If it is a RF line, if it is a right angle at the corner, there will be discontinuity, and discontinuity will easily lead to the generation of high-order modes, which will affect the radiation and conduction properties. If the RF signal line is right angled, the effective line width at the corner will increase, and the impedance will be discontinuous, causing signal reflection. In order to reduce the discontinuity, there are two ways to deal with the corner: corner cutting and fillet. The radius of arc angle should be large enough. Generally speaking, R > 3W should be ensured.

Acute angle and right angle routing

Sharp angle routing is generally prohibited when wiring. Right angle routing is generally avoided in PCB wiring, and it has almost become one of the standards to measure the quality of wiring. How much impact will right angle routing have on signal transmission?

In principle, the line width of transmission line will be changed when the line is laid at acute angle or right angle, resulting in the discontinuity of impedance.

The change of line width leads to the change of impedance

When the equivalent width of the line changes, the signal will be reflected. We can see that:

When we are routing, if the line width changes, it will lead to the change of routing impedance.

Microstrip line

It is composed of a strip conductor and ground plane, with dielectric in the middle. If the dielectric constant, the width of the line and the distance between the line and the ground plane are controllable, the characteristic impedance of the dielectric is also controllable, and its accuracy will be within ± 5%.

Stripline

The stripline is a copper strip 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, the characteristic impedance of the line is also controllable, and the accuracy is within 10%.

If the impedance is discontinuous, it will reflect

The acute angle is the worst, the right angle is the second, the obtuse angle is the second, the fillet is the second, and the straight line is 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 impedance of the transmission line. The initial voltage seen at the driver end is determined by the partial voltage of the internal resistance and line impedance.

reflection coefficient

Where – 1 ≤ ρ ≤ 1

When ρ = 0, no reflection occurs

Total positive reflection occurs when ρ = 1 (Z 2 = ∞, open circuit)

Total negative reflection occurs when ρ = – 1 (Z 2 = 0, short circuit)

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

Vinitial=1.33V

The subsequent reflectivity is calculated according to the reflection coefficient formula

The reflectivity of the source end is calculated as -0.33 according to the source end impedance (25 Ohm) and the transmission line impedance (50 ohm) according to the reflection coefficient formula;

The reflectivity of the terminal is calculated as 1 according to the terminal impedance (infinity) and the transmission line impedance (50 ohm) according to the reflection coefficient formula;

According to the amplitude and delay of each reflection, we superimpose on the initial pulse waveform to get this waveform, which is why the impedance mismatch causes poor signal integrity.

Because of the existence of connection, device pin, the change of wiring width, wiring bend and through hole, the impedance has to change. So reflection is inevitable.

Is there any reason other than reflection?

The influence of right angle routing on signal is mainly reflected in three aspects

First, the corner can be equivalent to the capacitive load on the transmission line to slow down the rise time;

Second, the discontinuous impedance will cause the reflection of the signal;

The third is EMI produced by right angle tip.

Fourth, there is another way of saying: acute angle will cause corrosion residue in the production process, which is not easy to process. It should not be difficult for the current PCB processing technology, so it is not a reason.

The parasitic capacitance caused by the right angle of the transmission line can be calculated by the following empirical formula:

C=61W(Er)1/2/Z0

In the above formula, C is the equivalent capacitance of the corner (unit: PF), W is the width of the line (unit: inch), ε R is the dielectric constant of the medium, and Z0 is the characteristic impedance of the transmission line.

For example, for a 4mils 50 ohm transmission line (ε r = 4.3), the electrical capacity brought by a right angle is about 0.0101pf, and then the rise time variation caused by this can be estimated

T10-90%=2.2*C*Z0/2 = 2.2*0.0101*50/2 = 0.556ps

Through calculation, it can be seen that the capacitance effect caused by right angle wiring is extremely small.

Due to the increase of right angle line width, the impedance at this point will decrease, so there will be a certain signal reflection phenomenon. We can calculate the equivalent impedance after the increase of line width according to the impedance calculation formula mentioned in the chapter of transmission line, and then calculate the reflection coefficient according to the empirical formula: ρ = (zs-z0) / (ZS + Z0). Generally, the impedance change caused by right angle line is between 7% and 20%, so the reflection coefficient is higher The maximum coefficient is about 0.1. Moreover, as can be seen from the figure below, the impedance of the transmission line changes to the minimum in a long time of W / 2 line, and then returns to the normal impedance after w / 2 time. The whole time of impedance change is very short, often within 10ps. Such a fast and small change is almost negligible for general signal transmission.

Many people have such an understanding of right angle wiring. They think that the tip is easy to transmit or receive electromagnetic waves and generate EMI. This has become one of the reasons why many people think that right angle wiring is not possible. However, many practical test results show that the right angle routing does not produce obvious EMI than the straight line. Perhaps the current performance of the instrument and the test level restrict the accuracy of the test, but at least it shows a problem that the radiation of the right angle line is less than the measurement error of the instrument itself.

On the whole, right angle routing is not as terrible as imagined. At least in the application of non RF and high-speed circuit, any effect such as capacitance, reflection, EMI can hardly be reflected in TDR test. High speed PCB design engineers should focus on layout, power / ground design, wiring design, via and other aspects. Of course, although the impact of right angle wiring is not very serious, it does not mean that we can use right angle wiring in the future. Paying attention to details is the basic quality of every excellent engineer. Moreover, with the rapid development of digital circuits, the signal frequency processed by PCB engineers will continue to improve. In the RF design field above 10GHz, these small right angles may become high-speed problems The key object of the question.

Source: Hardware 100000 why