Attention! 10 Questions in High-speed PCB Circuit Design
1. How to deal with some theoretical conflicts in actual wiring?
Basically, it is right to divide and isolate the analog/digital ground. It should be noted that the signal trace should not cross the moat as much as possible, and the returning current path of the power supply and signal should not be too large.
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 interfered. Even if ground guard traces is added, it may not be able to completely isolate the interference. If it is too far away, the noise on the ground plane will also affect the positive feedback oscillation circuit. Therefore, the distance between the crystal oscillator and the chip must be as close as possible.
Indeed, there are many conflicts between high-speed wiring and requirements of EMI . While, the basic principle is that the resistance and capacitance or ferrite bead added by EMI should not cause some electrical characteristics of the signal to fail to meet the specifications. Therefore, it is best to use the skills of arranging traces and PCB stacking to solve or reduce EMI problems, such as high-speed signals into the inner layer. Finally, a resistance-capacitance or ferrite bead method is used to reduce the damage to the signal.
2. How to solve the contradiction between manual wiring and automatic wiring of high-speed signals?
Most of the automatic routers of strong wiring software now have set constraints to control the winding method and the number of perforation. The winding engine capabilities and constraint setting items of various EDA companies sometimes differ greatly. For example, whether there are enough constraints to control the serpentine meandering, whether the spacing of the differential pair can be controlled, and so on. This will affect whether the routing method of the automatic routing can meet the designer's idea.
In addition, the difficulty of manually adjusting the wiring is also absolutely related to the ability of the winding engine. For example, the pushing ability of the wiring, the pushing ability of the perforation, and even the pushing ability of the wiring to the copper coating, etc. Therefore, choosing a router with strong winding engine capability is the solution.
3. In high-speed PCB design, the blank area of the signal layer can be coated with copper, and how should the copper coating of multiple signal layers be distributed on the ground and power supply?
Generally, the copper plating in the blank area is mostly grounded. Just pay attention to the distance between the copper and the signal line when applying copper next to the high-speed signal line, because the applied copper will reduce the characteristic impedance of the trace a little. Also be careful that not to affect the characteristic impedance of other layers, such as in the structure of dual strip line.
4. Is it possible to use the microstrip line model to calculate the characteristic impedance of the signal line on the power plane? Can the signal between the power supply and the ground plane be calculated by using the stripline model?
Yes, when the characteristic impedance is calculated, both the power plane and the ground plane must be regarded as reference planes. For example, a four-layer board: top layer-power layer-ground layer-bottom layer. At this time, the characteristic impedance model of the top layer is a microstrip line model with the power plane as the reference plane.
5. Can the test points be automatically generated by software on the high-density printed board, can it meet the test requirements of mass production under normal circumstances?
Generally, whether the test point automatically generated by the software meets the test requirements depends on whether the specification for adding test points meets the requirements of the test equipment. In addition, if the wiring is too dense and the rules for adding test points are too strict, there may be no way to automatically add test points to each segment of the line. Of course, you need to manually add in the places you want to test.
6. Will adding test points affect the quality of high-speed signals?
As to whether the signal quality will be affected, it depends on the method of adding test points and how fast the signal is. Basically, the additional test point does not need to use the existing online via or DIP pin as the test point. It may be added to the line or pulled a short line from the line. The former one is equivalent to adding a small capacitor on the line, and the latter one is adding an extra branch.
Both of these conditions will affect the high-speed signal more or less, and the degree of the effect is related to the frequency speed of the signal and EDGE rate. The magnitude of the impact can be known through simulation. In principle, the smaller the test point, the better (of course, it must meet the requirements of the test tool)and the shorter the branch, the better.
7. Several PCB form a system, and how should the ground wires between the boards be connected?
When the signal or power supply between each PCB board is connected to each other, for example, if board A has a power supply or a signal is sent to board B, there must be an equal amount of current flowing from the ground back to board A (this is Kirch off current law). The current on the ground will find the place with the least impedance to flow back. Therefore, at each interface, whether it is power or signal interconnection, the number of pins allocated to the ground layer should not be too small to reduce the impedance, which can reduce the noise on the ground layer.
In addition, you can also analyze the entire current loop, especially the part with larger current, and adjust the connection of the ground layer or ground wire to control the current flow (for example, make a low impedance somewhere so that most of the current flows from this place) to reduce the impact on other more sensitive signals.
8. What is the principle of properly selecting the grounding point between the PCB and the shell?
The principle of selecting the PCB board and the shell grounding point is to use the chassis ground to provide a low-impedance path for the returning current and to control the path of the returning current. For example, usually near high-frequency devices or clock generators, fixed screws can be used to connect the ground layer of the PCB board to the chassis ground to minimize the area of the entire current loop and reduce electromagnetic radiation.
9. Which aspects of circuit board DEBUG should start from?
As far as digital circuits are concerned, determine three things in order firstly:
Confirm that all power supply values meet the design requirements. Some systems with multiple power supplies may require certain specifications for the order and speed of power supplies;
Confirm that all clock signal frequencies are working properly, and there are no non-monotonic problems on the edges of the signal;
Confirm whether the reset signal meets the specification requirements.
If these are normal, the chip should send out the first cycle signal. Next, DEBUG according to the operating principle of the system and the busprotocol.
10. When the size of the circuit board is fixed, if more functions need to be accommodated in the design, it is often necessary to increase the trace density of the PCB, but this may increase the mutual interference of the traces, and at the same time, the impedance of the traces cannot be lowered if the wiring is too thin. So, what are the skills in high-speed (>100MHz) and high-density PCB design?
When designing high-speed and high-density PCB, cross talk interference really needs special attention, because it has a great impact on timing and signal integrity. Here are a few points to note:
Control the continuity and matching of the characteristic impedance of the trace. Control the size of the trace spacing. The commonly seen spacing is twice the line width. It is possible to know the influence of trace spacing on timing and signal integrity through simulation, and find the minimum tolerable spacing. The result of different chip signals may be different.
Choose the appropriate termination method. Avoid the wiring directions of two adjacent layers in the same direction, or even if the wiring overlaps up and down, because this kind of cross talk interference is greater than that of adjacent wiring on the same layer.
Use blind/buried via to increase the trace area. However, the manufacturing cost of the PCB board will increase. It is indeed difficult to achieve complete parallelism and equal length in actual implementation, but it is still necessary to do it as much as possible.
