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# Analysis of problems in PCB design (16-25)

Posted:05:23 PM October 18, 2018 writer: G

Applications for high-speed digital circuits today include communications networks and computers. In terms of communication networks, the working frequency of the PCB has reached GHz, and the number of layers is as much as 40 layers. Computer-related applications are also due to advances in chips. Whether it is a general PC or a server, the maximum operating frequency on the board has reached 400MHz (such as Rambus). In response to this high-speed, high-density wiring requirement, the demand for blind/buried vias, mircrovias, and build-up process processes is also increasing. These design requirements are available to manufacturers in large quantities.

17. Two characteristic impedance equations that are often referred to:

a. microstrip (microstrip) Z={87/[sqrt(Er+1.41)]}ln[5.98H/(0.8W+T)] where W is the line width and T is the copper thickness of the trace. H is the distance from the trace to the reference plane, and Er is the dielectric constant of the PCB material. This formula must be applied in the case of 0.1<(W/H)<2.0 and 1<(Er)<15.

b. Stripline Z=[60/sqrt(Er)]ln{4H/[0.67π(T+0.8W)]} where H is the distance between the two reference planes and the traces are in the two reference planes in the middle. This formula must be applied with W/H<0.35 and T/H<0.25.

18. Can the ground wire be added in the middle of the differential signal line?

In the middle of the differential signal, the ground line cannot be added. Because the most important point of application of differential signals is the benefits of mutual coupling between differential signals, such as flux cancellation, noise immunity, and the like. If the ground wire is added in the middle, the coupling effect will be destroyed.

19. Does the rigid flexible board design require special design software and specifications? Where can I undertake this type of circuit board processing in China?

A flexible printed circuit can be designed using software that generally designs PCBs. The same is done in the Gerber format for FPC manufacturers. Because the manufacturing process is different from the general PCB, each manufacturer will have restrictions on the minimum line width, minimum line spacing, and minimum aperture depending on their manufacturing capabilities. In addition, some copper can be reinforced at the turning point of the flexible circuit board. As for the manufacturer of the product, the online "FPC" can be found as a keyword query.

20. What is the principle of properly selecting the point at which the PCB and the case are grounded?

The principle of selecting the PCB and case ground point selection is to use chassis ground to provide a low impedance path to the return current and the path to control this return current. For example, the ground plane of the PCB can be connected to the chassis ground by a fixed screw near the high-frequency device or the clock generator to minimize the entire current loop area and reduce electromagnetic radiation.

21. The circuit board DEBUG should start from those aspects?

In the case of digital circuits, first determine three things in order:

21.1. Verify that all power supply values are as designed. Some systems with multiple power supplies may require some specification of the order and speed of some power supplies.

21.2. Verify that all clock signal frequencies are working properly and that there are no non-monotonic problems on the edges of the signal.

21.3. Verify that the reset signal meets the specifications.

If these are normal, the chip should signal the first cycle. Next, follow the system operation principle and the bus protocol to debug.

22. In the case of fixed circuit board size, if the design needs to accommodate more functions, it is often necessary to increase the PCB trace density, but this may lead to increased mutual interference of the traces, while the routing is too thin The impedance cannot be reduced. Can you introduce the techniques in high-speed (>100MHz) high-density PCB design?

Crosstalk interference is of particular concern when designing high-speed, high-density PCBs because it has a large impact on timing and signal integrity. Here are a few caveats:

22.1. Control the continuity and matching of the characteristic impedance of the trace.

22.2. The size of the trace spacing. The spacing commonly seen is twice the line width. The simulation can be used to know the influence of the trace spacing on timing and signal integrity, and to find the minimum tolerance that can be tolerated. The results of different chip signals may vary.

22.3. Select the appropriate termination method.

22.4. Avoid the same direction of the upper and lower adjacent layers, even if the traces are just overlapping up and down, because this crosstalk is larger than the adjacent lines in the same layer.

22.5. Use blind/buried via to increase the area of the trace. However, the manufacturing cost of the PCB will increase. It is really difficult to achieve full parallelism and equal length in actual implementation, but still try to do it. In addition, differential termination and common-mode termination can be reserved to mitigate the effects on timing and signal integrity.

23. The filtering at the analog power supply is often done with an LC circuit. But why is LC sometimes worse than RC filtering?

The comparison of LC and RC filtering effects must consider whether the selection of the frequency band and inductance value to be filtered is appropriate. Because the inductance of the inductor is related to the 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 resistor itself consumes energy, is inefficient, and pays attention to the power that the selected resistor can withstand.

24. What is the method of selecting the inductor and the capacitor value during filtering?

In addition to considering the noise frequency that you want to filter out, you should consider the response capability of the instantaneous current. If the output of the LC has a chance to output a large current instantaneously, the large value of the inductor will hinder the speed at which this large current flows through the inductor and increase the ripple noise. The value of the capacitor is related to the magnitude of the ripple noise specification that can be tolerated. The smaller the ripple noise value is, the larger the capacitance value will be. The ESR/ESL of the capacitor also has an effect. In addition, if the LC is placed at the output of the switching regulation power, pay attention to the effect of the pole/zero generated by the LC on the loop stability of the negative feedback control. .

25. How to achieve EMC requirements as much as possible without causing too much cost pressure?

The increased cost of PCBs due to EMC is usually due to increased number of formations to enhance shielding and increase the suppression of high frequency harmonics such as ferritebead and choke. In addition, it is usually necessary to match the shielding structure of other mechanisms to make the whole system pass the EMC requirements. The following is only a few of the PCB design techniques to provide electromagnetic radiation effects that reduce the circuit.

25.1. Use devices with slower slew rate as much as possible to reduce the high frequency components generated by the signal.

25.2. Pay attention to the position of the high-frequency device, not too close to the external connector.

25.3. Pay attention to the impedance matching of the high-speed signal, the trace layer and its return current path to reduce the high-frequency reflection and radiation.

25.4. Place enough decoupling capacitors on the power pins of each device to mitigate noise on the power plane and ground plane. Pay particular attention to whether the frequency response and temperature characteristics of the capacitor meet the design requirements.

25.5. The ground near the external connector can be properly segmented with the ground plane and the ground of the connector is connected to the chassis ground.

25.6. The appropriate use of ground guard / shunt traces next to some special high-speed signals. But pay attention to the effect of guard/shunttraces on the trace characteristic impedance.

25.7. The power supply layer is 20H smaller than the ground layer, and H is the distance between the power supply layer and the ground layer.

Tag: PCB design pcb design high-speed digital circuits
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