# Best Practices for Using LCR Meters

No electronic component is ideal, and in many instances it is necessary to get an accurate measure of component values from a direct measurement. Thankfully, there is a piece of equipment that is essential for any lab setup: an LCR meter.

These meters provide a quick way to take direct measurements of the inductance, resistance, or capacitance of a component without a requirement to build a test circuit. As with all meters or instruments, there are certain procedures and considerations that must be accounted for, and the value captured from an LCR meter may not be valid in all situations. Follow these tips for successfully using an LCR meter and make sure you understand the applicability of the results.

## LCR Meter Measurements

LCR meters are like simplified impedance analyzers; they provide direct measurements of inductance, capacitance, and resistance. This is done by sourcing a specific voltage and current at a desired frequency, and the output voltage and current are measured. The ratio of the source and dropped voltage/current gives the value the impedance. From these measurements, the resistance, capacitance, or inductance associated with the component could be determined.

Although the above points describe the core functions of LCR meters, these instruments can provide additional information via measurement:

• Series DC resistance (ESR) for a reactive component

• Total reactance

• Magnitude of impedance

• Phase shift at a specific frequency

In addition, multi-terminal components like transformers can be measured for their parasitics, such as leakage inductance, by terminating certain leads in various ways. These meters do not make an assumption about any circuit model for the component being tested, nor do they regress to any kind of circuit model. They only give a value for the desired measurement, it will be the designer’s job to determine how this appears in a particular circuit model.

## LCR Meter Capabilities

When selecting an LCR meter, the main factor that is used to select a meter is the frequency at which the component needs to be measured. Some components (such as capacitors) can exhibit non-ideal behavior at very high frequencies due to their package parasitics. Some components can exhibit nonlinear effects at high voltages (namely ferrites), so the applied voltage in the input signal should be kept low to maintain linear range.

LCR meters are available as bench instruments or as handheld meters (similar to DMMs). The cost of these meters can also vary greatly, with the handheld versions having the lowest cost.

Benchtop LCR meter.

Handheld meters are best for low-frequency measurements as they will not contain the specialized circuitry required to synthesize very high frequencies. For high-frequency measurements, a bench LCR meter will be needed, although the costs for these meters can be quite high. The high-end frequency sourcing capabilities of these meters can reach into the MHz range.

The capabilities of an LCR meter matter greatly when testing components that will be used in specific circuits. For example, in high-frequency/RF circuits, the frequency capabilities of an LCR meter are most important because the s are most important at these high frequencies. Another example is the ESL value of capacitors in high-speed digital boards; for capacitors targeting the MHz range in a system, the LCR meter needs to be able to measure into the MHz range, and the meter can return the ESR and ESL values of these components.

## Probes and SMD Test Fixtures

Through-hole components have long leads that can be clasped with a pair of probes to take measurements. Small SMD components may be too tiny to grab with a pair of alligator clip-style probes, so LCR meter manufacturers offer SMD text fixtures that are used with SMD components. 3rd parties also provide their own SMD fixtures that connect with BNC/coax cables, so they will be compatible with any LCR meter.

A typical SMD probe fixture is shown below. This fixture allows measurement of a passive package by driving two probe pins into the package pins. These fixtures are rated accurate up to specific frequencies, often reaching well into the MHz range. Many of these fixtures can provide measurements at much higher frequencies than could be generated with a high-end LCR meter.

SMD test fixture.

When much higher frequencies are needed for testing, such as into the GHz range, the components will need to be measured with an impedance analyzer. What is much more important is to realize that at these frequency ranges, the components should be measured in-circuit rather than with a frequency analyzer; this allows the PCB parasitics to be understood because circuit performance will depend heavily on the PCB layout.

Once you determine the parasitics in your component packages, you can fully qualify circuit behavior with the complete set of circuit simulation features in PSpice from Cadence. PSpice users can access a powerful SPICE simulator as well as specialty design capabilities like model creation, graphing and analysis tools, and much more.