LCR digital bridge is an instrument used to measure inductance, capacitance and resistance parameters. It usually includes Z, y, θ, RS, RP, x, G, B, LS, LP, CS, CP, Q, D and other measurement items. Why measure impedance? Today’s Antai test will give you an introduction to LCR digital bridge

What is impedance?

In a circuit with resistance, inductance and capacitance, the blocking effect on the current in the circuit is called impedance. Both digital circuit engineers and RF engineers are concerned about the impedance of various devices. For example, in audio equipment, impedance is often mentioned as an important parameter.

Impedance is usually represented by Z, which is a complex number. The real part is called resistance, and the imaginary part is called reactance. The blocking effect of capacitance on alternating current in circuit is called capacitive reactance. The blocking effect of inductance on alternating current in circuit is called inductive reactance. The blocking effect of capacitance and inductance on alternating current in circuit is called reactance.

Calculation formula of resistance, capacitance and inductance impedance:

XR=R；

XL=wL=2πfL；

XC==1/（wC）=1/（2πfC）；

Principle of LCR digital bridge

The earliest impedance measurement used the bridge method, as shown in the figure. With the development of modern analog and digital technology, modern impedance measurement mostly uses the automatic balance bridge method, which can not only measure the impedance, but also directly display the inductance and capacitance parameters of L-Q, C-D, R-D and z-q.

Selection method

LCR digital bridge selects the host according to the frequency and level required by the device under test;

LCR digital bridge carries out fixture selection according to the package of the device under test;

LCR digital bridge selects bias box and bias current box according to whether the device under test needs DC bias;

Testing techniques

1. When to add bias box?

When it is necessary to test the components with DC bias, such as the conduction capacitance of diode, it is necessary to add bias voltage test; similarly, when it is necessary to add DC current bias, it is possible to select bias box;

2. Electrical meaning of quality factor Q and loss factor D:

The quality factor Q of inductor indicates that the device is close to pure reactance. The higher the quality factor is, the larger the absolute value of reactance is, the smaller the real part series resistance is, and the smaller the energy loss is.

For capacitors, the term of purity is usually expressed by dissipation factor (d), d = 1 / Q; therefore, the smaller the D, the greater the capacitive reactance and the better the efficiency.

3. How to choose the internal resistance of LCR meter 30, 50, 100 Ω?

In order to reduce the non-linear of the core material, the inductance is generally measured with a large internal resistance of 100 ohm to reduce the test signal level; in order to compare the data with other models of testers, it must be ensured that they have the same internal resistance mode of the signal source. Other impedances are available, which are consistent with the internal resistance test conditions specified by the manufacturer.

30 Ω can be compared with ch107x / gr1689

100 Ω can be compared with hp4284a / E4980A / chroma3250

10 Ω / CC can be compared with ch106x / wk3245

50 Ω can be compared with: hk3532

In addition, 10 / 100 internal resistance mode: when users need to use 100kHz to measure the loss of 1uF ~ 10uF CBB, this internal resistance mode can be used to improve the stability of measurement results, especially the loss factor.

4. Why is it difficult for LCR bridge to test high Q and low D devices, such as MLCC chip multilayer ceramic capacitor?

The measurement of LCR bridge Q value and D value is based on the following formula:

Suppose q = 10, 0, then ESR is 1 / 100 XL. It can be seen that the L component of ESR in Z = ESR + J * x is relatively small, so the weak change of ESR caused by the instrument or measurement environment will be magnified by 100 times, and the phenomenon reflected is that the Q value is not stable or the measured data jumps greatly. The same principle applies to the measurement of capacitance D.

Users can do is to try to choose a better test fixture to improve the effect, the principle: can use 0 m, do not use 1 m, can use 4-end measurement, do not use 2-end measurement.

Editor in charge: GT