Common Issues in PCB Design

As signal rates continue to increase, many new problems in PCB design have plagued engineers. Smaller, faster devices make PCB design more complicated. Here are some common issues in the PCB design.

1. How to solve signal integrity problems in high speed design?

Answer: Signal integrity is basically a matter of impedance matching. The factors affecting the impedance matching are the signal source architecture and the output impedance, the characteristic impedance of the trace, the characteristics of the load end, and the topology of the trace and etc. The solution is to termination and adjust the topology of the traces.

2. How to avoid high frequency interference?

Answer: The basic idea of avoiding high-frequency interference is to minimize the interference of high-frequency signal electromagnetic fields, which is called crosstalk. You can increase the distance between the high-speed signal and the analog signal, or add ground guard or shunt traces next to the analog signal. And you also need to pay attention to the noise interference of the digital ground to the analog ground.

3. In high-speed PCB design, the blank are of the signal layer can be coated with copper. How should the copper of multiple signal layers be distributed on the ground and the power supply?

Answer: Generally, most of the copper in the blank area is grounded. When applying copper near the high-speed signal line, you only need to pay attention to the distance between the copper and the signal line, because the coated copper will reduce the characteristic impedance of the trace. Also be careful not to affect the characteristic impedance of other layers, such as in the structure of a dual strip line.

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

Answer: Impedance matching is one of the design elements when designing high-speed PCB circuit. The impedance value has an absolute relationship with the routing method. For example, the distance between the micro strip of the surface layer or the strip line or double strip line of the inner layer and the reference layer (power layer or ground layer), the trace width, the PCB material, etc. all affect the characteristic impedance value of the routing. That is to say, the impedance value can be determined after routing. In general, the simulation software cannot take into account some of the discontinuous routing conditions due to the limitations of the circuit model or the mathematical algorithms used. At this time, only some terminators can be reserved on the schematic, such as series resistors, to mitigate the discontinuity of the trace impedance. The only way to solve this problem is to avoid the occurrence of impedance discontinuities when routing.

5. More than 2G high-frequency PCB design, what should be paid attention to in the routing and typesetting?

Answer: High-frequency PCBs above 2G belong to radio-frequency circuit design and are not covered by high-speed digital circuit design. The layout and routing of the RF circuit should be considered together with the schematic because layout and routing can have a distribution effect. Moreover, some passive components of RF circuit design are realized by parameterized definition and special shape copper foil. Therefore, EDA tools are required to provide parameterized devices and edit special shape copper foil. HOYOGO's board station has a dedicated RF design module to meet these requirements.

6. How to deal with the layout and routing to ensure the stability of signals above 50M?

Answer: The key to high-speed digital signal routing is to reduce the impact of the transmission line on signal quality. Therefore, the high-speed signal layout above 100M requires the signal trace to be as short as possible. In digital circuits, high-speed signals are defined by the signal rise time. Moreover, different types of signals, such as TTL, GTL, LVTTL, have different methods of ensuring signal quality.

7. What aspects of EMC and EMI should designers consider in high-speed design?

Answer: In general EMI/EMC design, both radiated and conducted aspects need to be considered. The former belongs to the higher frequency part (>30MHz) and the latter is the lower frequency part (<30MHz), so you can't just pay attention to the high frequency and ignore the low frequency part. A good EMI/EMC design must take into account the location of the component at the beginning of the layout, the placement of the PCB stack, the important online routing, the choice of components, etc.

If these do not have better arrangements in advance, then the you will get half the result with twice the effort and even increase the cost. 

For example:

1) Keep the clock generator as far away as possible from the external connector.

2) high-speed signals should go as far as possible to the inner layer and pay attention to the characteristic impedance matching and the continuity of the reference layer to reduce reflection.

3) The slew rate of the signal pushed by the device is as small as possible to reduce the high frequency component.

4) When selecting a decoupling or bypass capacitor, please note whether its  frequency response meets the requirements for reducing power layer noise.

In addition, please pay attention to the return path of the high-frequency signal current, so that the loop area is as small as possible, that is, the loop impedance is as small as possible to reduce the radiation. It is also possible to divide the formation to control the range of high frequency noise. Finally, properly select the chassis ground of the PCB and the housing.