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PCB Layout

PCB Layout

PCB Layout- 3 special layout skills

The following is to describe the routing of PCB LAYOUT from 3 aspects: right-angle layout, differential layout and serpentine layout: 


Layout

 
I. Right Angle layout (3 Aspects)  

 
    The influence of right-angle routing on the signal is mainly reflected in three aspects: First, the corner can be equivalent to the capacitive load on the transmission pattern, which slows down the rise time;  Second, impedance discontinuity will cause the reflection of the signal;  The third is the EMI generated by the right Angle tip. To the RF design field above 10GHz, these small right angles may become the key object of high-speed problems.  
 
II. Differential layout  (" Equal Length, Equal Distance, Reference Graphic ")  
 
    What is a Differential Signal?  Generally speaking, the driver sends two equivalent and anti-phase signals, and the receiver judges the logical state of "0" or "1" by comparing the difference between the two voltages.  The pair of paths that carry the differential signal is called the differential path.  Compared with ordinary single-ended signal routing, the most obvious advantages of differential signal are reflected in the following 3 aspects:  
 
   1. Strong anti-interference ability, because the coupling between the two differential wires is very good, when the external noise interference exists, it is almost coupled to the two lines at the same time, and the receiving end only cares about the difference between the two signals, so the external common mode noise can be completely cancelled.  
   2. It can effectively inhibit EMI. In the same way, because the polarity of the two signals is opposite, the electromagnetic field radiated by them can cancel each other out.  
   3. Accurate timing positioning. Because the switch change of the differential signal is located at the intersection of two signals, unlike the ordinary single-ended signal, which depends on the high and low threshold voltage to judge, it is less affected by the process and temperature, which can reduce the error in timing, and is more suitable for circuits with low amplitude signals.  LVDS (Low Voltage Differential Signaling), which is popular at present, refers to this small amplitude differential signaling technology.  

III. Serpentine layout (adjust delay time)  
 
    Serpentine pattern is a kind of routing method often used in Layout.  Its main purpose is to adjust the delay, to meet the timing design requirements of the system.  The two most critical parameters are parallel coupling length (Lp) and coupling distance (S). Obviously, when the signal is transmitted on the serpentine route, the coupling between parallel line segments will occur in the form of differential mode. The smaller S is, the larger Lp is, and the greater the coupling degree is.  This may lead to reduced transmission delay and greatly reduced signal quality due to crosstalk, the mechanism of which can be referred to the analysis of common mode and differential mode crosstalk.  Here are some tips for Layout engineers when handling serpentine patterns:  
 
   1. Try to increase the distance of parallel pattern segment (S), which is at least greater than 3H. H refers to the distance from signal routing to the reference graphic.  In general, it means to go around a big bend, so as long as S is large enough, the coupling effect can be almost completely avoided.  
   2. Reduce the coupling length Lp, and when the two-fold Lp delay approaches or exceeds the signal rise time, the crosstalk generated will reach saturation.  
   3. The signal transmission delay caused by strip-patterns or Embedded micro-strips is less than that caused by micro-strips.  Theoretically, the ribbon line does not affect the transmission rate due to differential mode crosstalk.  
   4, High speed and strict timing requirements of the signal pattern, try not to go snake pattern, especially not in a small range of winding pattern.  
   5, can often use any Angle of snake routing, can effectively reduce the coupling between each other.  
   6. In high speed PCB design, serpentine has no so-called filtering or anti-jamming capability, which can only reduce the signal quality, so it is only used for timing matching and no other purpose.  
   7. Spiral winding can sometimes be considered. Simulation shows that the effect is better than that of normal snake winding.