PCB circuit board LAYOUT three special line skills
Update Time ： 2017-05-04 View ： 2624
The following line from the right angle, the difference between the line, snake line three aspects to elaborate PCB line LAYOUT line:
A right angle line (three aspects)
Right angle line of signal effect is mainly reflected in three aspects: one is the corner can be equivalent to the capacitive load of the transmission line, slow rise time; two is the impedance discontinuities will cause signal reflection; three is the angle from the tip of EMI, to the RF design field above 10GHz, these small angle may be the key problem of the high-speed object.
Two, differential line ("equal length, equidistant, reference plane")
What is the differential signal? Popular to say that the driver side to send two equivalent, inverted signal, the receiver by comparing the difference between these two voltages to determine the logical state of "0" or "1"". The pair of lines carrying the differential signal is called the differential line. Differential signal and the common single ended signal line, the most obvious advantage is reflected in the following three aspects:
1, strong anti-interference ability, because of poor coupling between two points line is very good, when the external noise, almost at the same time being coupled to the two line, and the receiver only cares about the two signal difference, so the common mode noise outside can be completely offset.
2, can effectively inhibit the EMI, the same reason, because of the opposite polarity of the two signals, their external radiation electromagnetic field can cancel each other, the more tightly coupled, the less the electromagnetic energy to the outside world.
3 timing, accurate positioning, due to poor signal switch changes is located in the two intersection signal, rather than relying on a single ended signal level two threshold voltage, thus by the process, the influence of temperature is small, can reduce the error of the timing circuit, but also more suitable for low amplitude signals, at present the popular LVDS (low voltage differential signaling) is the only small amplitude differential signal technology.
Three, snake line (adjust delay)
Serpentine line is one of the most frequently used methods in Layout. Its main purpose is to regulate the timing delay, meet the system design requirements, the two parameter is the key of parallel coupling length (LP) and coupling distance (s), obviously, the signal transmission in the snake to go online, the coupling between line segments parallel to each other, a differential form, the smaller the S the bigger the Lp. May cause the transmission delay is reduced, and the crosstalk and greatly reduce the quality of the signal, the mechanism analysis of the common mode and check mode crosstalk reference, here are a few points to Layout engineer to deal with the advice serpentine:
1, as far as possible to increase the distance between parallel lines (S), at least more than 3H, H refers to the signal line to the reference plane distance. That is popular around the bend line, as long as S is large enough, you can almost completely avoid the mutual coupling effect.
2, reduce the coupling length Lp, when the two times the Lp delay is close to or exceed the signal rise time, the crosstalk will reach saturation.
3, stripline (Strip-Line) or buried microstrip line (Embedded Micro-strip) of the serpentine line caused by the signal transmission delay is less than the microstrip line (Micro-strip). In theory, the transmission rate is not affected by the differential mode crosstalk.
4, high speed and timing requirements of the signal line is more stringent, try not to go snake line, especially in a small range of winding line.
5, can often use any angle snake line, can effectively reduce the mutual coupling.
6, high speed PCB circuit board design, the snake line does not have the ability to filter or interference, can only reduce the quality of the signal, so only for timing matching with no other purpose.
7, sometimes can be considered spiral winding way, simulation shows that the effect is better than the normal snake line.
