleshoot or characterize existing circuits.
● Identify unmarked components and sort components into bins with specified tolerances.
● Battery-operated, handheld LCR meters are convenient to carry into the field for testing away from the bench.
● Measure AC characteristics of a component as a function of frequency and amplitude to help you predict the AC behavior in a circuit.
● Characterize motors1 by inductance measurements and inductance as a function of rotor angle.
● Identify the coil leads of an unmarked relay by measuring the inductance.
● Select components for passive filters, then measure the filter's impedance at various frequencies.
● Measure components in-circuit. In particular, find capacitors with high ESR.
● Estimate the winding ratio of a transformer via inductance measurements.
● Measure the Q of coils. Sometimes a lossy inductor is desired and the LCR meter can tell you if you've made it correctly.
● If you maintain equipment, measure the equipments' characteristics with the LCR meter and write the values down for future reference.
LCR meters are available in various types, each suited for different applications and requirements. The most common types include:
Handheld LCR meters
These portable devices are ideal for field use and on-the-go measurements. They usually have a limited frequency range and lower accuracy compared to benchtop models, but they offer the convenience of mobility and ease of use.
Benchtop LCR meters
These are high-precision instruments designed for laboratory and production environments. Benchtop LCR meters offer a wide frequency range, high accuracy, and advanced features such as programmability and data logging. However, they are more expensive and less portable than handheld models.
PC-based LCR meters
These LCR meters connect to a computer via USB or other interfaces and use software to control the measurements and display results. PC-based LCR meters offer the flexibility of customization, data processing, and storage capabilities, making them suitable for research and development applications.
Key Features of LCR Meters
While LCR meters vary in design and functionality, there are several key features that are generally included in most models:
Test Frequencies
LCR meters offer a range of test frequencies. This is critical as the values of L, C, and R can vary with frequency.
Measurement Accuracy
Accuracy is a crucial factor. Higher-end models offer better accuracy, some measuring values to the sub-1% level.
Equivalent Circuit Models
LCR meters can model the device under test as a series or parallel equivalent circuit. This allows the user to match the measurement approach to the particular component or circuit being tested.
When choosing an LCR meter, it is essential to consider the following factors:
Measurement accuracy: Select an LCR meter with the required level of accuracy for your application. High-precision benchtop LCR meters typically offer better accuracy than handheld or PC-based models.
Frequency range: Consider the frequency range required for your measurements. Some applications may require LCR meters with a broader frequency range, while others may only need a limited range.
Test voltage/current: Ensure that the LCR meter can supply the appropriate test voltage or current for your component or device under test.
Portability: If you need an LCR meter for field use, opt for a handheld model that offers portability and battery operation.
Features and connectivity: Look for advanced features such as programmability, data logging, and computer connectivity if your application requires customization and data processing capabilities.
Budget: Finally, consider your budget and select an LCR meter that provides the best value for your specific requirements.
The automatic balance bridge method is a circuit design that is used in many LCR meters as the measurement circuit. The circuit has four terminals (Hc, Hp, Lp, and Lc), all of which are connected to the measurement target. See the right for an overview of the circuit and below for a description of the functionality of each terminal.
1. Hc:
Applies a measurement signal generated with controlled frequency and amplitude to the measurement target. The frequency can be controlled within the range of several millihertz to several megahertz, and the amplitude from 5 mV to 5 V.
2. Hp:
Detects the measurement target's Hi potential. The detection circuit's input impedance is extremely high, allowing accurate detection of the potential without a voltage drop.
3. Lp:
Detects the measurement target's Lo potential.
4. Lc:
Converts the current flowing to the measurement target into a voltage based on the detected resistance and detects the result. The Lc terminal's potential is always held to 0 V.
LCR meters that use the automatic balance bridge method use BNC connectors for all four of their terminals. These connectors incorporate a shielded coaxial design that protects the measurement signals and detection signals from external noise. In general, most detection circuits use the five-terminal method or the four-terminal-pair method.
1. Two-terminal method:
This design contacts the measurement target with two terminals. Measured values include wiring resistance and contact resistance and are significantly affected when the measurement target has low impedance. In addition, due to the existence of stray capacitance between the two cables, the measurement signal flows to the stray capacitance as well as the measurement target during measurement at high frequencies and high-impedance measurement, contributing a source of error.
2. Five-terminal method:
This method reduces the effects of wiring resistance and contact resistance by using separate cables for signal current and voltage detection. In addition, it reduces the effects of stray capacitance by using shielded cables and placing the shielding portion of the cables at the same potential. This method can be used to reduce measurement error for impedance values ranging from low to high.
3. Four-terminal-pair method:
This method can reduce measurement error from impedance values ranging from low to high by reducing the effects of the magnetic field caused by the measurement current. It can cancel the magnetic field by using shielded cables and overlapping the cables carrying the current to and from the measurement target.
Software automatically recognizes the LCR meter when connected to the PC and switched on. Only when the LCR meter is detected, calibration is enabled.
Manual Calibration
Press the Calibration button. A message displays, instructing you to use the Local/Lock key on the LCR meter to set the LCR meter into local control. The LCR meter is reset and a Processing message displays on the LCR meter front display. Proceed as follows:
● Open Calibration.
● From the instrument Gap Control panel, perform Zero Fixture.
● Set the gap to 25 mm and close the ETC doors.
● On the LCR meter front panel, press Meas Setup (to the left of the numbered keypad).
● From the LCR meter screen, select CORRECTION.
● Using the LCR meter directional keypad, scroll to CABLE and select 2 m.
● Using the LCR meter directional keypad, scroll to OPEN and select On.
● With the cursor still highlighting OPEN, select Meas Open from the
● LCR meter screen. A OPEN measurement in progress message displays.
● When the calibration is complete, press Display Format on the meter front panel. Confirm the following value: Cp < 0.01pF.
Short Calibration
● From the instrument Gap Control panel, command a zero gap to bring the two geometry plates together and apply 5 N to 10 N (500 gm to 1000 gm) of force from the instrument Axial Force Control panel.
● Disconnect the test fixtures from the BNC junction box and short the upper and lower junction box with the short cable supplied.
● On the meter keypad, press Meas Setup.
● From the LCR meter screen, select CORRECTION.
● Using the LCR meter directional keypad, scroll to SHORT and select On.
● With the cursor still highlighting SHORT, select Meas Short from the LCR meter screen. A SHORT measurement in progress message displays.
● When the calibration is complete, press Display Format on the meter front panel.
Automated calibration
● Select the Use calibration wizard option button to enable the automated calibration. Make sure that the dielectric test fixtures are installed and connected to the BNC junction boxes.
● Close the ETC doors and press the Calibration button. The text Resetting the meter – Please wait displays. At the same time, a Processing message is shown on the front display of the LCR meter.
● When finished, the text changes to Performing open measurement – zeroing the gap. The gap is zeroed automatically, and the upper plate is positioned at a gap of 25 mm. The text changes to Performing open measurement and an Open measurement is performed. An OPEN measurement in progress message displays on the LCR meter display.
● When finished successfully, an Open calibration result message displays in the calibration tab with the correction value (~1 e-10 µF) and a green check mark.
● The information text changes and instructs you to disconnect the cable from the upper and lower test fixtures and shorten the upper and lower BNC junction boxes with the short cable supplied.
● Press Continue to perform the short measurement. The text changes to Performing short measurement.
● The plates are now brought together and a compression force of 5 N is applied. The Short calibration is performed. A SHORT measurement in progress message displays on the LCR meter display.
● When finished successfully, a Short calibration result (uncorrected) message displays in the calibration tab with the correction value (~100 mW), and a Short calibration result (corrected) message displays in the calibration tab with the correction value (~50 mW). A green check mark indicates a successful calibration. If the short calibration is not performed successfully, a red X displays. Use the Repeat button to redo the Short calibration.
In general, LCR meters work on similar principals as impedance analyzers and the same parameters are to be considered. When measuring electrical impedance, main specifications are frequency and impedance measurement ranges, accuracy, speed and points per sweep. Just as important are requirements of the test setup like 2, 3 or 4 electrode configuration, bias control and scalability – when it comes to multichannel or high throughput problems. Since most applications require special care contacting a "device under test" the interface and extension possibilities of the instruments are also to be considered. Looking at instruments for impedance spectroscopy it is essential to look deeper than just the plain key specification from a product summary. Accuracy and speed both are at least dependent on frequency and absolute value of the measured impedance. Most manufacturers will specify across a very large range of impedances and frequencies, but a closer look into the datasheet will reveal that only certain range are covered in full accuracy. The distribution of them strongly varies across instruments. Thus, it is important to look at the accuracy-contour plots showing which accuracy can be obtained at what absolute value and frequency. For a detailed comparison of specifications for the most widely used impedance analyzers on the market, check out this article. In simplified terms, LCR meters oftentimes are characterized not in terms of impedance ranges, but in terms of L, C and R ranges. However, in most cases it is important to measure at a specific test frequency and for that purpose the accuracy contour plots are the best option to understand an instrument´s capabilities.
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