CadenceAllegro is now almost the actual industry standard in high-speed board design. The latest version is Allegro16.5 released in May 2011. Combined with its front-end product Capture, it can complete high-speed, high-density and multi-layer complex PCB design and wiring. Allegro has the advantages of easy operation, friendly interface and powerful functions (such as simulation, signal integrity simulation and power integrity simulation can be done. ), good integration and many other advantages, and it firmly occupies a dominant position in making high-speed pcb boards. In this world, 60% of computer motherboards and 40% of mobile phone motherboards are drawn by Allegro, which is widely used in the communication field and PC industry. It is known as the popular among high-end PCB tools.
1. What problems should be paid attention to when wiring high-frequency signals?
Answer: 1. Impedance matching of signal lines; 2. Space isolation from other signal lines; 3. For digital high-frequency signals, the effect of differential line will be better.
2. When laying the board, if the lines are dense, there may be more holes, which will of course affect the electrical performance of the board. How can we improve the electrical performance of the board?
Answer: For low-frequency signals, it doesn’t matter if there are vias. For high-frequency signals, minimize the number of vias. Multi-layer board can be considered if there are many lines.
3. Is it true that the more decoupling capacitors are added to the board, the better?
Answer: Decoupling capacitors need to be added with appropriate values at appropriate positions. For example, add it at the power supply port of your analog device, and you need to use different capacitance values to filter out stray signals with different frequencies.
4. What is the standard of a good board?
Answer: Reasonable layout, sufficient power redundancy of power lines, concise high-frequency impedance and low-frequency wiring.
5. How much does the through hole and blind hole affect the signal difference? What is the principle of application?
Answer: Using blind holes or buried holes is an effective method to increase the density of multilayer boards, reduce the number of layers and board size, and greatly reduce the number of plated through holes. However, in comparison, through holes are well realized in technology and low in cost, so through holes are generally used in design.
6. When it comes to the analog-digital hybrid system, some people suggest that the electrical layer should be divided, and the ground plane should be coated with copper. Others suggest that the electrical layer should be divided, and the different ground points should be connected at the end of the power source. However, the return path of the signal will be far away. How to choose the appropriate method in specific application?
Answer: If you have a high frequency > 20MHz signal line, and the length and number are relatively large, then you need at least two layers for this analog high frequency signal. One layer of signal line, one layer of large area ground, and the signal line layer needs to make enough vias to the ground. The purpose of this is:
1. For analog signals, this provides a complete transmission medium and impedance matching;
2. The ground plane isolates analog signals from other digital signals;
3. The ground circuit is small enough, because you have made many vias, and the ground is a large plane.
7. In the circuit board, the signal input plug-in is on the leftmost edge of PCB and the MCU is on the right. So, in layout, should the stabilized power supply chip be placed near the connector (the output of 5V from the power supply IC goes through a long path to the MCU), or should the power supply IC be placed to the right in the middle (the output of 5V from the power supply IC is shorter to the MCU, but the input power supply line passes through a long section of PCB)? Or have a better layout?
A: First of all, is your so-called signal input plug-in an analog device? If it is an analog device, it is recommended that your power supply layout should not affect the signal integrity of the analog part as much as possible. Therefore, there are several points to consider:
(1) First of all, whether your regulated power supply chip is a relatively clean power supply with small ripple. For the power supply of the analog part, the requirements for power supply are relatively high;
(2) Whether the analog part and your MCU are a power supply. In the design of high-precision circuit, it is recommended to separate the power supply of the analog part from that of the digital part;
(3) The power supply to the digital part needs to consider minimizing the influence on the analog circuit part.
8. In the application of high-speed signal chain, there are analog ground and digital ground for multiple ASIC. Is it divided by ground or not divided by ground? What are the existing criteria? Which effect is better?
A: So far, there is no conclusion. In general, you can consult the manual of the chip. All manuals of ADI’s hybrid chips recommend a grounding scheme, some of which recommend common ground and some of which recommend isolated ground. It depends on the chip design.
9. When should we consider the equal length of lines? If you want to consider using an equal length line, what is the maximum difference between the lengths of two signal lines? How to calculate?
Answer: The idea of differential line calculation: If you transmit a sinusoidal signal, your length difference is equal to half of its transmission wavelength, and the phase difference is 180 degrees, then the two signals will completely cancel out. So the length difference at this time is the maximum. By analogy, the signal line difference must be less than this value.
10. Serpentine route in high speed is suitable for that situation? Are there any disadvantages? For example, for differential routing, two sets of signals are required to be orthogonal.
Answer: Serpentine route has different functions because of different applications:
(1) If the serpentine wiring appears in the computer board, it mainly plays a role of filtering inductance and impedance matching to improve the anti-interference ability of the circuit. Serpentine traces in computer motherboards are mainly used in some clock signals, such as PCI-Clk,AGPCIK,IDE,DIMM and other signal lines.
(2) In the ordinary PCB, besides the function of filtering inductance, it can also be used as inductance coil of radio antenna, etc. For example, 2.4G walkie-talkies are used as inductors.
(3) The wiring length of some signals must be strictly equal in length. The equal length of high-speed digital PCB board is to keep the delay difference of each signal within a range and ensure the validity of data read by the system in the same cycle (when the delay difference exceeds one clock cycle, the data of the next cycle will be misread). For example, there are 13 HUBLink in the INTELHUB architecture, which use the frequency of 233MHz. They must be strictly of equal length to eliminate the hidden trouble caused by time delay, and winding is the only solution.
Generally, it is required that the delay difference should not exceed 1/4 clock cycle, and the delay difference per unit length of line is also fixed. The delay is related to the line width, line length, copper thickness and board structure, but too long line will increase the distributed capacitance and inductance, which will reduce the signal quality. Therefore, the clock IC pins are generally terminated, but the serpentine traces do not function as inductors. On the contrary, inductance will phase shift the higher harmonics in the rising edge of the signal, resulting in the deterioration of the signal quality, so it is required that the pitch of the serpentine should be at least twice the line width. The smaller the rise time of the signal, the more susceptible it is to the influence of distributed capacitance and inductance.
(4) Serpentine traces act as LC filters with distributed parameters in some special circuits.
1. When designing PCB, how to consider EMC/EMI, and what specific aspects should be considered? What measures are taken?
Answer: EMI/EMC design must take into account the location of devices, the arrangement of PCB stack, the way of important connection, the selection of devices, etc. at the beginning of layout.
For example, the position of the clock generator should not be close to the external connector as far as possible, the high-speed signal should go to the inner layer as far as possible and pay attention to the characteristic impedance matching and the continuity of the reference layer to reduce the reflection, the slewrate of the signal pushed by the device should be as small as possible to reduce the high-frequency component, and when selecting the decoupling/bypass capacitor, pay attention to whether its frequency response meets the requirements to reduce the noise of the power layer.
In addition, pay attention to the return path of high-frequency signal current to make its loop area as small as possible (that is, loopimpedance as small as possible) to reduce radiation. The range of high-frequency noise can also be controlled by dividing the stratum.
Finally, properly select the chassisground between PCB and shell.
12. What should I pay attention to in the transmission line design of RF broadband circuit PCB? How to set the ground hole of the transmission line properly? Do you need to design the impedance matching yourself or cooperate with PCB manufacturers?
A: There are many factors to consider in this question. For example, various parameters of PCB materials, transmission line model finally established according to these parameters, device parameters, etc. Impedance matching is generally designed according to the data provided by the manufacturer.
13. When analog circuits and digital circuits coexist, for example, half of them are digital circuits of FPGA or MCU, and the other half are analog circuits of DAC and related amplifiers. There are many power supplies with various voltage values. If you encounter power supplies with voltage values that both digital and analog circuits need, can you use a common power supply? What are your skills in wiring and bead arrangement?
A: Generally, it is not recommended to use it like this, which will be complicated and difficult to debug.
14. When designing a high-speed multilayer PCB, what is the main basis for selecting the packaging of devices such as resistors and capacitors? Can you give some examples of commonly used packages?
Answer: 0402 is commonly used in mobile phones; 003 is commonly used for modules of general high-speed signals; It is based on the fact that the smaller the package, the smaller the parasitic parameters. Of course, the same package from different manufacturers has great differences in high-frequency performance. It is recommended that you use high-frequency special components in key positions.
15. Generally, in the design of double panels, do you take the signal line or the ground line first?
Answer: This should be considered comprehensively. Considering the layout first, consider the routing.
16. What are the most important issues to pay attention to when designing high-speed multilayer PCB? Can you explain the solution to the problem in detail?
Answer: The most important thing to pay attention to is your design, that is, how you divide the signal line, power line, ground and control line on each floor. The general principle is that analog signals and analog signals should be grounded at least in a separate layer. It is also recommended to use a separate layer of power supply.
17. Excuse me, when to use 2-ply board, 4-ply board and 6-ply board. Are there any strict technical restrictions (except for the size)? Is it the frequency of CPU or the frequency of data interaction with external devices?
Answer: The multi-layer board can provide a complete ground plane at first, and can provide more signal layers to facilitate wiring. For applications where CPU is going to control external storage devices, the frequency of interaction should be taken into consideration. If the frequency is high, the complete ground plane must be guaranteed, and in addition, it is best to keep the signal lines of equal length.
18. How to analyze the influence of PCB wiring on analog signal transmission, and how to distinguish whether the noise introduced during signal transmission is caused by wiring or by operational amplifier devices?
Answer: It’s difficult to distinguish this one. PCB wiring can only be used to minimize the extra noise caused by wiring.
19. For high-speed multilayer PCB, what is the appropriate line width setting of power line, ground line and signal line? What are the common settings? Can you give an example? For example, how to set it when the working frequency is 300Mhz?
Answer: 300MHz signal must be impedance simulated to calculate the line width and the distance between the line and the ground; The line width of the power line needs to be determined according to the current. Generally, the whole plane is used instead of the “line” when the signal PCB is mixed, so as to ensure the minimum resistance of the loop and a complete plane under the signal line.
20. What kind of layout can achieve the best heat dissipation effect?
Answer: There are three main sources of heat in PCB:
(1) the heat of electronic components;
(2) the heat of 2)PCB itself;
(3) Heat from other parts. Among these three heat sources, the heat generated by components is the largest, which is the main heat source, followed by the heat generated by PCB board. The heat from outside depends on the overall thermal design of the system, so it will not be considered for the time being. Then the purpose of thermal design is to take appropriate measures and methods to reduce the temperature of components and PCB, so that the system can work normally at a suitable temperature. It is mainly achieved by reducing heat generation and accelerating heat dissipation.
