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PCB design tips 100, teach you to master PCB design easily (66-75)
Posted:02:37 PM June 20, 2018 writer: G

A circuit consists of several PCBs, most of which require a common ground because it is not practical to use several power supplies in a circuit. But if you have specific conditions, you can of course use different power sources and the interference will be less.


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

Hand-held products are also metal shells, ESD problems must be more obvious, LCD may also have more bad things. If there is no way to change the existing metal material, it is recommended to add anti-electric materials inside the mechanism to strengthen the ground of the PCB, and at the same time think of ways to ground the LCD. Of course, how to operate depends on the specific situation.


68. Design a system with DSP, PLD, considering ESD from those aspects?

As far as the general system is concerned, it is mainly necessary to consider the part of the human body that is in direct contact with it, and perform appropriate protection on the circuit and on the organization. As for the impact of ESD on the system, it depends on the circumstances. In the dry environment, the ESD phenomenon will be more serious. Compared with sensitive and delicate systems, the impact of ESD will be relatively obvious. Although large systems may not have obvious ESD effects, they must be designed with more care and try to prevent them from occurring.


69. PCB design, how to avoid crosstalk?

A changing signal (such as a step signal) propagates along the transmission line from A to B, and a coupling signal is generated on the transmission line CD. Once the changed signal is returned to the stable DC level, the coupled signal does not exist. Therefore, crosstalk occurs. It only occurs during signal transitions, and the faster the signal edge changes (switching rate), the greater the crosstalk generated. The coupled electromagnetic field in space can be extracted as a set of numerous coupling capacitors and coupled inductors. The crosstalk signals generated by the coupling capacitors can be divided into forward crosstalk and reverse crosstalk Sc on the victim network. The two signals have the same polarity; The crosstalk signal generated by the inductance is also divided into forward crosstalk and reverse crosstalk SL, and the two signals have opposite polarities. The forward crosstalk and reverse crosstalk generated by the coupled LC capacitor are present at the same time and are almost equal in size. In this way, the forward crosstalk signals on the victim network cancel each other due to the opposite polarity, and the polarity of the reverse crosstalk is the same. The modes of crosstalk analysis usually include the default mode, three-state mode and worst-case mode analysis. The default mode is similar to our actual cross-talk test method. That is, the infringing network driver is driven by the inversion signal. The victim network driver maintains the initial state (high or low) and then calculates the crosstalk value. This method is more effective for crosstalk analysis of unidirectional signals. The tri-state mode means that the infringing network driver is driven by the inversion signal, and the tri-state terminal of the victim network is set to a high-impedance state to detect the crosstalk size. This approach is more effective for two-way or complex topology networks. Worst-case analysis refers to maintaining the victim network driver's initial state, and the simulator calculates the sum of all default cross-talk networks for each victim network. This method generally only analyzes individual critical networks, because the number of combinations to be calculated is too slow and the simulation speed is slow.


70. Is there any provision for the copper area of the conduction band, that is, the ground plane of the microstrip line?

For microwave circuit design, the area of the ground plane has an effect on the parameters of the transmission line. The specific algorithm is more complex (please refer to the EESOFT related information of Angelon). In the simulation of the transmission line of a typical PCB digital circuit, the ground plane area has no influence on the transmission line parameters, or ignores the influence.


71. In the EMC test, it was found that the harmonics of the clock signal exceeded the standard, and only the decoupling capacitor was connected to the power pin. What aspects need attention in PCB design to suppress electromagnetic radiation?

The three elements of EMC are radiation sources, transmission routes and victims. The route of transmission is divided into spatial radiation propagation and cable conduction. So to suppress harmonics, first look at the way it spreads. Power supply decoupling is the solution to conduction propagation. In addition, necessary matching and shielding are also needed.


72. Why do some products that use 4-layer designs are double-sided and some are not?

There are several aspects of the role of paving: 1, shielding; 2, heat dissipation; 3, reinforcement; 4, PCB processing needs. So no matter how many floors are laid, first of all, we must look at the main reasons for it. Here we mainly discuss high-speed issues, so we mainly talk about shielding. Surface coating is good for EMC, but copper plating should be as complete as possible to avoid silos. In general, if there are many wirings on the surface devices, it is difficult to ensure the integrity of the copper foils, and the internal signal cross-segmentation problem is also caused. Therefore, it is recommended that the surface devices or boards with many traces be not copper-plated.


73. For a group of busses (addresses, data, commands) to drive multiple (up to 4, 5) devices (FLASH, SDRAM, other peripherals...), which way is used for PCB layout?

The effect of the routing topology on signal integrity is mainly reflected in the fact that the signal arrival time is inconsistent at each node, and the reflected signal arrives at the same node at the same time, which results in deterioration of the signal quality. In general, the star topology can achieve better signal quality by controlling the number of stubs of the same length so that signal transmission and reflection delay are consistent. In the use of topology, we must consider the signal topology node conditions, the actual working principle and wiring difficulty. Different buffers have different reflection effects on the signal. Therefore, the star topology does not solve the above problem that the data address bus is connected to flash and sdram, and the signal quality cannot be ensured. On the other hand, high-speed signals are generally Communication between dsp and sdram, the flash loading rate is not high, so in the high-speed simulation, as long as the actual high-speed signal to ensure effective operation of the node at the waveform, and do not have to pay attention to the waveform at the flash; star topology compared to the daisy chain topology It is difficult to route wiring, especially when a large number of data address signals use a star topology. The figure shows simulated waveforms using Hyperlynx emulated data signals at DDR-DSP-FLASH topology connection and DDR-FLASH-DSP connection at 150 MHz. It can be seen that in the second case, the signal quality at the DSP is better, and the waveform at the FLASH is worse, while the waveforms at the DSP and DDR are actually at work.


74. The frequency of 30M or more PCB, wiring using automatic routing or manual wiring; wiring software functions are the same?

Whether the high-speed signal is based on the rising edge of the signal rather than the absolute frequency or speed. Automatic or manual cabling depends on the support of software cabling functions. Some cabling may be better than automatic cabling. However, some cabling, such as inspection of distribution lines, bus delay compensation cabling, and automatic cabling, are far more effective and efficient than manual cabling. General PCB substrate is mainly composed of a mixture of resin and glass fiber cloth. Due to different proportions, the dielectric constant and thickness are different. Generally, the higher the resin content, the smaller the dielectric constant can be thinner. Specific parameters, you can consult the PCB manufacturer. In addition, with the advent of new processes, there are some special materials for PCB boards such as ultra-thin backplanes or low-loss RF boards.

 

75. In PCB design, ground lines are usually divided into protection grounds and signal grounds; power grounds are divided into digital grounds and analog grounds. Why should the ground wire be divided?

The purpose of the division is mainly due to EMC considerations. It is feared that the digital power supply and noise on the ground will interfere with other signals, especially analog signals through the conduction path. The division of signal and protection grounds is due to EMC ESD static discharge considerations, similar to the role of lightning rods in our lives. No matter what, there is only one final earth. It is just different ways of noise emission.


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