PCB design tips 100, teach you to master PCB design easily (19-24)

PCB Flexible software can be used to design flexible Printed Circuits. The same is used in Gerber format for the production of FPC manufacturers. Because the manufacturing process is different from the general PCB, the manufacturers will have their limitations on the minimum Xian Kuan, the minimum line distance, and the minimum Kong Jing (via) according to their manufacturing capability. In addition, it is possible to reinforce the flexible circuit board by spreading some copper skin. As for the manufacturer, the Internet can be found when the "FPC" is used.

20. What is the principle of choosing the point of PCB and shell earthing properly?

The principle of selecting PCB and shell location is to use chassis ground to provide a low impedance path to the reflux current (returning current) and the path to control the reflux current. For example, in the vicinity of a high frequency device or a clock generator, a fixed screw can be used to connect the PCB's formation to the chassis ground to minimize the entire current loop area and to reduce electromagnetic radiation.

21. Circuit board DEBUG should start from those aspects?

In terms of digital circuits, three things are first determined in order: 1. confirm that all power values are all up to design requirements. Some systems with multiple power sources may require a certain specification of the order and speed between certain power sources. 2. confirm that all clock signal frequencies are working normally and there is no nonmonotonic (non-monotonic) problem on the edge of the signal. 3. confirm whether the reset signal meets the requirements of the specification. If these are normal, the chip should issue the first cycle (cycle) signal. Next, according to the system operation principle and bus protocol to debug.

22. In the case of the fixed size of the circuit board, if the design needs to accommodate more functions, it often needs to improve the line density of PCB, but it may lead to mutual interference enhancement of the line, while the line is too fine and the impedance can not be reduced. Experts introduce the technology in the high density PCB design of high speed (>100MHz).  Coincidentally?

When designing high speed and high density PCB, the crosstalk (crosstalk interference) is really special attention, because it has a great influence on the timing (timing) and signal integrity (signal integrity). Here are a few places to pay attention to:

• 1> controls the continuity and matching of the characteristic impedance of the walk line.
• The size of the distance between the 2> lines. The commonly seen distance is two times the line width. The influence of line spacing on timing and signal integrity can be known through simulation, and the tolerable minimum spacing can be found. The results of different chip signals may be different.
• 3> selects the appropriate end connection.
• 4> avoids the same direction in the two adjacent layers, and even the line is overlapped just up and down, because the crosstalk is larger than the adjacent line of the same layer.
• 5> uses blind via (blind/buried) to increase the line area. But the cost of making the PCB board will increase. In practice, it is difficult to achieve full parallelism and equal length, but we must try our best to achieve it.

In addition, differential termination and common mode termination can be reserved to mitigate the effect of timing and signal integrity.

23. Analog power supply filter is often used LC circuit. But why is sometimes LC worse than RC filtering?

The comparison between LC and RC filtering effect must consider the appropriateness of the frequency band and inductance value to be filtered out.  Because inductance inductance (reactance) size is related to inductance value and frequency. If the noise frequency of the power supply is low, and the inductance value is not large enough, the filtering effect may not be as good as RC. However, the cost of using RC filtering is that the resistance itself will dissipate energy and efficiency is poor, and pay attention to the power that the selected resistors can bear.

24. What is the method of selecting inductance and capacitance when filtering?

The selection of inductance value should consider not only the noise frequency but also the response capability of instantaneous current. If the LC's output will have an opportunity to output a large current in a moment, the inductance is too high for the general current to increase the ripple noise (ripple noise) at the speed of the current flow through the inductor. The capacitance value is related to the tolerable ripple noise specification value. The smaller the ripple noise value is, the larger the capacitance will be. And the ESR/ESL of the capacitor will have an impact. In addition, if the LC is placed on the output of the switching power supply (switching regulation power), the effect of the pole-zero (pole/zero) generated by the LC on the stability of the negative feedback control (negative feedback control) loop is also noted.