PCB News - Poor characteristic impedance matching

Consequences of poor characteristic impedance matching and test methods

In the last issue, the editor introduced the concept of characteristic impedance in the circuit in detail in the article "Characteristic Impedance of God's Explanation" in simple and easy-to-understand language. Today we will continue to talk about the topic of characteristic impedance.

What happens if the PCB designer fails to handle the control of the square wave signal from the source to the receiver on a designed circuit board, or if a PCB circuit board has quality problems during the production process, which causes serious distortion? The consequences? How to measure this square wave signal?

Consequences caused by poor impedance matching

When there are no obvious defects in the process of PCB board production or design, the characteristic impedance Z0 value is controlled within the fluctuation range of ±10% or ±5%, which can effectively avoid the occurrence of misoperation.

Let's take a look at the calculation formula of Z0

Calculation formula of Z0 value in the line

It can be seen from the formula that any abnormality in any of the four variables (w, t, h, r) related to Z0 will affect the value of Z0.

If a certain section of the circuit board has a defect during the production process, Z0 will suddenly rise (Z0 is inversely proportional to W) and cannot continue to maintain its due stability, and the signal will produce noise and malfunction.

Watering flowers to illustrate the problem of poor Z0 matching

This is like when we water the flowers, our hose is suddenly stepped on by a child, and there will be abnormal conditions at both ends of the hose. This phenomenon appropriately explains the reason of poor characteristic impedance matching.

5. Test of characteristic impedance

Measurement of characteristic impedance Z0 value in PCB circuit board

Since the circuit board due to improper design or poor production will cause the square wave signal energy in the circuit to rebound, resulting in a good square wave signal in the original circuit, abnormal deformation and noise and misoperation, then how to control it What about testing?

Use PCB automatic impedance tester for measurement

In the detection link of PCB production plants, PCB automatic impedance testers are generally used to detect the characteristic impedance of the circuit in the circuit board.

The principle is: PCB automatic impedance tester will generate a stepped wave (Step Pulse or Step Wave). During the test, the stepped wave generated by the tester is sent to the transmission line of the PCB board to be tested and becomes an incident wave ( Incident Wave). When the width of the signal line changes, the instrument screen will show up and down oscillation curves of the Z0 ohm value. From this curve, the change and fluctuation range of ZO can be seen very intuitively.

Today's electronic equipment has extremely high requirements for circuit boards, especially high-frequency and high-speed PCB circuit boards. It is even more indispensable to use PCB automatic impedance tester for production test of its Z0 value.

The above is an introduction to the consequences of poor characteristic impedance matching and test methods. Ipcb is also provided to PCB manufacturers and PCB manufacturing technology.