How Much Influence Does PCB Right-angle Routing Have On Signal Transmission?

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How Much Influence Does PCB Right-angle Routing Have On Signal Transmission?
December 11, 2020 - 6:44am

 Right-angle routing is generally a situation that needs to be avoided as much as possible in PCB routing, and it has almost become one of the standards for measuring the quality of routing. So how much influence will the right-angle routing have on signal transmission? In principle, right-angle routing will change the track width of the transmission track, causing discontinuity in impedance. In fact, not only right-angled routing, but also corners and sharp-angled routing may cause impedance changes.

 

The influence of right-angle routing on the signal is mainly reflected in three aspects: one is that the corner can be equivalent to a capacitive load on the transmission track, which slows down the rise time; the other is that impedance discontinuity will cause signal reflection; the third is that the EMI generated from the right-angle tip.

 

The parasitic capacitance caused by the right angle of the transmission track can be calculated by the following empirical formula: C=61W(Er)1/2/Z0. In the above formula, C refers to the equivalent capacitance of the corner (unit: pF), and W refers to walking The width of the track (unit: inch), εr refers to the dielectric constant of the medium, and Z0 is the characteristic impedance of the transmission track.

 

As the track width of the right-angle trace increases, the impedance there will decrease, so a certain signal reflection phenomenon will occur. We can calculate the equivalent impedance after the track width increases according to the impedance calculation formula mentioned in the transmission track chapter, and then calculate the reflection coefficient according to the empirical formula: ρ=(Zs-Z0)/(Zs+Z0). Generally, the impedance change caused by the right-angle routing is between 7%-20%, so the maximum reflection coefficient is about 0.1.

 

Radio frequency, high-speed digital circuit: Prohibit sharp angles, try to avoid right angles. If it is a radio frequency track, if the corner is at a right angle, there will be discontinuities, and discontinuities will easily lead to the generation of high-order modes, which will affect the radiation and conduction performance. If the RF signal track runs at a right angle, the effective track 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: corner cutting and rounding. The radius of the arc angle should be large enough, generally speaking, to ensure: R>3W.

 

In the PCB routing rules, there is a "key signal track priority" principle, that is, priority routing of key signals such as power, analog signals, high-speed signals, clock signals, differential signals, and synchronization signals.

 

Analog signal routing requirements The main feature of analog signals is poor anti-interference, and the protection of analog signals is mainly considered when routing. The processing of analog signals is mainly reflected in the following points:

 

1. To increase its anti-interference ability, the routing should be as short as possible.

 

2. Some analog signals can give up impedance control requirements, and the routing can be thickened appropriately.

 

3. Limit the routing area, try to complete the routing in the analog area, away from the digital signal.

 

High-speed signal routing requirements:

 

Multilayer routing high-speed signal routing circuits tend to have higher integration and high routing density. The use of multilayer boards is not only necessary for routing, but also an effective means to reduce interference.