1、 Basic concepts of impedance testing

　　1. Impedance definition: Impedance is the total reaction of a component or circuit to a periodic AC signal.

　　AC AC test signal (amplitude and frequency).

　　Including real and imaginary parts.

　　Impedance is an important parameter for evaluating circuits, components, and materials used to make components. So what is impedance? Let's first take a look at the definition of impedance.

　　Usually, impedance refers to the resistance of a device or circuit to alternating current passing through it at a given frequency. It is represented by a complex number on the vector plane. An impedance vector includes the real part (resistance R) and the imaginary part (reactance X). as

　　Secondly, remember that the unit of impedance is the ohm. Additionally, it is important to consider the well-known positions of resistance (R), inductance (L), and capacitance (C) in the complex impedance plane.

　　2. Formula for impedance

　　What is admittance?

　　Admittance is the reciprocal of impedance, which can also be expressed as the real part (G conductivity) and the imaginary part (admittance), and its unit is Siemens.

　　Why are there two ways to express impedance and admittance? Mainly for a very simple description of two commonly used serial and parallel connection methods. When connecting resistors and reactances in series, the expression of impedance is very simple and easy to use. However, when connecting resistors and reactances in parallel, the expression of impedance is very complex, and in this case, using admittance is very simple and easy to use.

　　3. The relationship between impedance and admittance

　　The relationship between impedance, inductance L, and capacitance C:

　　There are two forms of reactance - inductive reactance (XL) and capacitive reactance (XC). Inductance corresponds to inductive reactance, while capacitance corresponds to capacitive reactance. For ideal inductors and capacitors, they satisfy proportional and inverse relationships with inductive reactance and capacitive reactance, respectively.

　　By definition,

　　XL=2pfL=wL

　　XC=1/2pfC=1/wC

　　F is the frequency of the AC signal, L is the inductance, and C is the capacitance. The unit of inductance is heng, and the unit of capacitance is fa.

　　W is the angular velocity, w=2pf.

　　4. Why test impedance?

　　The impedance of a component is influenced by many factors: frequency, test signal, DC bias, temperature, and so on. Due to the presence of parasitic parameters, frequency has an impact on all actual components. Not all parasitic parameters will affect the measurement results, but it is certain main parasitic parameters that determine the frequency characteristics of the component. When the impedance values of the main components are different, the main parasitic parameters will also vary.

　　2、 Impedance measurement methods and principles

　　There are multiple selectable methods for impedance measurement, each with its own advantages and disadvantages. It is necessary to first consider the measurement requirements and conditions, and then choose the appropriate method. Factors to consider include frequency coverage, measurement range, measurement accuracy, and ease of operation. There is no single method that can encompass all measurement capabilities, so it is necessary to consider trade-offs when selecting measurement methods. Below are three methods that focus on the characteristics of high-speed digital circuits. If only measurement accuracy and operational convenience are considered, the automatic balance bridge method is the best choice up to 110MHz frequency. For measurements from 100MHz to 3GHz, the RF I-V method has good measurement capabilities, while network analysis technology is recommended for others.

　　2.1 Automatic balance bridge method

　　The current flowing through the DUT also flows through resistor Rr. The potential at point 'L' remains at 0V (hence referred to as' virtual ground '). The I-V conversion amplifier balances the current on Rr with the current on DUT. By measuring the high-end voltage and the voltage on Rr, the impedance value of the DUT can be calculated.

　　The actual configuration of automatic balance bridges for various instruments may vary. The low-frequency range of a conventional LCR meter is generally lower than 100KHz, and a simple operational amplifier can be used as its I-V converter. Due to limitations in amplifier performance, this type of instrument has poor accuracy at high frequencies. The I-V converter used by the broadband LCR meter and Impedance analyzer includes complex geophones, integrators and vector modulators to achieve high accuracy in a wide frequency range above 1MHz. This type of instrument can reach a high frequency of 110MHz.

　　2.2 RF I-V method

　　The RF I-V method uses impedance matching measurement circuits (50 ohms) and precision coaxial test ports to achieve different configurations and can operate at higher frequencies. There are two ways to place Voltmeter and Ammeter to adapt to the measurement of low impedance and high impedance respectively. As shown in the figure, the impedance of the Device under test is derived from the measured values of voltage and current, and the current flowing through the DUT is calculated from the voltage on the low resistance resistor R with known resistance value. In actual measurements, a low loss transformer is placed at resistor R, but this transformer also limits the low end of the applicable frequency range.

　　2.3 Network segmentation

　　The reflection coefficient is obtained by measuring the ratio of the injected signal to the reflected signal. Use directional couplers or bridges to detect reflected signals, and use a network analyzer to provide and measure the signal. Due to the fact that this method measures reflection on the DUT, it can be used in a higher frequency range.

　　3、 Testing errors and calibration and compensation

　　3.1 Measurement error

　　For real * measurements, we must assume that errors are included in the measurement results. Common sources of error include:

　　The uncertainty of the instrument (including the uncertainty of DC bias and OSC level)

　　Residual parameters in test fixtures and cables

　　noise

　　The parasitic parameters of the DUT are not listed here, as they are a part of the DUT. We need to measure the impedance of the DUT, including its parasitic parameters. If the residual impedance of the test fixture and test cable is constant and stable among the listed error sources, they can be compensated.

　　3.2 Calibration

　　Calibration is defined by the "calibration plane" on which the specified measurement accuracy can be obtained. To calibrate the instrument, connect a "standard device" on the calibration plane, and then adjust the instrument (through calculation/data storage) to ensure that the measurement results are within the specified accuracy range.