Elements in PCB design:

　　1. select PCB sheet

　　Choose PCB plate must meet the design requirements and mass production and cost balance. Design requirements include electrical and institutional parts. This material problem is usually more important when designing very high-speed PCB boards (more than the frequency of GHz. The current FR-4 material, for example, the dielectric loss (dielectric loss) at several GHz frequencies will have a great impact on signal attenuation and may not be appropriate. As far as electricity is concerned, attention should be paid to the combination of dielectric constant (dielectric constant) and dielectric loss at the designed frequency.


　　2. avoid high frequency interference

　　A basic idea to avoid high frequency interference is to minimize the interference of high frequency signal electromagnetic field, that is, crosstalk (Crosstalk). Can widen the distance between high speed signal and analog signal, or add ground guard/shunt traces next to analog signal, but also pay attention to the noise interference of analog ground digitally.

　　3. solve the problem of signal integrity

　　signal integrity is basically a problem of impedance matching. The factors affecting impedance matching include the structure of signal source and output impedance (output impedance), the characteristic impedance of line, the characteristic of load end, the topology (topology). A solution is to rely on termination (te rmina ti on) and adjustment of the.

　　Implementation of Differential Distribution Line

　　There are two points to pay attention to, one is that the length of the two lines should be as long as possible, the other is that the distance between the two lines (which is determined by the differential impedance) should remain unchanged, that is, to keep parallel. There are two parallel ways, one is two lines walking in the same line layer (side-by-side), the other is two lines walking up and down adjacent two layers (over-under). Generally, the former side-by-side more ways to achieve.


　　5. differential wiring is implemented with only one output clock signal line

　　Differential wiring must be a signal source and receiver are also differential signals. Therefore, differential wiring can not be used for clock signals with only one output.

　　6、Matching resistance between differential line pairs at receiving end

　　The matching resistance between the differential line pairs at the receiving end is usually added, and its value should be equal to the value of the differential impedance. This signal quality will be better.

　　7. differential pair wiring should be close and parallel

　　The wiring of differential pairs should be properly close and parallel. A proper approach is called because this spacing affects the value of the differential impedance (differential impedance), which is an important parameter for designing differential pairs. Parallelism is also needed because of the consistency of differential impedance. When the two lines suddenly approach, the differential impedance will be inconsistent, which will affect the signal integrity (signal integrity) and time delay (timing delay).

　　Dealing with some theoretical conflicts in actual wiring

　　Basically, it is right to segment the modules/number. Note that the signal line should not cross the segmented place (moat), and do not let the power and signal reflux current path (returning current path) become too large. 

　　Crystal oscillator is an analog positive feedback oscillation circuit. In order to have a stable oscillation signal, the specification of loop g ai n and phase must be satisfied, and the oscillation specification of this analog signal is easy to be interfered, even if the ground guard traces may not be completely isolated. And too far away, the noise in the ground plane will also affect the positive feedback oscillation circuit. Therefore, we must close the distance between the crystal oscillator and the chip.

　　Do have many conflicts with EMI requirements. The basic principle is that some electrical characteristics of the signal are not in accordance with the specification because the resistor, capacitance or ferrite bead, added by the EMI can not cause the signal. Therefore, it is best to solve or reduce EMI problems by arranging wiring and PCB stacking techniques, such as high-speed signals to the inner layer. Finally, resistive capacitance or ferrite bead is used to reduce the damage to the signal.


　　9、Solve the contradiction between manual and automatic wiring of high speed signal

　　Most of the automatic cabling devices with strong cabling software now have constraints to control the winding mode and the number of holes. Each EDA company's winding engine capability and constraints are sometimes very different. For example, whether there are enough constraints to control the snake line (serpentine) winding way, whether to control the differential pair of line spacing and so on. This will affect whether the automatic wiring is in line with the designer's ideas. In addition, the difficulty of manually adjusting wiring is also absolutely related to the ability of the winding engine. For example, the push and squeeze ability of the line, the push and squeeze ability of the hole, and even the push and squeeze ability of the line to apply copper, etc. Therefore, the choice of a winding engine strong wiring device, is the solution.

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