Practical tips for high-frequency PCB circuit design

2017/3/6 14:53:31

1. How to choose PCB board?
The choice of PCB board must achieve a balance between design requirements, productivity and cost. The design requirements include electrical and institutional parts. This material problem is usually important when designing very high speed PCB boards (frequencies are higher than GHz). For example, the commonly used FR-4 material, whose dielectric loss at several GHz frequencies can have a significant effect on signal attenuation and may not be suitable. In the case of electric, it is vital to note whether the dielectric constant and the dielectric loss are appropriate at the designed frequency.
2. How to avoid high frequency interference?
The basic idea to avoid high-frequency interference is to minimize the interference of the high-frequency signal electromagnetic field, which is the so-called crosswalk. The distance between high-speed signal and the analog signal can be increased or ground guard traces can be added next to the analog signal. Besides, notice the noise interference from digital ground to the analog ground.
3. In high-speed design, how to solve signal integrity?
Signal integrity is essentially a matter of impedance matching. The factors that affect the impedance matching are the architecture of the signal source and the output impedance, the characteristic impedance of the trace, the characteristics of the load side, the trace topology architecture, and so on. The solutions rely on termination and alignment of trace topology.
4. How will differential pair routing be implemented?
There are two points to note for differential pair routing. Firstly, the length of the two lines should be the same. Secondly, the distance between the two lines (this distance determined by the differential impedance) remains the same, namely, to keep parallel. There are two parallel ways, one is the two lines walking in the same line layer (side-by-side); the other way is that two lines are in the upper and lower adjacent two layers (over-under). Generally the side-by-side one is frequently used.
5. For the clock signal line with only one output, how to achieve differential pair routing?
It is meaningful to use differential pair routing when the signal source and the receiving end are in differential signal. So for the clock signal line with only one output, the differential pair routing is not suitable.
6. Is a matching resistor necessary between differential pair routing in the receiving end?
It is usually necessary. The value of matching resistor should be equal to this of the differential impedance for better signal quality.
7. Why does the differential pair routing require being close and parallel?
Differential pair routing should be properly close and parallel. The so-called proper proximity is because this spacing will affect the value of differential impedance, which is an important parameter for designing differential pairs. Keeping parallel is necessary to maintain the consistency of the differential impedance. If the two lines are not paralleled, the differential impedance will be inconsistent affecting the signal integrity and time delay.
8. How to deal with some conflicting issues in actual wiring?
Basically, it is right to divide the analog and digital ground wire. It is important to note that the signal trace should not cross the moat, and avoid too large returning current path of the power supply and signal.Crystal oscillator is an analogue oscillator circuit of positive feedback, and must meet the loop gain and phase specifications for a stable oscillation signal. However, this analog signal oscillation specification is susceptible to interference which cannot be completely isolated even if the ground guard traces are added. Besides, the ground surface noise will also affect the positive feedback oscillator circuit if too far apart. Therefore, we must make the distance between crystal and chip close.
There are many conflicts with the requirements for high-speed wiring and EMI. The basic principle is the resistance capacitor or ferrite bead added in EMI shouldn’t cause fall short of the specification in the electrical characteristics of signal.Therefore, it is the best to solve or reduce EMI problems by the wiring alignment and PCB stacking techniques, such as high-speed signal walking in the inner layer. Finally you can use resistor-capacitor or ferrite bead to reduce the damage to the signal.
9. How to solve the high-speed signal between the manual wiring and automatic wiring between the contradictions?
Now most of the automatic router of cabling software has set the constraints to control the winding way and the number of vias. Each EDA company's winding engine capacity and the project set by constraints is sometimes quite different. For example, whether there are enough constraints to control the serpentine meandering way, and how to control the distance between differential pair routing. This will affect the automatic routing whether can meet the designer's ideas. In addition, the difficulty of manually adjusting the wiring and the ability of the winding engine has an absolute relationship. So, choosing a route engine with strong winding capable is the solution.
10. about test coupon:
Test coupon is to measure the characteristic impedance of the PCBs whether can meet the design requirements by TDR (Time Domain Reflectometer). Generally the impedance that have to be controlled includes a single line and differential pairs. Therefore, the line width and line spacing (differential pairs) on the test coupon are the same as the lines to be controlled. The most important thing is to measure the location of the ground. In order to reduce the inductance of the ground lead, the location of the TDR probe is usually very close to the probe tip, so the distance and mode of the signal-testing point and ground point at the test coupon should match the probe used.
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