Walk from rectangular wire, difference go line, serpentine three aspects to elaborate the walk line of PCB LAYOUT:
A, rectangular wire (three)
Orthogonal linear effect on the signal is mainly reflected in three aspects: one is the corner can be equivalent to the capacitive load on a transmission line, slow rise time; The second is the impedance discontinuity causing reflection in the signal; Three rectangular tip of EMI, to more than 10 GHZ RF design field, these small rectangular object can be the point.
B, the difference of (" isometric, isometric, reference plane ")
What is the difference Signal (Differential Signal)? Popular culture is the driving end send two equivalent, reverse phase signal, the receiver by comparing the difference of the two voltages to judge state of logic "0" or "1". And bearing that pair of differential signal lines to go go is called a difference. Differential signal compared with ordinary single-ended signals go line, one of the most obvious advantage is embodied in the following three aspects:
1, strong anti-jamming capability, because the coupling between the two difference walking thread is very good, when the outside noise interference, is almost at the same time was coupled to two line, and the receiving end concern only the difference of two signals, so the common-mode noise can be completely offset from the outside.
2, can effectively restrain EMI, in the same way, as a result of the two signal polarity on the contrary, their external radiation electromagnetic field can offset each other, the closer coupling, electromagnetic energy drain on the outside is less.
3, timing precision positioning, because of the difference signal switch is located at the intersection of two signals change, rather than relying on high and low two common single-ended signal threshold voltage, thus the process, the influence of temperature is small, can reduce the timing error, at the same time also more suitable for low amplitude signal circuit. Currently popular an LVDS (low voltage differential signaling) refers to the small amplitude difference signal technology.
C, serpentine (adjusting delay)
Serpentine is often used in the Layout of a class of linear way. Its main purpose is to adjust the time delay, timing design meet the system requirements. Two parameters is one of the most critical distance parallel coupling length (Lp) and coupling (S), and it is clear that the signal in the serpentine line, parallel line segments between coupling will happen, in the form of differential mode, S is smaller, the greater the Lp, the coupling degree. May lead to transmission delay is reduced, and the crosstalk and reduce the quality of the signal, its mechanism can be reference to the common mode and differential mode analysis of crosstalk. Below is for Layout engineer when handling serpentine several Suggestions:
1, try to increase the distance (S) segments, at least more than 3 H, H refers to the signal line to the distance of the reference plane. Popular said is to walk around the greater curvature line, as long as it S enough, almost can completely avoid the mutual coupling effect.
2, reduce the coupling length of Lp, when two times the Lp delay close to or more than one signal rise time, produce crosstalk will reach saturated.
3, stripline (Strip - Line) or buried decay with Line (Embedded Micro - Strip) serpentine caused by signal transmission delay is less than the microstrip Line (Micro - Strip). In theory, stripline won't because of differential mode crosstalk effects transmission rate.
4, high speed and more strict with temporal signal lines, don't go as far as possible serpentine, particularly can't go on a small scale winding line.
5, can often use any Angle of the serpentine line, can effectively reduce the coupling between each other.
6, high-speed PCB design, serpentine no filtering or anti-interference ability, can only reduce the signal quality, so only for timing matching and no other purpose.
7, sometimes consider the spiral wire winding in the form of the simulation shows that the effect is better than the normal serpentine line.
