Microwave Tech - High-Frequency PCB Circuit Design of Common Problems(5)

With the rapid development of electronic technology and the wide application of wireless communication technology in various fields, high frequency, high speed, and high density have gradually become one of the significant development trends of modern electronic products. High-frequency and high-speed digitization of signal transmission force PCB to micro-hole and buried/blind hole, fine conductor, medium layer uniform thin, high-frequency, high-density multilayer PCB design technology have become an important research field. Based on years of experience in hardware design, the author summarizes some design skills and matters needing the attention of High-Frequency PCB Circuit for your reference.

51. The design of a system containing DSP, PLD, which aspects to consider ESD?

For general systems, the main consideration should be given to the parts directly in contact with the human body, and appropriate protection should be carried out on the circuit and the mechanism. How much ESD will affect the system depends on the situation. In a dry environment, the ESD phenomenon will be more serious, and the impact of ESD will be relatively obvious for more sensitive and delicate systems. Although the impact of ESD may not be obvious in large systems, it is important to be careful in the design to prevent this from happening.

52. Safety issues: what is the specific meaning of FCC and EMC?

FCC: Federal Communication Commission

EMC: Electro Magnetic compatibility

FCC is a standards organization and EMC is a standard. Standards are issued for their respective reasons, standards, and test methods.

53. What is differential wiring?

Differential signals, some of which are also called differential signals, use two identical, opposite-polar signals to transmit data in one path and make decisions based on the level difference between the two signals. To ensure that the two signals are completely consistent, the wiring should be kept parallel, and the line width and line spacing should remain unchanged.

54. What is the PCB simulation software?

There are many kinds of simulation, high-speed digital circuit signal integrity analysis simulation analysis (SI) commonly used software icX, SignalVision, Hyperlynx, XTK, SpeectraQuest, and so on. Some also use Hspice.

55. How does THE PCB simulation software simulate the LAYOUT?

In the high-speed digital circuit, to improve the signal quality and reduce the wiring difficulty, the general use of multilayer board, the distribution of special power layer, stratum.

56. How to ensure the stability of signals over 50M in the layout and wiring?

The key of high-speed digital signal wiring is to reduce the influence of transmission lines on signal quality. Therefore, the signal wiring 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 time delay between signal rises. Moreover, different kinds of signals (such as TTL, GTL, LVTTL) have different methods to ensure signal quality.

57. The RF part, if part, and even the low-frequency circuit that monitors the outdoor unit is often deployed on the same PCB. What are the material requirements for such A PCB? How to prevent rf, IF, and even low-frequency circuits from interfering with each other?

Hybrid circuit design is a big problem. It's hard to have a perfect solution.

In general, the RF circuit is arranged and routed as an independent board in the system and even has a special shielding cavity. And the rf circuit is generally a single or double panel, the circuit is relatively simple, all these are to reduce the impact on the rf circuit distribution parameters, improve the consistency of the RF system. Compared with the general FR4 material, the RF circuit board tends to use a high Q value of the substrate, the dielectric constant of this material is relatively small, transmission line distribution capacitance is small, high impedance, signal transmission delay is small. In the hybrid circuit design, although the rf and digital circuits are done on the same PCB, they are generally divided into rf circuit area and digital circuit area, which are arranged and routed respectively. Between the grounding hole tape and shielding box shielding.

58. For the radio frequency part, if part and low-frequency circuit part deployed on the same PCB, Mentor has what solution?

Mentor board-level system design software, in addition to the basic circuit design functions, but also a dedicated RF design module. In the RF schematic design module, the parametric device model is provided, and the bidirectional interface with the RF circuit analysis and simulation tools such as EESOFT is provided. The RF LAYOUT module provides the pattern editing function specially used for the RF circuit LAYOUT and wiring and also has the bidirectional interface with the RF circuit analysis and simulation tools such as EESOFT so that the results after the analysis and simulation can be reverted to the schematic diagram and PCB. Also, Mentor's design management features enable easy design reuse, design derivation, and collaborative design. Greatly accelerate the process of hybrid circuit design. The mobile board is a typical hybrid circuit design, and many major handset designers and manufacturers are using Mentor and Jay's Eesoft as design platforms.

59. On a 12-layer PCB board, there are three power layers of 2.2V, 3.3V, and 5V. The three power sources are made in one layer respectively.

Generally speaking, the three power supplies are respectively in three layers, the signal quality is better. Because it is unlikely that the signal will split across the plane. Cross-segmentation is a key factor affecting signal quality, which is generally ignored by simulation software. For the power layer and the formation, the high-frequency signal is equivalent. In the real world, in addition to the signal quality, the power source plane coupling (using adjacent ground planes to reduce the AC impedance of the power source plane) and laminar symmetry are all factors that need to be considered.

60. How to check whether the PCB meets the design process requirements when it leaves the factory?

Many PCB manufacturers have to go through a power-on network on-off test before finishing PCB processing to ensure that all connections are correct. At the same time, more and more manufacturers are also using X-ray testing to check for etching or laminating some faults. For finished boards after patch processing, ICT test inspection is generally adopted, which requires adding ICT test points during PCB design. If there is a problem, a special X-ray inspection device can also be used to determine whether the processing caused the fault.

61. Do we need to consider the ESD problem of the chip itself when selecting the chip?

Whether double or multi-layer board should be as large as possible area. When selecting a chip, consider the ESD characteristics of the chip itself, which are generally mentioned in the chip specification, and the performance of the same chip can vary from manufacturer to manufacturer. Pay more attention to the design, consider a comprehensive point, make the performance of the circuit board will be guaranteed. However, ESD problems can still occur, so the protection of the organization is also very important to ESD protection.

62. When making PCB boards, to reduce interference, should the ground wire constitute a closed and form?

When making a PCB board, generally speaking, to reduce the area of the circuit, to reduce interference, when the grounding wire, should not be cloth into a closed form, but cloth into a dendritic better, there is to increase the area as far as possible.

63. If the emulator uses one power supply and the PCB board uses one power supply, should the ground of the two power supplies be connected?

If you can use the separation of power supply is of course better, because it is not easy to generate interference between the power supply, but most of the equipment has specific requirements. Since the emulator and PCB board use two power sources, in my opinion, they should not be shared.

64. A circuit consists of several PCB board, whether they should be common?

A circuit is made up of several PCBs, mostly requiring common ground, because it is not practical to use several power sources in a circuit. But if you have specific conditions, you can use a different power source and of course, it will be less intrusive.

65. Design a handheld product with LCD and metal shell. When testing ESD, ice-1000-4-2 cannot pass the test. CONTACT can only pass 1100V and AIR can pass 6000V. The ESD coupling test can only pass the 3000V horizontal test and 4000V vertical test. The CPU frequency is 33MHZ. Is there any way to pass the ESD test?

Handheld products are metal shells, ESD problems must be more obvious, LCD is also afraid that there will be a more bad phenomenon. If there is no way to change the existing metal material, it is suggested to add anti-electricity material inside the mechanism to strengthen the High-Frequency PCB ground, and at the same time to find a way to make the LCD ground. Of course, how to do it depends on the situation.