PCB design is very important in the whole circuit board, it determines the foundation of the whole PCB. This paper summarizes some points needing attention in PCB design for reference.
Select PCB board
PCB board selection must meet the design requirements and mass production and cost of the balance between. The design requirements include electrical and mechanical parts. This is usually important when designing very fast PCB boards (frequencies greater than GHz). For example, the fr-4 material commonly used today may not be suitable because the dielectric loss at several GHz has a great effect on signal attenuation. In the case of electricity, pay attention to the dielectric constant and dielectric loss at the designed frequency.
Avoid high-frequency interference
The basic idea of avoiding high frequency interference is to minimize the interference of high frequency signal electromagnetic field, also known as Crosstalk. You can increase the distance between the high speed signal and the analog signal, or add ground guard/shunt traces to the analog signal, and pay attention to the noise interference of the digital ground to the analog ground.
Solve the problem of signal integrity
Signal integrity is basically a matter of impedance matching. The factors that affect impedance matching include signal source architecture, output impedance, cable characteristic impedance, load side characteristic, and cable topology architecture. The solution is by termination and adjustment of the topology of the cable.
Realize differential wiring
The wiring of the difference pair has two points to pay attention to. One is that the length of the two lines should be as long as possible, and the other is that the distance between the two lines (determined by the difference impedance) should always remain constant, that is, to keep parallel. There are two parallel modes: one is that the two lines run on the same side-by-side layer, and the other is that the two lines run on two adjacent layers of the upper and lower layers. Generally, the former side-by-side implementation is more common.
In the case of only one output clock signal line, to achieve differential wiring
Want to use differential wiring must be signal source and receiving end are also differential signal is meaningful. So it is impossible to use differential wiring for clock signals with only one output.
Matching resistance between the differential line pairs at the receiving end
The matching resistance between the pair of differential lines at the receiving end is usually added, and its value should be equal to the value of the differential impedance. The signal quality will be better.
The wiring of the difference pair should be close and parallel
The wiring of difference pairs should be appropriately close and parallel. Proper proximity is because the spacing will affect the differential impedance value, which is an important parameter in designing difference pairs. Parallelism is also required to maintain the consistency of the differential impedance.If the two lines are either far or near, the differential impedance will be inconsistent, which affects signal integrity and timing delay.
Deal with some theoretical conflicts in the actual wiring
A. Basically, it is right to separate modules/numbers. Care should be taken not to cross the MOAT and not to let the power supply and signal return current path grow too large.
B. Crystal oscillator is a simulated positive feedback oscillation circuit. To have a stable oscillation signal, it must meet the specifications of loop gain and phase. And too far away, the noise in the plane will also affect the positive feedback oscillation circuit. Therefore, be sure to put the crystal oscillator and chip distance into possible close.
C. Indeed, there are many conflicts between high-speed wiring and EMI requirements. However, the basic principle is that due to the resistance capacitance or Ferrite Bead added by EMI, some electrical characteristics of the signal cannot be caused to fail to meet the specifications. Therefore, it is best to use the arrangement of wiring and PCB stacking skills to solve or reduce EMI problems, such as high-speed signal to the inner layer. Finally, resistor capacitance or Ferrite Bead method was used to reduce the damage to the signal.
Solve the contradiction between manual wiring and automatic wiring of high-speed signals
Nowadays, most of the automatic cabling devices in strong cabling software have set constraints to control the winding mode and the number of holes. EDA companies sometimes vary widely in setting the capabilities and constraints of winding engines. For example, whether there are enough constraints to control how serpentine lines wind, whether there are enough constraints to control the spacing of difference pairs, etc. This will affect whether the automatic wiring out of the wiring can conform to the designer’s idea. In addition, the difficulty of manual wiring adjustment is also absolutely related to the ability of the winding engine. For example, the wire pushing capacity, through the hole pushing capacity, and even the wire on copper coating pushing capacity and so on. So, choose a cabler with strong winding engine ability, it is the way to solve.