Rohs TG135 Digital Circuit Board

Why is it best to avoid acute angle and right angles when PCB wiring?

RF, high speed digital circuit: prohibit acute angle, try to avoid right angle.

If it is a radio frequency line, if it is a right angle at the corner, there will be discontinuity, and discontinuity will easily lead to the generation of higher order mode, which has an impact on the radiation and conduction performance.If RF signal line is at a right angle, the effective line width at the corner will be increased and impedance discontinuite,which will cause signal reflection.In order to reduce discontinuity, there are two ways to deal with corners: cut corners and rounded corners.The radius of the arc angle should be large enough, in general, to ensure R>3W.

Acute angle, right angle line

Acute angle wiring is generally prohibited when wiring, and try best to avoid right angle wiring in digital circuit board wiring, which almost become one of the standards to measure the quality of wiring, so how much influence will right-angle wiring have on signal transmission?

From the principle, acute angle, right angle line will make the transmission line width changes, resulting in impedance discontinuity.

A change in line width results in a change in impedance

When the equivalent width of the wire is changed, the signal will reflect.We can see that:

When we wire, if the line width changes, it will lead to wire impedance changes.

Microstrip line

It consists of a strip of wire and the ground plane, with dielectric in the middle.If the dielectric permittivity, the line width, and the distance from the ground plane of the dielectric are controllable, then its characteristic impedance is also controllable, and its accuracy will be controlled within 5 percent.

stripline

A stripline is a strip of copper that is placed in the dielectric between two conductive planes.If the thickness and width of the line, the dielectric constant of the medium, and the distance between the two grounding planes are controllable, then the characteristic impedance of the line is also controllable and the accuracy is within 10%.

Impedance discontinuity will reflect

Acute angles are the worst, right angles are the next, then obtuse angles, and then rounded angles, and straight lines are the best.

When the driver sends a signal into the transmission line, the amplitude of the signal depends on the voltage, the internal resistance of the buffer, and the resistance of the transmission line.The initial voltage seen at the driver end depends on the internal resistance and the partial voltage of the line impedance.

Reflection coefficient

-1≤ρ≤1

When ρ=0,no reflection occurs.

Whenρ=1(Z2 =∞, open circuit), it's going to be a full positive reflection.

Whenρ=-1(Z 2 =0, short circuit), all negative reflection occurs.

The initial voltage is the source voltage Vs(2V) divided by the Zs(25ohm) and the transmission line impedance (50ohm).

Vinitial = 1.33 V

The subsequent reflectivity is calculated according to the formula of reflection coefficient.

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