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What is characteristic impedanc
PCB News
What is characteristic impedanc

What is characteristic impedanc

2021-11-10
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Author:Kavie

Characteristic impedance

Before the grand debut of the "protagonist", let's first understand what a square wave signal is.

According to the explanation of "Baidu Encyclopedia", a square wave signal refers to a signal in a circuit system that can be transmitted from the source to the receiver without distortion within the required time.

To ensure that the signal transmitted in the circuit system is not distorted, PCB designers must consider how to ensure the integrity of the signal, minimize the transmission noise, and reduce the misoperation caused by signal distortion.

If we want to understand these clearly, we first have to take a look at the signal transmission process in the circuit board. Just like you, Codey does not like obscure words and sentences, so let's make an analogy.

We can compare the circuit in the multi-layer PCB board to a hose used by a gardener to water the flowers, and the current signal is the water flowing in the pipe. When we water the flowers, one end is connected to the faucet, and the other end is pressurized at the water outlet, applying appropriate pressure, so that the water column just sprays on the flower area.

Use the principle of watering flowers to explain the principle of characteristic impedance


When the pressure is too high, the water column will exceed the range and the flower area will not be watered, which will cause the water pipe to fall off from the faucet; vice versa.

 When the pressure is too high, the water column will exceed the range and the flower area will not be watered, which will also cause the water pipe to fall off from the faucet.

When the pressure is too high, the water column will exceed the range and the flower area will not be watered, which will also cause the water pipe to fall off from the faucet.

    This vivid metaphor vividly illustrates the process of current transmission in the circuit board circuit (Transmission Line, which is composed of the signal line, the dielectric layer, and the ground layer). As we said above, the hose is equivalent to the circuit. The wire circuit in the board, the water flow is equivalent to a square wave signal, the pressure applied by the water pipe is the "source end", the flower area is the "receiving end", and the pressure applied by the hand is a resistor that can adjust the resistance of the circuit board.

The characteristic impedance circuit diagram of the circuit board


PCB


Explain the principle of characteristic impedance with the principle of watering flowers

It can be seen that in order to keep the signal from the source end to the receiving end intact and not distorted during transmission, the "characteristic impedance" of the wires in the circuit board must match the electronic impedance inside the terminal assembly. Row. From a professional point of view, it is "correctly execute instructions, reduce noise interference, and avoid wrong actions." Once they fail to match each other, there will be a “water flow” that bounces back toward the “transmitting end”, and then forms reflected noise (Noise), causing the water pipe at the faucet to fall off.

 

Characteristic impedance in the pcb circuit board

    When the characteristic impedance (Z0) of the circuit itself is set as 28 ohm by the designer, the grounding resistance (Zt) of the terminal control must also be 28 ohm, so that the overall design value can be stabilized at 28 ohm. Only when Z0=Zt, signal transmission is the most efficient, and "Signal Integrity" (signal integrity, a special term for signal quality) is also the best. The "water column" can just be poured into the "flower area".

Watering flowers with water pipes to illustrate the principle of characteristic impedance

When a square wave signal is pushed forward by a high-level positive pressure signal in the PCB circuit, the reference layer closest to it (such as the ground layer) must theoretically be affected by the electrical field. The induced negative pressure signal accompanies the forward movement, so that the overall loop system can be completed. If the “signal” moves forward and freezes its flight time for a short time, you can imagine it suffers from the instantaneous impedance value (Instantanious Impedance) presented by the signal line, the dielectric layer and the reference layer. This is the so-called "Characteristic impedance".

The above is an introduction about what the characteristic impedance is. Ipcb is also provided to PCB manufacturers and PCB manufacturing technology.