A Preliminary Study on Calibration of LCR Impedance Meter

0 Introduction

In the current manufacturing industry, compared with the traditional manual AC bridge, the digital LCR impedance meter has been increasingly used in the measurement of AC impedance parameters because of its stable and reliable measurement performance, no need for repeated and complex manual balancing, and can also reduce measurement errors and result calculations. To ensure the measurement accuracy of the LCR impedance meter, it is particularly important to assess its performance. This article makes a preliminary discussion on the calibration of the capacitance, inductance, resistance and loss parameters of the LCR impedance meter at a frequency of lkHz.

1 Principle and performance overview

The LCR impedance meter with microprocessor is a digital measuring instrument with multiple measurement functions, wide frequency and wide measurement range. It is mainly used to measure the main and secondary parameters such as inductors, capacitors, resistors, losses, Q values, etc. Its working principle is based on the built-in standard and traces back to the capacitance reference at 1kHz. It uses the method of measuring the phase (vector) voltage ratio of the measured component and the built-in standard range resistor under the condition of automatic balance and other current conditions to give the value of the measured component. Generally speaking, it consists of a sinusoidal AC excitation power supply, a comparator, a phase voltage ratio detector, a digital control and display circuit.

In actual work, the measurement standard and the measured quantity are measured using the same circuit, and the time interval is very short (about only 0.01s), and the measurement circuit has almost no changes. The proportion of the standard, the measured value and the phase are all quickly displayed after precise calculation inside the microprocessor.

It can also display the connection mode of the measuring circuit and the L, Q, C, and D values ​​at the same time, overcoming the cumbersome balance and difficult calculation of the traditional manual AC bridge.

In addition, since the LCR impedance meter can conveniently perform three-terminal or four-terminal measurements, the influence of open circuit capacitance and short circuit inductance can be eliminated through open circuit zeroing and short circuit zeroing; the influence of the unshielded terminal at the measured end can also be eliminated. These influences can be automatically adjusted by the nonlinearity of the LCR meter to control the nonlinear part of the changing electromagnetic wave. The bias voltage of the bridge body can also be provided internally or externally.

2 Calibration of main parameters

2.1 Calibration of capacitance parameters

Select a standard capacitor that is two levels higher than the LCR impedance meter being calibrated (or a standard impedance meter and transition gauge that is two levels higher) as the measured value, mainly to examine the basic error of the capacitance indication. However, the performance of the standard capacitor is relatively complex. The capacitor with the best stability in the audio range should be a quartz capacitor and a three-terminal air capacitor, and its value can be accurately calculated. In precision measurement, a three-terminal capacitor is generally selected as a standard device, and an equipotential shield is used, that is, the low potential end and the shield are made equipotential, but not directly connected. When measuring with a bridge, let the standard capacitor and the capacitor being measured flow through the same current. Since the measurement of the three-terminal capacitor is carried out on the basis of equipotential shielding, the shielding eliminates the influence of parasitic leakage and has a high accuracy. If other interference is excluded, a capacity of 10pF-1uF can be selected as the standard capacitor to examine the basic error of the capacitance indication. However, when examining the capacitance indication range above luF, when using a large capacitor as a standard, it is necessary to pay attention to the measurement frequency. If the measurement frequency is higher than the audio range, the measurement result will change greatly, and the frequency effect correction must be made at this time. If mica capacitors are selected as the assessment standard, the temperature coefficient is small but the loss is poor, so only the LCR impedance meter with low accuracy can be assessed. The three-terminal capacitor adopts the two-time measurement method: first connect the capacitor to the LCR impedance meter for measurement, then disconnect the high potential end to measure the open circuit capacitance, and the increment of the two measured values ​​is the actual value of the capacitor under test.

2.2 Calibration of resistance parameters

A precision AC resistor that is two levels higher than the LCR impedance meter being calibrated (or a standard impedance meter and transition measuring tool that is two levels higher) is used as the standard. The performance of the LCR impedance meter being calibrated in measuring resistance is mainly assessed. During the assessment, attention must be paid to understanding the stability of the standard resistor used. The deviation between the instrument display value and the actual value should meet the resistance measurement index of the LCR impedance meter being calibrated. In addition, the measurement of AC resistance can be carried out using two-terminal, three-terminal, four-terminal, and five-terminal wiring methods. The specific measurement wiring depends on the resistance value. Standard resistors of different values ​​are selected as standards. When assessing different ranges of the LCR impedance meter, attention must be paid to the measurement status of the standard resistor. When ZX<100Ω, the influence of the parallel capacitance between the two ends of the connecting wire can be ignored, and the instrument under test generally adopts a series circuit; when measuring high resistance (Zx>1kΩ), the instrument under test adopts a parallel circuit, and the parallel capacitance between the ends of the connecting wire has a greater influence. If the shield is not connected, its AC resistance value and time constant are related to the surrounding electromagnetic field. The changing electromagnetic energy will cause the AC characteristics of the AC resistor to change significantly. Therefore, the precision AC resistor should be shielded, and the measurement needs to be carried out according to the three-terminal wiring. When evaluating the small resistance range of the bridge, pay attention to the influence of the series inductance on the measurement, and use four-terminal or five-terminal wiring. AC resistors below 10Ω can use the five-terminal wiring method. The purpose of shield grounding is to eliminate the influence of lead admittance.

When measuring resistors, a secondary measurement must be performed to deduct the lead resistance from the LCR impedance meter to the resistor being measured. First, short-circuit the measuring end to balance it, and then perform the main measurement of the resistance value. The increment of the two measured values ​​is the actual value of the resistor being measured.

3 Calibration of inductance parameters

The range and accuracy of the inductance measured by the LCR impedance meter are the same as those for the C and R tests. However, due to the influence of the additional effect, the precise measurement of the inductance is difficult. The main reason is the nonlinearity of the electromagnetic field, which is caused by the material of the inductor itself. The electric field strength around a energized coil can be determined, but the electric field strength around it is nonlinear, especially the nonlinear response of magnetic materials to different voltage amplitudes and different frequencies. These factors make the inductance measurement complex. The selected standard inductor must have frequency response measurement data. The LCR impedance meter should give the equivalent circuit of the inductor being measured. The modern automatic bridge uses the principle of double-T network and selects component matching to meet the measurement of inductance over a wider frequency range.

Inductance measurement is greatly affected by frequency, so when evaluating the performance of LCR impedance meter in measuring inductance, attention should be paid to the resonant frequency of the selected standard inductor itself, at which the inductance measurement accuracy is highest. For example, when a high-frequency inductor is measured at 100MHz, it is 3uH and Q is 100, while the resistance measured at 1kHz is 18mΩ and Ω is 0.1, so the inductance measured at 100MHz becomes the inductive resistance at 1kHz.

Sullivan's standard inductor (100uH-1H) can be used as a 0.01% standard inductor to test LCR impedance meters at 1kHz, but the accuracy changes significantly at 10kHz. When testing the performance of LCR impedance meters for measuring small inductors, the four-terminal measurement method should be used. In order to obtain accurate measurement results, the coupling between the voltage terminal and the current terminal should be as small as possible.

Inductors cannot be shielded with metal because they will cause eddy currents, causing the measured value of the inductor to change with the operating current and frequency. However, the connecting wire for measuring the inductor should be shielded, and the length of the connecting wire is generally 80cm to 100cm. Regardless of the value of the inductor, the lead end connected to the inductor cannot be shielded, and the lead end should be as short as possible to reduce the influence of distributed capacitance. It is also necessary to note that:

There should be no metal or ferromagnetic material around the inductor being measured. The inductor measurement must also be measured twice, deducting the lead resistance from the LCR impedance meter to the inductor being measured. First, short-circuit the measuring end for balance, and then do the main measurement of the inductance value. The increment of the two measured values ​​is the actual value of the inductor being measured.

4 Calibration of capacitance loss factor

The loss factor standard is composed of a combination of capacitors and resistors, and there are two types: series equivalent circuit and parallel equivalent circuit. The value range is (10-1~10-5) ten-step progressive multi-disc loss box or 1, 2, 5, 8 jump step loss box. The single-value standard capacitor with a capacitance value of 100pF~1uF. The calibration range of the capacitance loss factor is generally (10-4~1).

5. Frequency characteristics assessment.

The accuracy of AC impedance measurement is related to frequency, and the sensitivity of impedance measurement is proportional to frequency. Each AC impedance has a dedicated frequency, and the sensitivity of impedance measurement is highest at its dedicated frequency. Therefore, the frequency range of the LCR impedance meter should be wide to meet the measurement of various AC impedances. At present, the frequency of the LCR impedance meter is between 10Hz and 10MHz, such as 1689 (M) produced by GR Company of the United States, 7600 of Quadtech Company, and 4284 of HP and Agillent Company. The frequency of the LCR impedance meter is mainly assessed by directly measuring the various frequencies of the LCR impedance meter through a digital frequency meter to see whether it meets its frequency index. For example, to assess the frequency of the GRl689 digital bridge, a frequency meter is used to monitor between the "-" end of the test clip and the ground. The instrument programming can select test frequencies of 12Hz, 100Hz, 1kHz, 10kHz, 100kHz, etc. The deviation of the frequency measurement should be less than the instrument's frequency index requirements.

6. Voltage measurement test

AC impedance measurement requires a special voltage amplitude. Iron core inductors and semiconductor capacitors are nonlinear components. The voltage cannot be too small during measurement. The noise caused by the AC signal in the measurement lead will increase the distortion during the entire measurement process. In addition, the nonlinearity of the measured impedance will also cause harmonic distortion. Increasing the voltage amplitude can overcome the above phenomenon. Therefore, the LCR impedance meter is required to have an adjustable voltage within a certain range. The measurement voltage is generally between 5mV and 1.25V, which is conducive to the measurement of AC impedance. The measurement voltage is related to the frequency. Therefore, the frequency of the measurement must be indicated when assessing the measurement voltage. The specific method is to use the direct measurement method. The LCR impedance meter is not connected to the test, and a digital voltmeter is used to monitor between the "-" end of the test fixture and the ground: the instrument frequency can be selected to measure 5mV~1.25V in three cases of 12Hz, lkHz and 100kHz. The voltage deviation should meet the bridge measurement voltage index requirements; it can also be selected to assess the change of the voltage at a certain voltage at different frequencies, and its voltage deviation should also meet the voltage index requirements.

In short, the calibration of an LCR impedance meter, in addition to assessing its measurement frequency range, voltage amplitude and other performance, is more importantly to assess its combined measurement, that is, the performance of measuring L, C, and R. When calibrating its performance of measuring L, C, and R, pay attention to the measurement frequency and connection method. When an LCR impedance meter can perform both three-terminal and four-terminal measurements, in order to obtain more accurate measurement results, the measurement connection method should be selected according to the following principles: (1) Use four-terminal measurement for impedances below 10Ω; (2) Use three-terminal measurement for impedances above 10kΩ; (3) For impedances between 10Ω and 10kΩ, the influence of open-circuit capacitance and series resistance of the wire must be eliminated, and shield grounding should be added to the four-terminal measurement, that is, the five-terminal measurement method should be used.