LCR Meter: LCR bridge

The LCR Meter or LCR Bridge is used for measuring inductance, L, capacitance, C, and resistance, R.

LCR Meter / Bridge Includes:
LCR meter introduction

LCR meters or LCR bridges are items of test equipment or test instrumentation used to measure the inductance, capacitance, and resistance of components.

LCR meters tend to be specialist items of test equipment, often used for inspection to ensure that the components arriving are correct. They can also be used in a development laboratory where it is necessary to test and measure the true performance of particular components.

The LCR meter or LCR bridge takes its name from the fact that the inductance, capacitance and resistance are denoted by the letters L, C, and R respectively. Some versions of the LCR meter use a bridge circuit format as the basis of its circuit giving the name that is often used.

A variety of meters are available. Simpler versions of LCR meters provide indications of the impedance only converting the values to inductance or capacitance.

More sophisticated designs of LCR bridge are able to measure the true inductance or capacitance, and also the equivalent series resistance and tanδ of capacitors and the Q factor of inductive components. This makes them valuable for assessing the overall performance or quality of the component.

Example of a typical LCR meter as used in laboratories and production environments, etc. This one manufactured by Thurlby Thandar Instruments — A typical LCR Meter

LCR meter basics

Two main circuit techniques are used to form the basis of an LCR meter.

Bridge method: This method uses the familiar Wheatstone bridge concept as the basis of its operation. The aim is to aim for a condition where the bridge is balanced and no current flows through the meter. At the balance point the bridge component positions can be used to determine the value of the component under test. This method is typically used for lower frequency measurements - often measurement frequencies of up to 100 kHz or so are used.

In bridge method the device under test, DUT, is placed in a bridge circuit as shown, and its value can be determined from the settings for the other elements in the bridge. It is the LCR meters using this technique that are known as LCR bridges.

Basic bridge based LCR meter circuit

The DUT impedance is represented by Zu in the circuit. The impedance Z2 and Z3 are known. The oscillator circuit generally operates at frequencies up to about 100 kHz and can usually be selected before the test.

Then Z1 can be changed until no current flows through D. This is the balance position for the bridge. AT this point the four impedances in the circuit obey the equation:

$Z_{U} = (\frac{Z_{3}}{Z_{2}}) Z_{1}$

This basic bridge circuit is sometimes used on its own in very primitive LCR meters. Some very old instruments actually have the elements that are manually balanced. However technology has moved on and higher levels of integration coupled with operational amplifier circuitry enable accurate automated versions of the circuit to be used.
Current-voltage measurement: The current voltage approach is normally used for components that are to be used for higher frequency applications. It provides a highly accurate measurement technique that can be used at high frequencies and over a wide range of values.

Often known as the RF I-V measurement method, this technique for LCR measurement uses measurements of current and voltage as the name implies. However, as the frequencies involved are high, it uses an impedance matched measurement circuit. In some cases for very high frequency and high precision measurements a precision coaxial test port may be employed.

There are two types of the voltmeter and current meter arrangements: one suited to low impedance and the other for high impedance measurements.

IV LCR Measurement for low impedance circuits

IV LCR Measurement for high impedance circuits

Using the voltage and current values from the measurements, the impedance of the device under test can easily be derived. By using a phase sensitive detector to make these measurements the relative phase of the voltage and current can be used to determine the impedance of the device under test in terms of resistance, capacitance and inductance. The inductance or capacitance and the resistance may then be displayed as separate values.

Often a transformer is used in the circuit to enable these measurements to be made and to isolate the measurements from ground. However this can limit the lower frequencies or the frequency range over which measurements can be made.

LCR bridge measurement guidelines

To make the best measurements using an LCR meter or LCR bridge, a few simple guidelines of hints and tips can be employed.

Effect of lead length: At frequencies above 1 MHz or so the lead length can start to have an effect. As a rough guide a good estimate for lead inductance is around 10 nH per cm of lead. For the best measurements kep the leads as short as possible.
Measure at operational frequency: When making measurements using an LCR meter it helps to use a test frequency as close to the actual operational frequency as possible. This means that the effects of any stray effects or changes due to frequency are minimised - for example inductor cores may have different properties at different frequencies. This can make a noticeable difference in some instances.
Adjust test amplitude: In the same way that it is good practice to measure at a frequency that is as close to the operational frequency as possible, the same is true for the test amplitude. This is because component values may vary with the signal applied. This is particularly true for inductors that use cores such as ferrite that may introduce losses. These may be amplitude dependent.
Discharge capacitors before measurement: Some capacitors may carry a residual charge under some circumstances. It is best to discharge them before any measurements. As charge on some capacitors can linger for some time, it is always best to discharge them before any tests.

LCR meters / bridges are very useful test instruments. They may not be as widely used as in previous years where they were often found in goods-inwards areas for sample testing incoming components. However these days LCR meters tend to be used in some laboratories for testing the performance of components likely to be used in development as well as in may workshops where they can be used as an aid for fault finding.

LCR meters / bridges are most often used to display the capacitance, inductance and resistance, but may also be used to measure the Q of an inductor, or the tan δ of a capacitor. As such they are particaulrly useful items of test equipment.