In high-speed PCB design, these issues must be paid attention to

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In high-speed PCB design, these issues must be paid attention to [Copy link]

When designing PCB, we often encounter various problems, such as impedance matching, EMI rules, etc. This article has compiled some difficult questions and answers related to high-speed PCB, hoping to be helpful to everyone.

1. How to consider impedance matching when designing high-speed PCB schematics?

When designing high-speed PCB circuits, impedance matching is one of the design elements. The impedance value is absolutely related to the routing method, such as whether it is on the surface layer (microstrip) or the inner layer (stripline/double stripline), the distance from the reference layer (power layer or ground layer), the routing width, and the PCB material, which will affect the characteristic impedance value of the routing.

That is to say, the impedance value can only be determined after routing. General simulation software cannot take into account some routing situations with impedance discontinuity due to the limitations of the line model or the mathematical algorithm used. At this time, only some terminators (terminations), such as series resistors, can be reserved on the schematic diagram to alleviate the effect of routing impedance discontinuity. The real fundamental solution to the problem is to try to avoid the occurrence of impedance discontinuity during routing.

2. When there are multiple digital/analog functional blocks in a PCB board, the conventional practice is to separate the digital/analog grounds. Why?

The reason for separating the digital/analog grounds is that digital circuits will generate noise in the power supply and ground when switching between high and low potentials. The amount of noise is related to the speed of the signal and the size of the current.

If the ground plane is not divided and the noise generated by the digital area circuit is large and the analog area circuit is very close, even if the digital and analog signals do not cross, the analog signal will still be interfered by the ground noise. In other words, the method of not dividing the digital and analog ground can only be used when the analog circuit area is far away from the digital circuit area that generates large noise.

3. When designing high-speed PCBs, from which aspects should designers consider EMC and EMI rules?

Generally, EMI/EMC design needs to consider both radiation (radiated) and conduction (conducted). The former belongs to the higher frequency part (>30MHz) and the latter belongs to the lower frequency part (<30MHz). Therefore, we cannot only pay attention to the high frequency and ignore the low frequency part.

A good EMI/EMC design must consider the location of the device, the arrangement of the PCB stack, the routing of important connections, the selection of devices, etc. at the beginning of the layout. If these are not arranged better in advance, solving them afterwards will be ineffective and increase costs.

For example, the location of the clock generator should be as far away as possible from the external connector, high-speed signals should be routed on the inner layer as much as possible, and attention should be paid to characteristic impedance matching and continuity of the reference layer to reduce reflections. The slew rate of the signal pushed by the device should be as small as possible to reduce high-frequency components. When selecting decoupling (decoupling/bypass) capacitors, pay attention to whether their frequency response meets the requirements to reduce power layer noise.

In addition, pay attention to the return path of the high-frequency signal current to make its loop area as small as possible (that is, the loop impedance is as small as possible) to reduce radiation. You can also use the method of splitting the ground layer to control the range of high-frequency noise. Finally, choose the grounding point (chassis ground) of the PCB and the shell appropriately.

4. When making PCB boards, in order to reduce interference, should the ground wire be closed?

When making PCB boards, the loop area should generally be reduced to reduce interference. When laying out the ground wire, it should not be laid in a closed form, but rather in a tree-like shape. In addition, the ground area should be increased as much as possible.

5. How to adjust the routing topology to improve signal integrity?

The signal direction of this network is more complicated, because the topology has different effects on unidirectional and bidirectional signals, and signals of different levels and types. It is difficult to say which topology is beneficial to signal quality. In addition, when doing pre-simulation, it is very demanding for engineers to know which topology to use, and they must understand the circuit principles, signal types, and even wiring difficulties.

6. How to ensure the stability of signals above 100M during layout and wiring?

The key to high-speed digital signal routing is to reduce the impact of transmission lines on signal quality. Therefore, when laying out high-speed signals above 100M, signal routing is required to be as short as possible. In digital circuits, high-speed signals are defined by signal rise delay time.

Moreover, different types of signals (such as TTL, GTL, LVTTL) have different methods for ensuring signal quality.