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How to design characteristic impedance
In the high-speed PCB design, we often control the characteristIC impedance of high-speed signal lines to optimize the signal quality What is the characteristic impedance?
1. Principle of transmission line
Before introducing the characteristic impedance, let's review the basic principle of the transmission line introduced in the "Signal Integrity Video". As shown in the figure on the left, the RLGC model with lumped parameters is used for low-frequency circuits, while the RLGC model with distributed parameters is used for high-frequency circuits on the right.
Solving the transmission line calculus equation is easy for you, and I won't introduce it here. The result of this equation is that the attenuation constant, characteristic impedance and phase shift constant of PCB transmission line can be obtained. The specific meaning is described in detail in "Signal Integrity Video". Here we mainly introduce the application of PCB characteristic impedance. The following is the characteristic impedance formula obtained by solving the equation, but it is not very useful because you cannot know R, L, G and C corresponding to the tissue length.
2. Effect of characteristic impedance miSMAtch
The characteristic impedance of the transmission line refers to the impedance corresponding to each point on the line when high-speed signals are transmitted on the PCB line. We hope that the characteristic impedance of the transmission line will not change suddenly, because the sudden change of the transmission line will cause the signal reflection, thus affecting the signal quality.
3. Selection of characteristic impedance
The characteristic impedance of a PCB is determined by the matching resistance ZL at the load end and depends on the specifications of various interfaces Characteristic impedance error is mainly affected by Pcb factory process, which can be controlLED between 5% and 10% The following is a general-purpose interface PCB impedance control
USB differential cable 90 Î)
PCIE differential line 100 Î)
RF single ended transmission line 50 Î)
Common single ended transmission line 50 Î)
4. PCB characteristic impedance design
At present, PCB manufacturers usually use polar si9000 software to calculate characteristic impedance. Let's take a single ended signal line as an example. There are four setting parameters: dielectric constant, dielectric thickness, line width and copper foil thickness.
Dielectric constant Er1: the dielectric constant of the plate. The FR4 plate is usually 4.2-4.5.
Medium thickness H1: the thickness of plate or PP.
Line width W1/W2: Line width of PCB wiring.
Copper foil thickness: 0.5/1/2oz, etc. according to the actual situation.
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