Working Principle and Test Method of Impedance Analyzer

Impedance analyzers and LCR meters are very common electronic instruments for measuring devices. According to the different impedance ranges and frequency ranges, there are a series of instruments with different principles to meet the test requirements. Figure 1 shows the impedance measurement methods for different impedance ranges and different frequency ranges.

Figure 1 Impedance measurement method

Figure 2 is a block diagram of the principle of the automatic balancing bridge method. By accurately measuring the voltage and current loaded on the DUT, the DUT impedance value can be accurately measured. As can be seen from Figure 2, the current through the DUT is equal to the current through the resistor Rr, and the current through Rr can be calculated by measuring V2.

Usually, in low frequency (<100KHz) LCR meters, a simple operational amplifier is used as an IV converter. The disadvantage is that the frequency response of the operational amplifier is poor in the high frequency band. For LCR meters or impedance analyzers with frequencies above 1MHz, the IV converter consists of a precision zero detector, a phase detector and an integrator (loop filter). This instrument can measure a frequency range up to 110MHz.

Figure 2 Principle diagram of the automatic balancing bridge method

Figure 3 is a block diagram of the RF IV method. The RF IV method is an extension of IV technology in the high frequency range, and can closely measure impedance values ​​up to a frequency range of 3GHz. The RF IV circuit and path must be carefully designed to ensure that it can be connected to the DUT with a 50ohm impedance. If the impedance of the connection path is not 50ohm, unwanted reflections will occur, which will increase the measurement errors of current and voltage. The RF IV method is subdivided into two measurement modes: high resistance and low resistance. In fact, the measuring instrument remains unchanged, only the test head is changed to meet the requirements of the two measurement modes. In the high resistance measurement mode, the test current is very small. In order to correctly detect the current, the current probe should be as close to the DUT as possible; in the low resistance measurement mode, in order to sensitively obtain the voltage value, the voltage probe should be as close to the DUT as possible.

Figure 3 RF IV method principle block diagram

The network reflection method is the network analyzer method and will not be introduced here.

The advantages and disadvantages of each method are as follows.

1. Advantages and disadvantages of automatic balancing bridge method:

1) Most accurate, basic test accuracy 0.05%;

2) The widest impedance measurement range: C, L, D, Q, R, X, G, B, Z, Y, O...;

3) The widest range of electrical measurement conditions;

4) Easy to use;

5) Low frequency: f < 110MHz.

2. Advantages and disadvantages of RF IV method:

1) Wide frequency range: 1MHz