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Method to solve PCB design impedance discontinuity
Method to solve PCB design impedance discontinuity

# Method to solve PCB design impedance discontinuity

2021-10-24
View：296
Author：Downs

As a PCB design engineer, everyone knows that the impedance must be continuous. However, as Luo Yonghao said, "There are always times when I step on my stool in my life", there are always times when the impedance cannot be continuous in PCB design. What should I do at this time?

Let's clarify a few concepts first. We often see impedance, characteristic impedance, and instantaneous impedance. Strictly speaking, they are different, but they are always the same. They are still the basic definition of impedance:

A) The input impedance at the beginning of the transmission line is referred to as impedance for short;

B) The instantaneous impedance encountered by the signal at any time is called the instantaneous impedance;

C) If the transmission line has a constant instantaneous impedance, it is called the characteristic impedance of the transmission line.

Characteristic impedance describes the transient impedance experienced by the signal as it propagates along the transmission line. This is a major factor affecting the integrity of the signal in the transmission line circuit.

If there is no special instructions, the characteristic impedance is generally used to collectively refer to the transmission line impedance. The factors that affect the characteristic impedance are: dielectric constant, dielectric thickness, line width, and copper foil thickness.

The impedance is continuously similar:

The water flows steadily in a uniform ditch, and suddenly the ditch turns and widens.

Then the water will sway at the corners, and water waves will propagate.

This is the result of impedance mismatch.

The method to solve the impedance discontinuity

1, corner

If the RF signal line goes at a right angle, the effective line width at the corner will increase, and the impedance will be discontinuous, causing signal reflection. In order to reduce the discontinuity, to deal with the corners, there are two methods: chamfering and rounding. The radius of the arc angle should be large enough, generally speaking, to ensure: R>3W.