Three special routing methods for PCB

- Jun 06, 2016-

Before explaining the PCB wiring to complete the inspection work, the first to introduce you to the three kinds of PCB special line. Will take the line from the angle difference, walk the line, three aspects to elaborate the serpentine line PCB LAYOUT:

First, the right angle to walk the line (three aspects)

Right down the line to signal the impact is mainly reflected in three aspects: one corner can be equivalent to transmission line of capacitive load, slow the rise in time; the second is impedance discontinuities will cause the signal reflection; the third is angled tip of EMI, to above 10GHz RF design field, these small rectangular may become the focus of high-speed problems.

Two, the differential line ("the same length, distance, reference plane")

What is the difference signal (Signal Differential)? Popular to say that the driver is sending two equivalent, reverse phase of the signal, the receiver by comparing the difference between the two voltages to determine the logical state of 0 or 1". While carrying the differential signal on the line is called the line of difference. Differential signal and ordinary single ended signal compared to the most obvious advantage is reflected in the following three aspects:

1, anti-interference ability is strong, because the two differential go coupling between the lines is very good, when outside the presence of noise interference, almost is also coupled to two lines, and the receiving end care of just the difference between the two signals, so outside the common mode noise can is completely offset.

2, can effectively inhibit the EMI, the same reason, because of the polarity of the two signals on the contrary, their external radiation electromagnetic fields can be offset, the more closely coupled, the less the electromagnetic energy into the outside world.

3, timing accurate positioning, due to the poor signal changes in the switch is located in the intersection of the two signals, and unlike the ordinary single ended signal depend on both high and low threshold voltage judgment, thus subject to process, small temperature effects, reduce the timing error, and it is more suitable to low amplitude signal circuit. The current popular LVDS (voltage differential signaling low) is refers to the small amplitude differential signal technology.

Three, the snake line (delay)

The serpentine is taking a class of frequently used in Layout line. Its main purpose is to adjust the delay, to meet the design requirements of the system timing. Which is the key of the two parameters of parallel coupling length (LP) and coupling distance (s), it is clear, the signal in the serpentine go on a transmission line, parallel to each other between the line will occur in the coupling, a poor form, s is small, the greater the LP, coupling degree. The transmission delay can be reduced, and the quality of the signal can be greatly reduced due to crosstalk, and the mechanism can be referenced to the analysis of the common mode and differential mode crosstalk. Below are a few points to Layout engineer to deal with the advice serpentine:

1, as far as possible to increase the distance of parallel line segments (S), at least more than H, 3H refers to the distance between the signal line to the reference plane. 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 of Lp, when 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 the serpentine line microstrip line (dded Micro-strip) induced signal transmission delay is less than microstrip line (Micro-strip). Theoretically, the ribbon wire will not affect the transmission rate due to the differential mode crosstalk.

4, speed and timing requirements more stringent signal line, do not go as far as possible the serpentine, especially not in the small range winding wire.

5, the snake can often arbitrary angle line, can effectively reduce the coupling between each other.

6 and high speed PCB design, serpentine no so-called filtering or anti-interference ability, can only reduce the signal quality, so only for timing matching and without any other purpose.

7, sometimes can consider spiral way wire winding, simulation results show that the effect is better than the normal serpentine line.

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