Understand the principle and application of TDR impedance measurement in signal integrity

With the continuous development of science and technology, the requirements for the speed of each digital interface are getting higher and higher, and the requirements for signal integrity are becoming more and more stringent. Controlling impedance is one of the important requirements for signal integrity, and TDR is the basic technique for measuring characteristic impedance. Today, I will introduce the basic principles and applications of TDR measurement.

With the continuous development of science and technology, the requirements for the speed of each digital interface are getting higher and higher, and the requirements for signal integrity are becoming more and more stringent. Controlling impedance is one of the important requirements for signal integrity, and TDR is the basic technique for measuring characteristic impedance. Today, I will introduce the basic principles and applications of TDR measurement.

TDR (Time Domain Reflectometry) is called Time Domain Reflectometry, which uses signal reflection to evaluate the degree of impedance change in the link. Its basic working principle is shown in the following figure: The output impedance of the TDR test equipment is 50 ohms, and it is connected to the DUT link under test through a 50 ohm cable. The equipment outputs a step signal with a very jittery rising edge to the transmission environment under test. If the impedance of the transmission environment to be measured is discontinuous, then reflection (positive or negative reflection) will occur. The reflected signal is sampled. The device compares the reflection of the environment to be measured with the reflection produced by the standard impedance to obtain the impedance change. And then realize impedance measurement.

Typical time-domain waveform diagram when impedance is discontinuous. In the figure, regular reflection occurs. The green signal is the ideal signal, and the red signal is the actual reflected signal. It can be seen that ringing occurs on the rising/falling edge of the signal.

Here is an extra introduction. When using an oscilloscope to measure signals, if a longer ground wire is used, the loop inductance in the measurement link will increase, resonance will occur, and it will also cause oscillations, making the test inaccurate. Therefore, the measurement requirements are more stringent. When using a ground spring for signal measurement.

The following specifically introduces the application of TDR based on the actual acquired waveform. The TDR device is connected to the open-circuit terminal according to the reflection coefficient formula:

Z1=50Ω, Z2 is an open circuit, at this time Z2 impedance is infinite, it is regular reflection, reflection coefficient=1, then the amplitude of the incident voltage Vinc, when it reaches the terminal, the amplitude will be totally reflected, and the superimposition of the incident signal and the reflected signal is 2Vinc, In the figure below, the 250mV signal becomes 500mV after reaching the terminal. This is the time-domain wave voltage waveform when the terminal is open and the impedance is infinite.

According to the principle of TDR, in addition to measuring impedance, TDR can of course also measure distance, such as measuring line length. Let’s look at the impedance waveform when the terminal is open. Reflection 1 is the connection point where the device is connected to the link to be tested. From the impedance curve, it can be seen that the impedance of the connection point changes suddenly and the signal will reflect. At the position of total reflection, the link is In the open state, the impedance is infinite at this time.

It can be seen that the time difference between reflection 1 and total reflection is about 500ps. Because the measurement process of TDR is the process of the device output signal to the terminal and then reflected back, the signal has doubled the line length, so the one-way time of the signal is 500/2=250ps. In ordinary FR4 type PCB, the signal speed is greater than 6 inch/ns (0.0124 cm/ps), the dielectric constant of the high-speed board is a bit smaller, and the signal speed is slightly higher. If it is FR4 board, then the length of the trace can be estimated, which is about 0.01524*(500/2)=3.81cm.

The above is the basic principle and application of TDR impedance measurement.