PCB (Printed Circuit Board), the Chinese name is printed circuit board, also known as printed circuit board, is an important electronic component, a support for electronic components, and a carrier for electrical connections of electronic components. Because it is made using electronic printing, it is called a “printed” circuit board.
1. How to choose PCB board?
The choice of PCB board must strike a balance between meeting design requirements, mass production and cost. Design requirements contain both electrical and mechanical components. Usually this material issue is more important when designing very high-speed PCB boards (frequency greater than GHz).
For example, the FR-4 material that is commonly used today may not be suitable because the dielectric loss at a frequency of several GHz will have a great impact on signal attenuation. As far as electricity is concerned, it is necessary to pay attention to whether the dielectric constant (dielectric constant) and dielectric loss are suitable for the designed frequency.
2. How to avoid high frequency interference?
The basic idea of avoiding high-frequency interference is to minimize the interference of high-frequency signal electromagnetic fields, which is the so-called crosstalk (Crosstalk). You can increase the distance between the high-speed signal and the analog signal, or add ground guard/shunt traces next to the analog signal. Also pay attention to the noise interference of the digital ground to the analog ground.
3. In high-speed design, how to solve the signal integrity problem?
Signal integrity is basically a matter of impedance matching. The factors that affect impedance matching include the structure and output impedance of the signal source, the characteristic impedance of the trace, the characteristics of the load end, and the topology of the trace. The solution is to rely on termination and adjust the topology of the wiring.
4. How is the differential distribution method realized?
There are two points to pay attention to in the wiring of the differential pair. One is that the length of the two lines should be as long as possible. There are two parallel ways, one is that the two lines run on the same wiring layer (side-by-side), and the other is that the two lines run on the upper and lower adjacent layers (over-under). Generally, the former side-by-side (side by side, side by side) is used in many ways.
5. For a clock signal line with only one output terminal, how to implement differential wiring?
To use differential wiring, it is only meaningful that the signal source and receiver are both differential signals. So it is not possible to use differential wiring for a clock signal with only one output.
6. Can a matching resistor be added between the differential line pairs at the receiving end?
The matching resistance between the differential line pairs at the receiving end is usually added, and its value should be equal to the value of the differential impedance. This way the signal quality will be better.
7. Why should the wiring of differential pairs be close and parallel?
The routing of differential pairs should be properly close and parallel. The so-called proper proximity is because the distance will affect the value of differential impedance, which is an important parameter for designing a differential pair. The need for parallelism is also due to the need to maintain the consistency of the differential impedance. If the two lines are far or near, the differential impedance will be inconsistent, which will affect the signal integrity (signal integrity) and time delay (timing delay).
8. How to deal with some theoretical conflicts in actual wiring
Basically, it is right to separate the analog/digital ground. It should be noted that the signal traces should not cross the divided place (moat) as much as possible, and the return current path (returning current path) of the power supply and signal should not become too large.
The crystal oscillator is an analog positive feedback oscillation circuit. To have a stable oscillation signal, it must meet the specifications of loop gain and phase. However, the oscillation specification of this analog signal is easily disturbed, and even adding ground guard traces may not be able to completely isolate the interference. And if it is too far away, the noise on the ground plane will also affect the positive feedback oscillation circuit. Therefore, the distance between the crystal oscillator and the chip must be as close as possible.
Indeed, there are many conflicts between high-speed routing and EMI requirements. But the basic principle is that the resistors and capacitors or ferrite beads added due to EMI cannot cause some electrical characteristics of the signal to fail to meet the specifications. Therefore, it is best to use the techniques of arranging wiring and PCB stacking to solve or reduce EMI problems, such as routing high-speed signals to the inner layer. Finally, use resistor capacitor or ferrite bead to reduce the damage to the signal.
9. How to solve the contradiction between manual wiring and automatic wiring of high-speed signals?
Most of the automatic routers of the stronger routing software now have set constraints to control the routing method and the number of vias. The setting items of winding engine capabilities and constraint conditions of various EDA companies sometimes differ greatly.
For example, are there enough constraints to control the way the serpentine snakes, can the spacing of the differential pairs be controlled, and so on. This will affect whether the routing method obtained by automatic routing can meet the designer’s idea.
In addition, the difficulty of manually adjusting the wiring also has an absolute relationship with the ability of the winding engine. For example, the pushability of traces, the pushability of vias, and even the pushability of traces to copper, etc. Therefore, choosing a router with a strong winding engine capability is the solution.
10. About test coupons.
The test coupon is used to measure whether the characteristic impedance of the produced PCB meets the design requirements with TDR (Time Domain Reflectometer). Generally, the impedance to be controlled has two cases: a single line and a differential pair. Therefore, the line width and line spacing (when there are differential pairs) on the test coupon should be the same as the lines to be controlled.
The most important thing is the position of the ground point when measuring. In order to reduce the inductance value of the ground lead (ground lead), the place where the TDR probe (probe) is grounded is usually very close to the place where the signal is measured (probe tip). Therefore, the distance and method between the point where the signal is measured on the test coupon and the ground point To match the probe used
11. In high-speed PCB design, the blank area of the signal layer can be covered with copper, but how should the copper of multiple signal layers be distributed on grounding and power supply?
Generally, most of the copper in the blank area is grounded. Just pay attention to the distance between the copper and the signal line when depositing copper next to the high-speed signal line, because the deposited copper will reduce the characteristic impedance of the trace a little. Also be careful not to affect the characteristic impedance of other layers, such as in the structure of a dual strip line.
12. Is it possible to use the microstrip line model to calculate the characteristic impedance of the signal line above the power plane? Can the signal between power and ground plane be calculated using stripline model?
Yes, both the power plane and the ground plane must be considered as reference planes when calculating the characteristic impedance. For example, a four-layer board: top layer-power layer-ground layer-bottom layer. At this time, the model of the characteristic impedance of the top layer trace is the microstrip line model with the power plane as the reference plane.
13. In general, can automatic generation of test points by software on high-density printed boards meet the test requirements of mass production?
Whether the test points automatically generated by the general software meet the test requirements depends on whether the specifications for adding test points meet the requirements of the test equipment. In addition, if the wiring is too dense and the specification for adding test points is relatively strict, it may not be possible to automatically add test points to each segment of the line. Of course, it is necessary to manually fill in the places to be tested.
14. Will adding test points affect the quality of high-speed signals?
As for whether it will affect the signal quality, it depends on the way of adding test points and how fast the signal is. Basically, additional test points (not using the existing via or DIP pin as test points) may be added to the line or pulled out from the line. The former is equivalent to adding a small capacitor online, while the latter is an extra branch.
These two situations will affect the high-speed signal more or less, and the degree of influence is related to the frequency speed of the signal and the edge rate of the signal (edge rate). The size of the impact can be known through simulation. In principle, the smaller the test point, the better (of course, it must also meet the requirements of the test equipment). The shorter the branch, the better.
15. Several PCBs form a system, how should the ground wires between the boards be connected?
When the signal or power between the various PCB boards is connected to each other, for example, board A has power or signals sent to board B, there must be an equal amount of current flowing from the ground layer back to board A (this is Kirchoff current law).
The current on this formation will find the place of least resistance to flow back. Therefore, the number of pins assigned to the ground plane should not be too small at each interface, no matter whether it is a power supply or a signal, so as to reduce the impedance, which can reduce the noise on the ground plane.
In addition, it is also possible to analyze the entire current loop, especially the part with a large current, and adjust the connection method of the formation or ground wire to control the current flow (for example, create a low impedance somewhere, so that most of the current flows from this places), reduce the impact on other more sensitive signals.
16. Can you introduce some foreign technical books and data on high-speed PCB design?
Now high-speed digital circuits are used in related fields such as communication networks and calculators. In terms of communication networks, the operating frequency of the PCB board has reached GHz, and the number of stacked layers is as many as 40 layers as far as I know.
Calculator-related applications are also due to the advancement of chips. Whether it is a general PC or a server (Server), the maximum operating frequency on the board has also reached 400MHz (such as Rambus).
In response to the high-speed and high-density routing requirements, the demand for blind/buried vias, mircrovias and build-up process technology is gradually increasing. These design requirements are available for mass production by manufacturers.
17. Two frequently referenced characteristic impedance formulas:
Microstrip line (microstrip) Z={87/[sqrt(Er+1.41)]}ln[5.98H/(0.8W+T)] where W is the line width, T is the copper thickness of the trace, and H is The distance from the trace to the reference plane, Er is the dielectric constant of the PCB material (dielectric constant). This formula can only be applied when 0.1≤(W/H)≤2.0 and 1≤(Er)≤15.
Stripline (stripline) Z=[60/sqrt(Er)]ln{4H/[0.67π(T+0.8W)]} where, H is the distance between the two reference planes, and the trace is located in the middle of the two reference planes . This formula can only be applied when W/H≤0.35 and T/H≤0.25.
18. Can a ground wire be added in the middle of the differential signal line?
Generally, the ground wire cannot be added in the middle of the differential signal. Because the most important point of the application principle of differential signals is to take advantage of the benefits brought by mutual coupling (coupling) between differential signals, such as flux cancellation, noise immunity, etc. If a ground wire is added in the middle, the coupling effect will be destroyed.
19. Does rigid-flex board design require special design software and specifications?
The flexible printed circuit (FPC) can be designed with general PCB design software. Also use the Gerber format to produce for FPC manufacturers.
20. What is the principle of properly selecting the grounding point of the PCB and the case?
The principle of selecting the ground point of the PCB and the shell is to use the chassis ground to provide a low-impedance path for the return current (returning current) and control the path of the return current. For example, usually near the high-frequency device or the clock generator, the ground layer of the PCB can be connected with the chassis ground by fixing screws to minimize the area of the entire current loop, thereby reducing electromagnetic radiation.
21. What aspects should we start with for circuit board DEBUG?
As far as digital circuits are concerned, first determine three things in sequence:
1. Verify that all supply values are sized for the design. Some systems with multiple power supplies may require certain specifications for the order and speed of certain power supplies.
2. Verify that all clock signal frequencies are working properly and there are no non-monotonic issues on the signal edges.
3. Confirm whether the reset signal meets the specification requirements. If all these are normal, the chip should send out the signal of the first cycle (cycle). Next, debug according to the system operation principle and bus protocol.
22. When the size of the circuit board is fixed, if more functions need to be accommodated in the design, it is often necessary to increase the trace density of the PCB, but this may lead to enhanced mutual interference of the traces, and at the same time, the traces are too thin to increase the impedance. It can’t be lowered, please experts introduce the skills in high-speed (≥100MHz) high-density PCB design?
When designing high-speed and high-density PCBs, crosstalk interference should be paid special attention because it has a great impact on timing and signal integrity.
Here are a few things to pay attention to:
Control the continuity and matching of the trace characteristic impedance.
The size of the trace spacing. Generally, the spacing that is often seen is twice the line width. The impact of trace spacing on timing and signal integrity can be known through simulation, and the minimum tolerable spacing can be found. Results may vary from chip to chip.
Choose the appropriate termination method.
Avoid the same direction of the traces on the upper and lower adjacent layers, or even overlap the upper and lower traces, because this kind of crosstalk is greater than that of adjacent traces on the same layer.
Use blind/buried vias to increase the trace area. But the manufacturing cost of the PCB board will increase. It is indeed difficult to achieve complete parallelism and equal length in actual implementation, but it is still necessary to do it as much as possible.
In addition, differential termination and common-mode termination can be reserved to mitigate the impact on timing and signal integrity.
23. The filter at the analog power supply is often LC circuit. But why sometimes LC filters less effectively than RC?
The comparison of LC and RC filter effects must consider whether the frequency band to be filtered out and the selection of inductance value are appropriate. Because the inductive reactance (reactance) of the inductor is related to the inductance value and frequency.
If the noise frequency of the power supply is low and the inductance value is not large enough, the filtering effect may not be as good as RC. However, the price to pay for using RC filtering is that the resistor itself dissipates power, is less efficient, and pays attention to how much power the selected resistor can handle.
24. What is the method of selecting inductance and capacitance value when filtering?
In addition to the noise frequency you want to filter out, the selection of the inductance value also considers the response capability of the instantaneous current. If the output terminal of the LC has the opportunity to output a large current instantaneously, a too large inductance value will hinder the speed of the large current flowing through the inductor and increase ripple noise. The capacitance value is related to the size of the ripple noise specification value that can be tolerated.
The smaller the ripple noise value requirement, the larger the capacitor value. The ESR/ESL of the capacitor will also have an impact. In addition, if the LC is placed at the output of a switching regulation power, it is also necessary to pay attention to the influence of the pole/zero generated by the LC on the stability of the negative feedback control loop. .
25. How to meet the EMC requirements as much as possible without causing too much cost pressure?
The increased cost due to EMC on the PCB is usually due to the increase in the number of ground layers to enhance the shielding effect and the addition of ferrite bead, choke and other high-frequency harmonic suppression devices. In addition, it is usually necessary to cooperate with shielding structures on other mechanisms to make the entire system pass the EMC requirements. The following are just a few PCB board design tips to reduce the electromagnetic radiation effect generated by the circuit.
Choose a device with a slower slew rate as much as possible to reduce the high-frequency components generated by the signal.
Pay attention to the placement of high-frequency components, not too close to external connectors.
Pay attention to the impedance matching of high-speed signals, the wiring layer and its return current path (return current path) to reduce high-frequency reflection and radiation.
Place sufficient and appropriate decoupling capacitors at the power pins of each device to moderate noise on the power and ground planes. Pay special attention to whether the frequency response and temperature characteristics of the capacitor meet the design requirements.
The ground near the external connector can be properly separated from the formation, and the ground of the connector should be connected to the chassis ground nearby.
Appropriately use ground guard/shunt traces next to some particularly high-speed signals. But pay attention to the effect of guard/shunt traces on the characteristic impedance of the trace.
The power layer is 20H inward than the formation, and H is the distance between the power layer and the formation.
26. When there are multiple digital/analog function blocks in one PCB board, the common practice is to separate the digital/analog ground. What is the reason?
The reason for separating the digital/analog ground is because the digital circuit will generate noise on the power supply and ground when switching between high and low potentials. The magnitude of the noise is related to the speed of the signal and the magnitude of the current. If the ground plane is not divided and the noise generated by the circuit in the digital area is large and the circuit in the analog area is very close, then even if the digital and analog signals do not cross, the analog signal will still be interfered by the ground noise. That is to say, the method of not dividing the digital and analog grounds can only be used when the analog circuit area is far away from the digital circuit area that generates large noise.
27. Another approach is to ensure that the digital/analog separate layout and the digital/analog signal lines do not cross each other, the entire PCB board is not divided, and the digital/analog ground is connected to this ground plane. What’s the point?
The requirement that the digital-analog signal traces cannot cross is because the return current path (return current path) of the slightly faster digital signal will try to flow back to the source of the digital signal along the ground near the bottom of the trace. cross, the noise generated by the return current will appear in the analog circuit area.
28. How to consider the impedance matching problem when designing the schematic diagram of high-speed PCB design?
When designing high-speed PCB circuits, impedance matching is one of the design elements. The impedance value has an absolute relationship with the routing method, such as walking on the surface layer (microstrip) or inner layer (stripline/double stripline), the distance from the reference layer (power layer or ground layer), trace width, PCB material, etc. Both will affect the characteristic impedance value of the trace.
That is to say, the impedance value can only be determined after wiring. General simulation software will not be able to consider some wiring conditions with discontinuous impedance due to the limitation of the line model or the mathematical algorithm used. At this time, only some terminators (terminations), such as series resistors, can be reserved on the schematic diagram. to mitigate the effect of trace impedance discontinuities. The real fundamental solution to the problem is to try to avoid impedance discontinuity when wiring.
29. Where can I provide a more accurate IBIS model library?
The accuracy of the IBIS model directly affects the simulation results. Basically, IBIS can be regarded as the electrical characteristic data of the equivalent circuit of the actual chip I/O buffer, which can generally be obtained by converting the SPICE model, and the data of SPICE has an absolute relationship with the chip manufacturing, so the same device is provided by different chip manufacturers. The data in SPICE is different, and the data in the converted IBIS model will also be different accordingly.
That is to say, if the devices of manufacturer A are used, only they have the ability to provide accurate model data of their devices, because no one else knows better than them which process their devices are made of. If the IBIS provided by the manufacturer is inaccurate, the only solution is to continuously ask the manufacturer to improve.
30. When designing high-speed PCBs, from what aspects should designers consider the rules of EMC and EMI?
In general, EMI/EMC design needs to consider both radiated and conducted aspects. The former belongs to the higher frequency part (≥30MHz) and the latter belongs to the lower frequency part (≤30MHz).
So you can’t just pay attention to the high frequency and ignore the low frequency part. A good EMI/EMC design must take into account the position of the device, the arrangement of the PCB stack, the way of important connections, the selection of the device, etc. at the beginning of the layout. If there is no better arrangement in advance, it can be resolved afterwards It will get twice the result with half the effort and increase the cost.
For example, the position of the clock generator should not be close to the external connector as much as possible, the high-speed signal should go to the inner layer as far as possible and pay attention to the continuity of the characteristic impedance matching and the reference layer to reduce reflection, and the slope (slew rate) of the signal pushed by the device should be as small as possible to reduce the high When selecting a decoupling/bypass capacitor, pay attention to whether its frequency response meets the requirements to reduce power plane noise.
In addition, pay attention to the return path of the high-frequency signal current to make the loop area as small as possible (that is, the loop impedance is as small as possible) to reduce radiation. It is also possible to control the range of high-frequency noise by dividing the formation. Finally, properly select the grounding point of the PCB and the case (chassis ground).
31. How to choose EDA tools?
In the current pcb design software, thermal analysis is not a strong point, so it is not recommended to use it. For other functions 1.3.4, you can choose PADS or Cadence, and the performance and price ratio are good. Beginners in PLD design can use the integrated environment provided by PLD chip manufacturers, and single-point tools can be used when designing more than one million gates.
32. Please recommend an EDA software suitable for high-speed signal processing and transmission.
For conventional circuit design, INNOVEDA’s PADS is very good, and there are matching simulation software, and this type of design often accounts for 70% of the applications. For high-speed circuit design, analog and digital mixed circuits, the Cadence solution should be a software with better performance and price. Of course, the performance of Mentor is still very good, especially its design process management should be the best.
33. Explanation of the meaning of each layer of PCB board
Topoverlay —- the name of the top-level device, also called top silkscreen or top component legend, such as R1 C5,
IC10.bottomoverlay–similarly multilayer—–If you design a 4-layer board, you place a free pad or via, define it as multilay, then its pad will automatically appear on the 4 layers, if You only define it as top layer, then its pad will only appear on the top layer.
34. What aspects should be paid attention to in the design, routing and layout of high-frequency PCBs above 2G?
High-frequency PCBs above 2G belong to the design of radio frequency circuits, and are not within the scope of discussion of high-speed digital circuit design. The layout and routing of the RF circuit should be considered together with the schematic diagram, because layout and routing will cause distribution effects.
Moreover, some passive devices in RF circuit design are realized through parametric definition and special-shaped copper foil. Therefore, EDA tools are required to provide parametric devices and edit special-shaped copper foil.
Mentor’s boardstation has a dedicated RF design module that meets these requirements. Moreover, general radio frequency design requires special radio frequency circuit analysis tools, the most famous in the industry is agilent’s eesoft, which has a good interface with Mentor’s tools.
35. For high-frequency PCB design above 2G, what rules should the microstrip design follow?
For the design of RF microstrip lines, it is necessary to use 3D field analysis tools to extract transmission line parameters. All rules should be specified in this field extraction tool.
36. For a PCB with all digital signals, there is an 80MHz clock source on the board. In addition to using wire mesh (grounding), what kind of circuit should be used for protection in order to ensure sufficient driving capability?
To ensure the driving capability of the clock, it should not be realized through protection. Generally, the clock is used to drive the chip. The general concern about clock drive capability is caused by multiple clock loads. A clock driver chip is used to convert one clock signal into several, and a point-to-point connection is adopted. When selecting the driver chip, in addition to ensuring that it basically matches the load and the signal edge meets the requirements (generally, the clock is an edge-effective signal), when calculating the system timing, the delay of the clock in the driver chip must be taken into account.
37. If a separate clock signal board is used, what kind of interface is generally used to ensure that the transmission of the clock signal is less affected?
The shorter the clock signal, the smaller the transmission line effect. Using a separate clock signal board will increase the signal routing length. And the ground power supply of the board is also a problem. For long-distance transmission, it is recommended to use differential signals. L size can meet the drive capacity requirements, but your clock is not too fast, it is not necessary.
38, 27M, SDRAM clock line (80M-90M), the second and third harmonics of these clock lines are just in the VHF band, and the interference is very large after the high frequency enters from the receiving end. In addition to shortening the line length, what other good ways?
If the third harmonic is large and the second harmonic is small, it may be because the signal duty cycle is 50%, because in this case, the signal has no even harmonics. At this time, it is necessary to modify the signal duty cycle. In addition, if the clock signal is unidirectional, the source end series matching is generally used. This suppresses secondary reflections without affecting the clock edge rate. The matching value at the source end can be obtained by using the formula in the figure below.
39. What is the topology of the wiring?
Topology, some are also called routing order. For the wiring order of the multi-port connected network.
40. How to adjust the topology of the wiring to improve the integrity of the signal?
This kind of network signal direction is more complicated, because for one-way, two-way signals, and signals of different levels, the topology has different influences, and it is difficult to say which topology is beneficial to the signal quality. Moreover, when doing pre-simulation, which topology to use is very demanding for engineers, and requires an understanding of circuit principles, signal types, and even wiring difficulties.
41. How to reduce EMI problems by arranging stackup?
First of all, EMI should be considered from the system, and the PCB alone cannot solve the problem. For EMI, I think that stacking is mainly to provide the shortest signal return path, reduce the coupling area, and suppress differential mode interference. In addition, the ground layer and the power layer are tightly coupled, and the extension is appropriately larger than the power layer, which is good for suppressing common-mode interference.
42. Why is copper laid?
Generally, there are several reasons for laying copper.
1. EMC. For large-area ground or power supply copper, it will play a shielding role, and some special ones, such as PGND, will play a protective role.
2. PCB process requirements. Generally, in order to ensure the effect of electroplating or lamination without deformation, copper is laid on the PCB layer with less wiring.
3. Signal integrity requirements, give high-frequency digital signals a complete return path, and reduce the wiring of the DC network. Of course, there are also reasons for heat dissipation, special device installation requires copper laying, and so on.
43. In a system, dsp and pld are included, what problems should be paid attention to when wiring?
Look at the ratio of your signal rate to the length of the wiring. If the delay of the signal on the transmission line is comparable to the time of the signal change edge, the signal integrity problem should be considered. In addition, for multiple DSPs, clock and data signal routing topology will also affect signal quality and timing, which needs attention.
44. In addition to the protel tool wiring, are there other good tools?
As for tools, in addition to PROTEL, there are many wiring tools, such as MENTOR’s WG2000, EN2000 series and powerpcb, Cadence’s allegro, zuken’s cadstar, cr5000, etc., each with its own strengths.
45. What is the “signal return path”?
Signal return path, that is, return current. When a high-speed digital signal is transmitted, the signal flows from the driver along the PCB transmission line to the load, and then the load returns to the driver end along the ground or the power supply through the shortest path.
This return signal on the ground or power supply is called the signal return path. Dr.Johnson explained in his book that high-frequency signal transmission is actually a process of charging the dielectric capacitance sandwiched between the transmission line and the DC layer. What SI analyzes is the electromagnetic properties of this enclosure and the coupling between them.
46. How to conduct SI analysis on connectors?
In the IBIS3.2 specification, there is a description of the connector model. Generally use the EBD model. If it is a special board, such as a backplane, a SPICE model is required. You can also use multi-board simulation software (HYPERLYNX or IS_multiboard). When building a multi-board system, input the distribution parameters of the connectors, which are generally obtained from the connector manual. Of course, this method will not be accurate enough, but as long as it is within the acceptable range.
47. What are the methods of termination?
Termination (terminal), also known as matching. Generally, according to the matching position, it is divided into active end matching and terminal matching. Among them, source matching is generally resistor series matching, and terminal matching is generally parallel matching. There are many ways, including resistor pull-up, resistor pull-down, Thevenin matching, AC matching, and Schottky diode matching.
48. What factors determine the way of termination (matching)?
The matching method is generally determined by the BUFFER characteristics, topology conditions, level types and judgment methods, and the signal duty cycle and system power consumption should also be considered.
49. What are the rules for the way of termination (matching)?
The most critical issue in digital circuits is the timing problem. The purpose of adding matching is to improve the signal quality and obtain a determinable signal at the judgment moment. For level effective signals, the signal quality is stable under the premise of ensuring the establishment and holding time; for delayed effective signals, under the premise of ensuring the signal delay monotonicity, the signal change delay speed meets the requirements. There is some material on matching in the Mentor ICX product textbook.
In addition, “High Speed Digital design a hand book of blackmagic” has a chapter dedicated to the terminal, which describes the role of matching on signal integrity from the principle of electromagnetic waves, which can be used for reference.
50. Can I use the IBIS model of the device to simulate the logic function of the device? If not, how can board-level and system-level simulations of the circuit be performed?
IBIS models are behavioral level models and cannot be used for functional simulation. For functional simulation, SPICE models or other structural-level models are required.
51. In a system where digital and analog coexist, there are two processing methods. One is to separate the digital ground from the analog ground. Beads are connected, but the power supply is not separated; the other is that the analog power supply and digital power supply are separated and connected with FB, and the ground is a unified ground. I would like to ask Mr. Li, whether the effect of these two methods is the same?
It should be said that it is the same in principle. Because power and ground are equivalent to high-frequency signals.
The purpose of distinguishing between analog and digital parts is for anti-interference, mainly the interference of digital circuits to analog circuits. However, segmentation may result in an incomplete signal return path, affecting the signal quality of the digital signal and affecting the EMC quality of the system.
Therefore, no matter which plane is divided, it depends on whether the signal return path is enlarged 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, when laying out, separate the layout and wiring according to the digital part and the analog part to avoid cross-regional signals.
52. Safety regulations: What are the specific meanings of FCC and EMC?
FCC: federal communication commission American Communications Commission
EMC: electro magnetic compatibility electromagnetic compatibility
FCC is a standards organization, EMC is a standard. There are corresponding reasons, standards and test methods for the promulgation of standards.
53. What is differential distribution?
Differential signals, some of which are also called differential signals, use two identical, opposite-polarity signals to transmit one channel of data, and rely on the level difference of the two signals for judgment. In order to ensure that the two signals are completely consistent, they must be kept in parallel during wiring, and the line width and line spacing remain unchanged.
54. What are the PCB simulation software?
There are many types of simulation, high-speed digital circuit signal integrity analysis simulation analysis (SI) commonly used software are icx, signalvision, hyperlynx, XTK, spectraquest, etc. Some also use Hspice.
55. How does PCB simulation software perform LAYOUT simulation?
In high-speed digital circuits, in order to improve the signal quality and reduce the difficulty of wiring, multi-layer boards are generally used to assign special power layers and ground layers.
56. How to deal with layout and wiring to ensure the stability of signals above 50M
The key to high-speed digital signal wiring is to reduce the impact of transmission lines on signal quality. Therefore, the layout of high-speed signals above 100M requires that the signal traces be as short as possible. In digital circuits, high-speed signals are defined by signal rise delay time. Moreover, different types of signals (such as TTL, GTL, LVTTL) have different methods to ensure signal quality.
57. The RF part of the outdoor unit, the intermediate frequency part, and even the low-frequency circuit part that monitors the outdoor unit are often deployed on the same PCB. What are the requirements for the material of 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 is difficult to have a perfect solution.
Generally, the radio frequency circuit is laid out and wired as an independent single board in the system, and there is even a special shielding cavity. Moreover, the RF circuit is generally single-sided or double-sided, and the circuit is relatively simple, all of which 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 high-Q substrates. The dielectric constant of this material is relatively small, the distributed capacitance of the transmission line is small, the impedance is high, and the signal transmission delay is small. In 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, which are laid out and wired separately. Use ground vias and shielding boxes between them.
58. For the RF part, the intermediate frequency part and the low frequency circuit part are deployed on the same PCB, what solution does 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 RF circuit analysis and simulation tools such as EESOFT is provided; in the RF LAYOUT module, a pattern editing function specially used for RF circuit layout and wiring is provided, and there is also a The two-way interface of RF circuit analysis and simulation tools such as EESOFT can reverse-label the results of analysis and simulation back to the schematic diagram and PCB.
At the same time, using the design management function of Mentor software, design reuse, design derivation, and collaborative design can be easily realized. Greatly speed up the hybrid circuit design process. The mobile phone board is a typical mixed circuit design, and many large mobile phone design manufacturers use Mentor plus Angelon’s eesoft as the design platform.
59. What is the product structure of Mentor?
Mentor Graphics’ PCB tools include WG (formerly veribest) series and Enterprise (boardstation) series.
60. How does Mentor’s PCB design software support BGA, PGA, COB and other packages?
Mentor’s autoactive RE, developed from the acquisition of Veribest, is the industry’s first gridless, any-angle router. As we all know, for ball grid arrays, COB devices, gridless, and any-angle routers are the key to solving the routing rate. In the latest autoactive RE, functions such as pushing vias, copper foil, REROUTE, etc. have been added to make it more convenient to apply. In addition, he supports high-speed routing, including signal routing and differential pair routing with time delay requirements.
61. How does Mentor’s PCB design software handle differential line pairs?
After the Mentor software defines the properties of the differential pair, the two differential pairs can be routed together, and the line width, spacing and length of the differential pair are strictly guaranteed. They can be separated automatically when encountering obstacles, and the via method can be selected when changing layers.
62. On a 12-layer PCB board, there are three power supply layers 2.2v, 3.3v, 5v, and each of the three power supplies is on one layer. How to deal with the ground wire?
Generally speaking, the three power supplies are respectively arranged on the third floor, which is better for signal quality. Because it is unlikely that the signal will be split across plane layers. Cross-segmentation is a critical factor affecting signal quality that is generally ignored by simulation software. For power planes and ground planes, it is equivalent for high-frequency signals. In practice, in addition to considering the signal quality, 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.
63. 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 continuity test before the PCB processing is completed to ensure that all connections are correct. At the same time, more and more manufacturers are also using x-ray testing to check some faults during etching or lamination.
For the finished board after patch processing, ICT test inspection is generally used, 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 rule out whether the fault is caused by processing.
64. Is the “protection of the mechanism” the protection of the casing?
Yes. The casing should be as tight as possible, use less or no conductive materials, and be grounded as much as possible.
65. Is it necessary to consider the esd problem of the chip itself when selecting the chip?
Whether it is a double-layer board or a multi-layer board, the area of the ground should be increased as much as possible. When choosing a chip, the ESD characteristics of the chip itself should be considered. These are generally mentioned in the chip description, and even the performance of the same chip from different manufacturers will be different.
Pay more attention to the design and consider it more comprehensively, and the performance of the circuit board will be guaranteed to a certain extent. But the problem of ESD may still appear, so the protection of the organization is also very important for the protection of ESD.
66. When making a pcb board, in order to reduce interference, should the ground wire form a closed form?
When making PCB boards, generally speaking, it is necessary to reduce the area of the loop to reduce interference. When laying the ground wire, it should not be laid in a closed form, but in a dendritic shape. The area of the earth.
67. If the emulator uses one power supply and the pcb board uses one power supply, should the grounds of the two power supplies be connected together?
It would be better if a separate power supply can be used, because it is not easy to cause interference between power supplies, but most of the equipment has specific requirements. Since the emulator and the PCB board use two power supplies, I don’t think they should share the same ground.
68. A circuit is composed of several pcb boards. Should they share the ground?
A circuit consists of several PCBs, most of which require a common ground, because it is not practical to use several power supplies in one circuit. But if you have specific conditions, you can use a different power supply, of course the interference will be smaller.
69. Design a handheld product with an LCD and a 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 horizontal can only pass 3000V, and the vertical can pass 4000V. CPU frequency is 33MHZ. Is there any way to pass ESD test?
Handheld products are metal casings, so ESD problems must be more obvious, and LCDs may also have more adverse phenomena. If there is no way to change the existing metal material, it is recommended to add anti-electric material inside the mechanism to strengthen the ground of the PCB, and at the same time find a way to ground the LCD. Of course, how to operate depends on the specific situation.
70. When designing a system containing DSP and PLD, what aspects should ESD be considered?
As far as the general system is concerned, the parts in direct contact with the human body should be mainly considered, and appropriate protection should be carried out on the circuit and mechanism. As for how much impact ESD will have on the system, it depends on different situations.
Post time: Mar-19-2023