66 Common Problems in PCB High Frequency Board Design Part VI

PCB manufacturers, PCB designers and PCBA manufacturers explain 66 common problems in PCB high-frequency board design Part VI

51. In the system where digital and analog coexist, there are two processing methods. One is to separate digital and analog. For example, in the stratum, digital and analog are independent, and single points are connected by copper sheet or FB magnetic beads, while the power supply is not separated; The other is that the analog power supply and digital power supply are connected by FB separately, and the ground is unified. Are the two methods equally effective?

It should be said that it is the same in principle. Because the power and ground to high frequency (PCB high frequency board) signals are equivalent.

The purpose of distinguishing analog and digital parts is to resist interference, mainly the interference of digital circuits to analog circuits. However, segmentation may cause incomplete signal return path, affect the signal quality of digital signals, and affect the EMC quality of the system. Therefore, no matter which plane is divided, it depends on whether the signal return path is increased and how much the return signal interferes with the normal working signal. Now there are also some mixed designs, regardless of power supply and ground. During the layout, the digital part and analog part are laid out separately to avoid cross zone signals.

52. Safety regulation: What is the specific meaning of FCC and EMC?

FCC: federal communication commission EMC: electromagnetic compatibility FCC is a standard organization and EMC is a standard. There are corresponding reasons, standards and test methods for the promulgation of standards.

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53. What is the difference distribution line?

Differential signals, some also called differential signals, use two identical signals with opposite polarity to transmit one channel of data, and make decisions based on the level difference between the two signals. In order to ensure that the two signals are completely consistent, the wiring shall be kept parallel, and the line width and line spacing shall remain unchanged.

54. What are the PCB high-frequency board simulation software?

There are many kinds of simulations. Common software for signal integrity analysis, simulation and analysis (SI) of high-speed digital circuits include icx, signalvision, hyperlynx, XTK, specraquest, etc. Some also use Hspice.

55. How does the PCB high-frequency board simulation software perform the LAYOUT simulation?

In high-speed digital circuits, in order to improve signal quality and reduce wiring difficulty, multilayer circuit boards are generally used to allocate special power layers and layers.

56. How to deal with the layout and wiring to ensure the stability of signals above 50M?

The key of high-speed digital signal wiring is to reduce the influence of transmission line on signal quality. Therefore, the signal routing shall be as short as possible for high-speed signal layout above 100M. In digital circuit, high-speed signal is defined by signal rise delay time. Moreover, different kinds of signals (such as TTL, GTL, LVTTL) have different methods to ensure signal quality.

57. The RF part, IF part of the outdoor unit, and even the low-frequency circuit part for monitoring the outdoor unit are often deployed on the same PCB high-frequency board. What are the material requirements for such PCB high-frequency board? How to prevent interference between RF, IF and LF circuits?

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

Generally, RF circuits are laid and routed as an independent board in the system, and even have a special shielding chamber. The RF circuit is generally single panel or double-sided circuit board, and the circuit is relatively simple. All these are to reduce the impact on the distribution parameters of the RF circuit and improve the consistency of the RF system. Compared with the general FR4 material, RF circuit boards tend to use the substrate with high Q value. This material has smaller dielectric constant, smaller distributed capacitance of transmission line, high impedance and small signal transmission delay. In the hybrid circuit design, although RF and digital circuits are built on the same PCB, they are generally divided into RF circuit area and digital circuit area for layout and wiring. The grounding through hole tape and shielding box are used for shielding.

58. Do you also need to consider the esd of the chip itself when selecting chips?

Whether it is double-sided PCB board or multi-layer circuit board, the ground area should be increased as much as possible. The ESD characteristics of the chip itself should be considered when selecting the chip. These are generally mentioned in the chip description, and even the performance of the same chip from different manufacturers will be different. When designing, pay more attention and consider more comprehensively. The performance of the circuit board will also be guaranteed. However, ESD problems may still occur, so the protection of institutions is also very important for ESD protection.

59. On a 12 layer PCB high-frequency board, there are three power supply layers of 2.2v, 3.3v and 5v. How to deal with the ground wire when the three power supplies are placed on each layer?

Generally speaking, the three power supplies are located on the third floor, which is good for signal quality. Because it is unlikely that the signal will be segmented across the plane layer. Cross segmentation is a key factor affecting signal quality, which is generally ignored by simulation software. For power supply layer and stratum, it is equivalent to high-frequency (PCB high-frequency board) signal. In practice, in addition to considering the signal quality, the power plane coupling (using adjacent ground planes to reduce the AC impedance of the power plane) and the symmetry of the layers are all factors that need to be considered.

60. How to check whether the PCB high-frequency board meets the design process requirements when leaving the factory?

Many high-frequency board manufacturers must pass the power on and off test of the network before the PCB high-frequency board is processed and delivered, to ensure that all connections are correct. At the same time, more and more manufacturers also use x-ray testing to check some faults during etching or lamination. For the finished boards after chip processing, ICT test is generally used for inspection, which requires adding ICT test points when designing PCB high-frequency boards.

In case of any problem, a special X-ray inspection equipment can also be used to eliminate whether the fault is caused by processing.

61. If the simulator uses one power supply and the PCB high-frequency board uses one power supply, should the ground of the two power supplies be connected together?

It is better to use separate power supply, because it is not easy to generate interference between power supplies, but most equipment has specific requirements. Since the emulator and the PCB high-frequency board use two power supplies, in my opinion, they should not be co grounded.

62. When making a PCB high-frequency board, should the ground wire form a closed form to reduce interference?

When making a PCB high-frequency board, generally speaking, the circuit area should be reduced to reduce interference. When laying ground wires, they should not be laid in a closed form, but preferably in a tree shape. In addition, the area of the ground should be increased as much as possible.

63. For the RF (PCB high-frequency board) part, the IF part and the low-frequency circuit part are deployed on the same PCB high-frequency board. What solution does mentor have?

In addition to the basic circuit design functions, Mentor's board level system design software also has special RF design modules. In the RF schematic design module, parametric device models are provided, and two-way interfaces with RF circuit analysis and simulation tools such as EESOFT are provided; In the RFLAYOUT module, the pattern editing function specially used for the layout and routing of RF circuits is provided. It also has two-way interfaces with RF circuit analysis and simulation tools such as EESOFT. The analysis and simulation results can be back labeled to the schematic diagram and PCB high-frequency board. At the same time, design reuse, design derivation, and collaborative design can be easily realized by using the design management function of Mentor software. Greatly accelerate the process of hybrid circuit design. Mobile phone board is a typical hybrid circuit design. Many large mobile phone design manufacturers use Mentor plus Jason's eesoft as their design platform.

64. A circuit consists of several high frequency PCB boards. Should they share the ground?

A circuit is composed of several high frequency PCB boards, most of which require common grounding, because it is not practical to use several power supplies in a circuit after all. But if you have specific conditions, you can use different power supplies, of course, the interference will be smaller.

65. Design a hand-held product with LCD and metal shell. When testing ESD, the test of ICE-1000-4-2 cannot be passed. CONTACT can only pass 1100V, and AIR can pass 6000V. During ESD coupling test, only 3000V can be passed horizontally and 4000V can be passed vertically. The main frequency of CPU is 33MHz. What methods can pass the ESD test?

The hand-held product is also a metal shell, so the ESD problem must be obvious, and LCD may also have many undesirable phenomena. If there is no way to change the existing metal materials, it is recommended to add anti electric materials inside the mechanism to strengthen the grounding of PCB, and at the same time, find a way to ground the LCD. Of course, how to operate depends on the specific situation.

66. To design a system with DSP and PLD, what aspects should ESD be considered?

For general systems, the parts directly contacted by human body shall be mainly considered, and proper protection shall be provided on the circuit and mechanism. As for the impact of ESD on the system, it depends on different situations. In a dry environment, the ESD phenomenon will be more serious, and the impact of ESD will be relatively obvious for more sensitive and fine systems. Although the ESD effect of large systems is not obvious sometimes, more attention should be paid to the design to prevent problems as far as possible. PCB manufacturers, PCB designers and PCBA manufacturers will explain 66 common problems in PCB high-frequency board design Part VI.