Right Angle, difference and serpentine in PCB Layout

- Aug 18, 2018-

Right Angle, difference and serpentine in PCB Layout

Layout is one of the most basic working skills of PCB design engineers.The quality of wiring will directly affect the performance of the whole system. Most high-speed design theories should finally be realized and verified through Layout. Therefore, wiring is of vital importance in high-speed PCB design.The following part will analyze the rationality of some possible situations in the actual wiring, and give some more optimized routing strategies.

It is mainly illustrated from three aspects: right-angle routing, differential routing and serpentine.

1. Right-angle routing

Right-angle routing is generally required to avoid as much as possible in PCB wiring, and almost becomes one of the standards to measure whether the wiring is good or not. Then how much impact will right-angle routing have on signal transmission?In principle, right-angle routing changes the width of a transmission line, resulting in a discontinuity of impedance.In fact, not only the right Angle, but also the square Angle and the acute Angle may cause the impedance changes.

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, slowing down the rise time;

The other is the impedance discontinuity will cause the signal reflection;

The third is the EMI produced by the rectangular tip.

The parasitic capacitance resulting from the right Angle of the transmission line can be calculated by the following empirical formula:

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

In the above formula, C refers to the equivalent capacitance of the corner (unit: pF), W refers to the width of the moving line (unit: inch), travellers refers to the dielectric constant of the medium, and Z0 refers to the characteristic impedance of the transmission line.For an example, for a 50-ohm transmission line of 4Mils (i.e., Angela r = 4.3), a right Angle brings about an electric capacity of approximately 0.0101pF, which can then be used to estimate the change in rise time caused by it:

T10-90%=2.2*C* z0/2 =2.2* 0.0101*50/2 = 0.556ps

It can be seen from the calculation that the capacitance effect caused by right-angle routing is extremely small.

As the line width of the right-angle traverse increases, the impedance at this point will decrease, thus a certain signal reflection phenomenon will occur. We can calculate the equivalent impedance after the line width increases according to the impedance calculation formula mentioned in the transmission line section, and then calculate the reflection coefficient according to the empirical formula:

Rho = (Zs - Z0)/(Zs + Z0)

Generally, the impedance variation caused by right-angle routing is between 7% and 20%, so the maximum reflection coefficient is about 0.1.Moreover, as can be seen from the figure below, the impedance of the transmission line changes to the minimum within a long time of W/2 line, and then returns to the normal impedance after W/2 time. The whole time of impedance changes is very short, usually within 10ps. Such quick and small changes are almost negligible for ordinary signal transmission.Many people have this understanding of right-angle routing, and believe that the tip is easy to transmit or receive electromagnetic waves, resulting in EMI, which is also one of the reasons why many people think that right-angle routing is impossible.However, the results of many practical tests show that a straight line does not produce a noticeable EMI than a straight line.Perhaps the current instrument performance, test level limits the accuracy of the test, but at least illustrates the problem that the radiation of the right-angle traverse is less than the instrument's own measurement error.

In general, right - Angle alignment is not as scary as it seems.In applications below GHz at least, any effects such as capacitance, reflection, EMI, etc., which are almost invisible in TDR testing, high-speed PCB design engineers should focus on the layout, power/ground design, routing design, and other aspects.Although, of course, the effects of rectangular go line is not very serious, but is not to say that we can walk right Angle line, attention to detail is the essential quality for every good engineers, and, with the rapid development of digital circuits, PCB engineers processing of signal frequency also will continue to improve, to more than 10 GHZ RF design field, these small rectangular object can be the point.

2. Differential routing

Because Differential Signal is used more and more widely in high-speed circuit design, the most important signals in the circuit are usually designed with Differential structure.How can you guarantee good performance in PCB design?With these two questions in mind, let's move on to the next section.

What is a difference signal?Generally speaking, the driving end sends two equivalent and opposite signals, and the receiving end judges the logical state of "0" or "1" by comparing the difference between the two voltages.The pair of lines carrying differential signals are called differential lines.

Compared with ordinary single-end signal routing, the most obvious advantages of difference signal lie in the following three aspects:

A. Strong anti-interference ability, because the coupling between two difference lines is good, when there is noise interference from outside, it is almost coupled to two lines at the same time, while the receiver only CARES about the difference between two signals, so the external common-mode noise can be completely offset.

B. It can effectively inhibit EMI. Similarly, since the two signals have opposite polarity, their external electromagnetic fields can offset each other.

C. Accurate timing positioning. As the switching change of difference signals is located at the intersection of two signals, instead of the common single-end signals relying on high and low two threshold voltages, it is less affected by process and temperature, which can reduce the error in the timing sequence, and is more suitable for circuits with low amplitude signals.The current popular LVDS, low voltage differential signaling, is a small amplitude differential signal.

For PCB engineers, the most important concern is how to ensure that the advantages of differential routing can be fully exploited in practical routing.Perhaps anyone who has been exposed to Layout will understand the general requirement of difference routing, which is "equal length, equal distance".The equal length is to ensure the two difference signals to maintain opposite polarity and reduce the common module component.The equidistant is mainly to ensure the same differential impedance and reduce reflection."As close as possible to the principle" is sometimes one of the requirements of differential routing.But all these rules are not meant to be learned by rote, and many engineers do not seem to understand the nature of high-speed differential signal transmission.The following focuses on several common mistakes in PCB differential signal design.

Myth 1: the difference signal does not need the ground plane as the backflow path, or the difference line for each other to provide the backflow path.The reason for this misunderstanding is that we are confused by the surface phenomenon, or we do not have a deep understanding of the mechanism of high-speed signal transmission.As can be seen from the structure of the receiving end of figure 1-8-15, the emitter current of transistor Q3 and Q4 is equivalent and reverse, and the current they are connected is exactly offset (I1=0), so the difference circuit is not sensitive to similar projectiles and other noise signals that may exist in the power supply and ground plane.Ground plane offset part of the return does not represent a differential circuit is not returned as reference plane as a signal path, actually on the signal flow analysis, differential line and common single-ended walk line is consistent, the mechanism of the high frequency signal is always along the circuit of the minimum inductance for reflow, the biggest difference lies in the difference line besides there are coupled to the ground, there are mutual coupling, which kind of strong coupling, which is become the main reflux pathway.

In PCB circuit design, generally the coupling between difference routing is small, which takes up only 10-20% of the degree of coupling. More of the coupling is to the ground, so the main backflow path of difference routing still exists in the ground plane.When the local plane is discontinuous, there is no region of the reference plane, and the coupling between the differential routing provides the main reflow path, as shown in figure 1-8-17.Although the discontinuity of the reference plane has no serious effect on the difference routing than the ordinary single-end routing, it still reduces the quality of the difference signal, increases the EMI, and should be avoided as much as possible.Some designers think that the reference plane of the differential off-line side can be removed to suppress part of the common mode signal in the differential transmission, but it is not desirable theoretically. How can the impedance be controlled?Failure to provide a ground impedance loop for common mode signals is bound to cause EMI radiation, which does more harm than good.

Myth 2: it is more important to keep the equal spacing than to match the length of the line.In actual PCB wiring, the requirements of difference design cannot be satisfied.Due to factors such as pin distribution, overhole, and routing space, it is necessary to make proper winding to achieve line length matching, but the result must be that part of the difference pair cannot be parallel. How should we choose?Before coming to the conclusion, let's look at the following simulation results.

It can be seen from the above simulation results that the waveforms of scenario 1 and scenario 2 are almost coincident, that is, the effect caused by different spacing is minimal, compared with the effect of line length mismatch on the timing sequence (scenario 3).From the perspective of theoretical analysis, although the difference spacing is inconsistent, it will cause the difference impedance to change, but because the coupling between the difference pairs is not significant in itself, the range of impedance change is also very small, usually within 10%, which is only equivalent to the reflection caused by a hole, which will not have a significant impact on the signal transmission.When the line length is not matched, the deviation will occur in the time sequence, and the common mode is also introduced into the differential signal to reduce the signal quality and increase the EMI.

It can be said that the most important rule in the design of PCB differential routing is matching line length. Other rules can be flexibly handled according to design requirements and practical application.

Mistake three: think of the difference line must be very close.The reason for the difference routing is to enhance their coupling, which can not only improve their immunity to noise, but also make full use of the opposite polarity of magnetic field to offset the electromagnetic interference to the outside.Although this practice is very favorable in most cases, it is not absolute. If we can ensure that they are fully shielded from external interference, then we do not need to make them anti-interference and anti-emi through strong coupling with each other.How can the differential routing be guaranteed with good isolation and shielding?Increasing the spacing with other signals is one of the most basic approaches. The energy of electromagnetic field decreases with the square relation of distance. When the distance between general lines is more than 4 times the width, the interference between them is extremely weak, which can be basically ignored.In addition, the isolation of ground plane can also play a good role in shielding. This structure is commonly used in high frequency (over 10G)IC package PCB design and is called CPW structure, which can guarantee strict differential impedance control (2Z0).

Differential routing can also be carried in different signal layers, but this route is not generally recommended, as differences such as impedance and through holes generated by different layers can damage the effect of differential mode transmission and introduce common mode noise.In addition, if the two adjacent layers are not coupled enough, the ability of differential routing to resist noise will be reduced. However, crosstalk is not a problem if the proper spacing between adjacent routing lines is maintained.In general frequency (below GHz), the EMI is not a serious problem. Experiments show that the radiation energy attenuation at a distance of 3 meters away from the differential alignment of 500Mils has reached 60dB, which is enough to meet the electromagnetic radiation standard of FCC. Therefore, the designer should not worry too much about the electromagnetic incompatibility caused by insufficient coupling of difference lines.