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PCB design tips 100, teach you to master PCB design easily (56-65)
Posted:02:24 PM June 20, 2018 writer: G

The key to high-speed digital signal routing is to reduce the impact of transmission lines on signal quality. Therefore, the signal traces should be as short as possible when the high-speed signal layout is over 100M.

In digital circuits, high-speed signals are defined by the signal rise time. And, different kinds of signals (such as TTL, GTL, LVTTL), the method to ensure signal quality is different.

57. The RF section of the outdoor unit, the IF section, and even the low-frequency circuit section that monitors the outdoor unit are often deployed on the same PCB. What are the requirements for such a PCB on the material? How to prevent interference between radio frequency, intermediate frequency and even low-frequency circuits?

Mixed circuit design is a big problem. It is difficult to have a perfect solution.

Generally, RF circuits are arranged and wired as a single board in the system, and even there will be special shielded cavities. Moreover, RF circuits are generally single-sided or double-sided, and the circuit is relatively simple. All these are to reduce the influence on the distribution parameters of the RF circuit and improve the consistency of the RF system. Compared with the general FR4 material, the RF circuit board tends to use substrates with high Q values. The material has a relatively small dielectric constant, a small distributed capacitance of the transmission line, high impedance, and a small signal transmission delay.

In the hybrid circuit design, although the RF circuit and the digital circuit are on the same PCB, they are generally divided into the RF circuit area and the digital circuit area, respectively. Use a grounded via strip and shield box to shield.

58. For the RF part, the IF part and the low-frequency circuit part are deployed on the same PCB. What solution does the mentor have?

Mentor's board level system design software, in addition to basic circuit design functions, also has a dedicated RF design module. In the RF schematic design module, a parameterized device model is provided, and a bidirectional interface with EESOFT and other RF circuit analysis and simulation tools is provided; in the RF LAYOUT module, pattern editing functions dedicated to RF circuit placement and routing are provided. The bi-directional interface of the RF circuit analysis and simulation tool such as EESOFT can be used to back-analog the schematic and PCB for analyzing the simulation results. At the same time, the use of Mentor software's design management functions can facilitate design reuse, design derivation, and collaborative design. It greatly accelerates the hybrid circuit design process.

The mobile phone board is a typical hybrid circuit design. Many large mobile phone design and manufacture companies use Mentor's eesoft as their design platform.

59. How does Mentor's PCB design software handle the differential team?

After the Mentor software defines the properties of the differential pair, the two differential pairs can be routed together, strictly ensuring the differential pair line width, spacing and length difference, and can be automatically separated when encountering obstacles. The via mode can be selected when changing layers.

60. On a 12-layer PCb board, there are three power layers 2.2v, 3.3v, 5v, and each of the three power supplies is on a layer. How to handle the ground?

In general, the three power supplies are each on the third floor, and the signal quality is better. Because it is unlikely that the signal splits across the plane layer. Cross-segmentation is a key factor affecting signal quality, and simulation software generally ignores it. For power layers and ground layers, it is equivalent to high-frequency signals. In practice, in addition to considering the signal quality, the power plane coupling (using the adjacent ground plane to reduce the AC impedance of the power plane) and stacking symmetry are all factors that need to be considered.

61. How to check the PCB at the factory to meet the design process requirements?

Many PCB manufacturers must pass power-on network continuity test before PCB processing is completed to ensure that all wiring is correct. At the same time, more and more manufacturers also use x-ray testing to check for some failures during etching or lamination. For the finished board after patch processing, ICT test and inspection are generally used, which requires adding ICT test points during PCB design. If there is a problem, it can also be caused by a special X-ray inspection device to eliminate the cause of the failure.

62. Is the “protection of the organization” a protection of the enclosure?

Yes. The chassis should be as tight as possible, with little or no conductive material, and grounded as much as possible.

63. Does the esd problem of the chip itself need to be considered when the chip is selected?

Whether it is a double deck or a multilayer deck, the area should be increased as much as possible. When selecting a chip, the ESD characteristics of the chip itself must be considered. These are generally mentioned in the chip description, and even if the same chip of different manufacturers performs differently. Careful design, consider the overall point, make the performance of the circuit board will also be a certain guarantee. However, ESD problems may still occur, so the protection of the organization against ESD is also very important.

64. When doing pcb board, in order to reduce interference, should the ground line form a closed form?

When doing PCB board, generally speaking, the circuit area should be reduced in order to reduce interference. When grounding, it should not be placed in a closed form. Instead, it should be clothed in a dendritic shape, and it should be increased as much as possible. The area of the earth.

65. If the emulator uses a power supply and the pcb board uses a power supply, should the two power supplies be connected together?

If it is possible to use separate power supplies, of course, it is better because the interference between the power supplies is less likely to occur, but most equipments have specific requirements. Since the simulator and the PCB board use two power supplies, it is not my idea to share them together.

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