Your Guide to LCR Meters

LCR meters measure inductance (L), capacitance (C), and resistance (R) – hence the name. LCR meters can be powerful tools for supporting the development of electronic circuits. However, to get the best from your meter, you must understand precisely how they work and the benefits they bring to the development process.

In this article, we'll look at how LCR meters work, how to choose the best meter for your needs, and what benefits they can bring you as you design and test your electronic devices.

What is an LCR Meter

At the basic level, these are potent instruments for measuring a circuit's inductance, capacitance, and resistance with precision and speed. They achieve this function by measuring current flow through the component or circuit you're testing, the voltage across it, and the phase angle between the two measurements. The meter will then calculate the impedance values from these three measurements. However, their application goes beyond simple measurements to supporting precision evaluation and validation of components to optimize your design.

A good LCR meter can measure inductance, capacitance, and resistance along with more complex parameters, including phase angle, dissipation factor, reactance, and conductance. These measurements are vital in characterizing components' real-world impedance, precision, and performance when integrated into your circuit design. 

LCR meters go beyond the capabilities of typical digital multimeters by providing more accurate and precise measurements across broader frequency ranges. The critical difference is that this higher frequency performance allows you to test components in situ on a circuit board at their intended operating frequencies to validate their performance in real-world conditions. In this application, the LCR meter supports the test phase of iterative development processes to optimize device performance.

However, it's essential to recognize that LCR meters are available in various form factors across various price points. Therefore, choosing the suitable meter for your application requires understanding the advantages and disadvantages of each type.

LCR Meter Options

Performance

Performance can vary significantly across the different LCR meters available. The key characteristics to watch out for are the test voltage, frequency range, and response time.

The test voltage of the signal generated by the meter should ideally be adjustable to match the actual voltage your circuit under test will work at in normal operation to produce representative results. The accuracy and stability of the test voltage will vary between different meter options.

The frequency range is the minimum to maximum test signal frequency range over which the meter can operate. Ideally, this range should exceed the range of frequencies that your electronic devices operate over to get the maximum value from your meter. Test frequency range and the signal's accuracy and stability are the main differentiators between lower-cost and more expensive meter options.

The response time is the period for the meter to display a stable reading after initiating the measurement. An extended response time can make testing time consuming if you make numerous measurements. However, manufacturers typically achieve faster response times by compromising accuracy, so balancing these requirements in your meter selection is vital.

Measurement Options

While all LCR meters offer readings of inductance, capacitance, and resistance, higher-end meters will also provide additional measurements. These options typically include impedance (Z), phase angle (θ), conductance (G), susceptance (B), dissipation factor (D), quality factor (Q), and equivalent series resistance (ESR).

Measurement Initiation

LCR meters are available that take measurements in response to an operator-initiated event, such as a button press and automatic options. The latter include remote triggering, typically in computer-controlled automated test equipment, or a standalone cyclically triggered operation.

Operator Displays

LCR meters have various display types, from analog needles to digital readouts. The clarity of the display and the number of displayed digits are essential considerations when choosing a suitable meter. However, remember that digital displays that show readings to a large number of digits to the right of the decimal point do not necessarily mean the measurement is as accurate as the display implies.

Meter Form

Handheld and portable LCR meters offer a compact, typically low-cost solution where portability is a critical selection criterion. These meters are ideal for field testing and maintenance, where the higher accuracy and frequency ranges available from benchtop LCR meters are not required.

Manually operated benchtop LCR meters offer higher precision and more measurement capabilities than handheld meters. They are also more likely to provide a higher maximum test frequency. 

Why LCR Meters are Useful

Look at the accompanying video to this article for a practical demonstration of using an LCR meter to take measurements of discrete components.

Resistor Inductance

To demonstrate the usefulness of LCR meters, consider the example of measuring the current draw of a circuit with a current shunt featured in the accompanying video. At low frequencies, the measured impedance will be the same as the specified resistance of the current shunt. However, the impedance will dramatically change when the circuit operates at higher frequencies. For example, at 200kHz, the impedance will be 10% greater than the specified resistance. At 1MHz, the impedance will be more than double the resistance.

This change in impedance with frequency is due to the inductance of the resistor, a parameter determined by the construction method used to produce the resistive component. Adding current bias will also affect the measurement. The video shows that just 200mA of current will result in an impedance three times the resistance.

By contrast, the video demonstrates that the current sense resistor only has an impedance that is 10% greater than the specified resistance at 1MHz due to the lower inherent inductance of the component.

These differences between resistance and impedance at different bias currents and frequencies can create significant challenges when measuring higher frequencies. In addition, a lack of detailed information in the data sheets of the resistive components often compounds this challenge. These issues hinder the prediction of the impact of component inductance on circuit performance. The ability to measure impedance at the required operating frequencies using an LCR meter enables you to quantify the effects and select the best component for your application.

Capacitor Resistance

Another example of LCR meters' usefulness is looking at the inherent resistance of different capacitor types. The video shows the measurement of the properties of three different capacitor types operating at the typical rectification frequency for mains AC to DC conversion in North America.

An aluminum electrolytic capacitor has a high series resistance and lower-than-expected capacitance for its specified rating. By contrast, an aluminum polymer capacitor of the same rating at the same frequency delivers better performance. However, at higher operating frequencies, the polymer capacitor offers a significantly lower series resistance than the polymer capacitor affecting circuit operations, and the nominal capacitance decreases as the frequency increases.

Finally, the demonstration shows that a tantalum capacitor offers better high-frequency performance than the aluminum electrolytic and polymer equivalents.

Using an LCR Meter Correctly

Getting the best out of your LCR meter requires more preparation and attention to detail than using a digital multimeter.

Firstly, the meter should be allowed to reach its normal operating temperature before making measurements. This practice ensures the internal components have stabilized, ensuring your measurements are as accurate as possible.

Secondly, the test fixtures' accuracy will directly affect measurements, including flying leads and connectors between the meter and the component or circuit under the test. Consequently, the meter will require compensation for the parasitic capacitance and inductance of the test fixtures before taking any measurements to maximize the accuracy of the results. This compensation process is essential when measuring high-impedance components or testing at high frequencies where the effects can significantly impact precision.

You should carefully select your testing frequencies to prevent component behavior from adversely affecting results and causing incorrect interpretation.

Before using an LCR meter, you must understand the differences between measurement parameters, such as resistance, impedance, capacitance, and inductance, so you choose to measure the correct parameters for your particular application.

Always use the highest available test signal level from your LCR meter to gain the best measurement accuracy and resolution. However, the level must be within your circuit's limits and the test setup's constraints. This advice is especially crucial when working with high-impedance components.

Finally, use good quality fixtures, test leads, and connectors and check their condition before using them. Any poor quality or damaged elements and dirty, corroded, or loose connections can introduce signal noise and generate measurement errors, reducing the accuracy of readings.

Conclusion

LCR meters are essential for accurately measuring the electrical properties of components under operational conditions. Using an LCR meter to evaluate discrete components before incorporating them into a critical circuit lets you ensure your board will perform as expected and allows you to optimize its performance before committing to manufacture.