PCB Blog - Analysis of common problems in PCB board design skills

1. How to choose a PCB board
The choice of PCB board must strike a balance between meeting design requirements and mass producibility and cost. Design requirements include both electrical and mechanical components. Usually when designing very high-speed PCB boards (frequency greater than GHz) this material problem will be more important. For example, the commonly used FR-4 material, the dielectric loss at several GHz frequencies will have a great influence on signal attenuation, and may not be suitable. As far as electrical is concerned, attention should be paid to whether the dielectric constant (dielectric constant) and the dielectric loss are suitable for the designed frequency.

2. How to avoid high frequency interference
The basic idea of avoiding high-frequency interference is to minimize the interference of the electromagnetic field of high-frequency signals, which is the so-called crosstalk (Crosstalk). You can increase the distance between the high-speed signal and the analog signal, or add ground guard/shunt traces next to the analog signal. Also be aware of the noise interference from the digital ground to the analog ground.

3. In high-speed design, how to solve the problem of signal integrity
Signal integrity is basically a matter of impedance matching. The factors affecting impedance matching include the structure and output impedance of the signal source, the characteristic impedance of the trace, the characteristics of the load end, and the topology of the trace. The solution is to rely on termination (termination) and adjust the topology of the trace.

4. How is differential wiring implemented
There are two points to pay attention to in the wiring of the differential pair, 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 (this distance is determined by the differential impedance) should always remain unchanged, that is, it should be kept parallel. There are two parallel ways, one is for two lines to run on the same wiring layer (side-by-side), and the other is for two lines to run on two adjacent layers (over-under). Generally, there are many ways to realize the former side-by-side. 

5. For a clock signal line with only one output, how to implement differential wiring
To use differential wiring, it only makes sense that both the signal source and the receiving end are differential signals. Therefore, differential routing cannot be used for clock signals with only one output.

6. Can a matching resistor be added between the differential line pairs at the 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 will make the signal quality better.

7. Why should the wiring of the differential pair be close and parallel
The differential pairs should be routed appropriately close and parallel. The so-called appropriate proximity is because this spacing will affect the value of the differential impedance (differential impedance), which is an important parameter for designing differential pairs. Parallelism is also required to maintain the consistency of the differential impedance. If the two lines are suddenly far and near, the differential impedance will be inconsistent, which will affect the signal integrity (signal integrity) and time delay (timing delay).

8. How to deal with some theoretical conflicts in actual wiring
1) Basically, it is right to isolate the analog/digital ground. It should be noted that the signal traces should not cross the divided places (moat) as much as possible, and do not let the return current path of the power supply and the signal (returning current path) become too large. 
2) The crystal oscillator is an analog positive feedback oscillation circuit. To have a stable oscillation signal, it must meet the specifications of loop gain and phase, and the oscillation specification of this analog signal is easily disturbed. Even if ground guard traces are added, the interference may not be completely isolated. . And too far away, the noise on the ground plane will also affect the positive feedback oscillator circuit. Therefore, the distance between the crystal oscillator and the chip must be as close as possible.
3) Indeed, there are many conflicts between high-speed wiring and EMI requirements. However, the basic principle is that some electrical characteristics of the signal cannot meet the specifications due to the resistance, capacitance or ferrite bead added by EMI. Therefore, first use the skills of arranging traces and PCB stack-up to solve or reduce EMI problems, such as high-speed signals going through inner layers. Only use the resistor-capacitor or ferrite bead method to reduce the damage to the signal.

9. How to solve the contradiction between manual wiring and automatic wiring of high-speed signals
Most of the automatic routers of strong routing software now have set constraints to control the routing method and the number of vias. The winding engine capability and constraint setting items of various EDA companies are sometimes quite different. For example, whether there are enough constraints to control the way the serpentine is meandering, whether to control the trace spacing of the differential pair, etc. This will affect whether the automatic routing method can meet the designer's idea. In addition, the difficulty of manually adjusting the wiring is also related to the ability of the winding engine. For example, the pushing ability of the trace, the pushing ability of the via, and even the pushing ability of the trace to the copper coating, etc. Therefore, choosing a router with strong winding engine capability is the solution.

10. About the test coupon
The test coupon is used to measure whether the characteristic impedance of the produced PCB board meets the design requirements by TDR (Time Domain Reflectometer). Generally, the impedance to be controlled has two cases: single line and differential pair. Therefore, the line width and line spacing (when there is a differential pair) on the test coupler should be the same as the line to be controlled. What is important is the location of the ground point when measuring. In order to reduce the inductance value of the ground lead, the grounding place of the TDR probe is usually very close to the probe tip. to match the probe used on PCB board.