Rigid PCB three special wiring sharing and inspection methods detailed
In the explanation of Rigid PCB wiring after the completion of the inspection work, first for you to introduce three kinds of Rigid PCB special walking line skills. will be from the right angle line, differential line, serpentine line three aspects to explain the Rigid PCB layout line:
First, right angle walking line (three aspects)
The influence of the right angle line on the signal is mainly embodied in three aspects: one is that the corner can be equivalent to the capacitive load on the transmission line, slowing the rise time, and the second is that the impedance discontinuity will cause the reflection of the signal; third, the right angle of the cutting-edge generation of EMI, to 10GHz above the RF design field, These small angles can be the focus of high speed problems.
II. Differential line ("Equal length, isometric, reference plane")
What is a differential signal (differential Signal)? In layman's way, the driver sends two of the equivalent and inverse phase signals, and the receiver compares the difference between the two voltages to determine the logical state "0" or "1". The pair of lines that carry the differential signal are called differential lines. The most obvious advantages of the differential signal and the common single-ended signal line are as follows:
1, anti-interference ability, because the coupling between two differential line is very good, when there is noise interference, almost at the same time is coupled to two lines, and the receiving end of concern is only the difference between two signals, so the external common mode noise can be completely offset.
2, can effectively restrain EMI, the same reason, because two of the polarity of the signal opposite, their external radiation electromagnetic field can cancel each other, the closer the coupling, the less electromagnetic energy released to the outside world.
3. Accurate timing positioning, as the difference signal switch is located in the intersection of two signals, and unlike the common single terminal signal depending on the height of two threshold voltage judgment, so by the process, the temperature is small, can reduce the timing of the error, but also more suitable for low amplitude signal circuit. The current popular LVDS (low voltage differential signaling) refers to this small amplitude differential signal technique.
Three, serpentine line (adjust delay)
Serpentine lines are often used in layout. Its main purpose is to adjust the delay to meet the system timing design requirements. The key two parameters are the parallel coupling length (Lp) and the coupling distance (S), it is obvious that when the signal is transmitted on the Serpentine line, the coupling between the parallel segments will occur, and the smaller the S, the larger the Lp, the greater the coupling degree. May cause the transmission delay to decrease, and because of crosstalk and greatly reduces the signal quality, its mechanism may refer to the common mode and the differential mode crosstalk analysis. Here are some suggestions for layout engineers to handle serpentine lines:
1, as far as possible to increase the distance between the parallel segment (S), at least greater than 3H, H refers to the signal line to the reference plane distance. The popular way is to go around the Big Bend line, as long as S is big enough, can almost completely avoid the mutual coupling effect.
2, reduce the coupling length of LP, when the two times the LP delay close to or exceed the signal rise time, the resulting crosstalk will reach saturation.
3. The signal transmission delay caused by the serpentine line of the strip line (Strip-line) or The Buried Microstrip line (Embedded Micro-strip) is less than the micro-strip line (Micro-strip). In theory, strip lines do not affect the transmission rate because of differential mode crosstalk.
4, high-speed and the timing of the more stringent signal lines, as far as possible not to walk the serpentine line, especially in a small range of winding line.
5, can often adopt arbitrary angle of serpentine line, can effectively reduce the coupling between.
6, high-speed Rigid PCB design, the Serpentine line does not have the so-called filter or anti-interference ability, only may reduce the signal quality, therefore only makes the time series match the use but has no other goal.
7, can sometimes consider the way of spiral wire winding, simulation shows that its effect is better than the normal serpentine line.