High-Frequency PCB Circuit Design Experience Sharing

PCB(Printed Circuit Board), the Chinese name is called printed circuit board, also known as printed circuit board, is an important electronic part, is the support body of electronic components, is the carrier of electrical connection of electronic components. Because it is produced by electronic printing, it is called a "printed" circuit board.

1. how to choose PCB board?

The selection of PCB board must strike a balance between meeting the design needs and mass production and cost. The design requirements include both electrical and institutional parts. This material issue is usually more important when designing very high speed PCB boards (frequencies greater than GHz). For example, the FR-4 material commonly used now has a dielectric loss at several GHz frequencies that has a great impact on signal attenuation and may not be suitable. For electrical purposes, pay attention to whether the dielectric constant and dielectric loss are suitable at 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. 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 digital ground to analog ground.

3. in high-speed design, how to solve the problem of signal integrity?

Signal integrity is basically a matter of impedance matching. The factors that affect the impedance matching are the signal source architecture and output impedance, the characteristic impedance of the track, the characteristics of the load side, and the topology architecture of the track. The solution is to terminate and adjust the topology of the routing.

4. How is the differential wiring method realized?

The wiring of the difference pair has two points to pay attention to, one is that the length of the two lines should be as long as possible, and the other is that the distance between the two lines (the distance between the two lines is determined by the differential impedance) should always remain unchanged, that is, to maintain parallel. There are two ways of parallel, one is two lines walk in the same line layer (side-by-side), and the other is two lines walk in the upper and lower adjacent two layers (over-under). There are many ways to implement the former side-by-side(side by side).

5. for only one output clock signal line, how to achieve differential wiring?

To use differential wiring must be the signal source and the receiving end are also differential signals to make sense. Therefore, it is impossible to use differential wiring for clock signals with only one output.

6. Can a matching resistance be added between the receiving end difference line pairs?

The matching resistance between the differential pairs at the receiving end is usually added, and its value should be equal to the value of the differential impedance. The signal quality will be better.

7. why the differential pair of wiring should be close and parallel?

The wiring mode of the differential pairs should be appropriately close and parallel. The so-called appropriate proximity is because this spacing will affect the value of the differential impedance, which is an important parameter in the design of the differential pair. Parallelism is also required to maintain the consistency of the differential impedance. If the two lines are far and near, the differential impedance will be inconsistent, which will affect signal integrity and timing delay.

8. how to deal with some theoretical conflicts in the actual wiring

Basically, it is right to segregate modulo/numerically. It should be noted that the signal route should not cross the separated area (moat), and do not make the power and signal return current path (return current path) too large.

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 specifications of the analog signal are easy to be interfered with, and the interference may not be completely isolated even if ground guard traces are added. And too far away, ground plane noise will also affect the positive feedback oscillation circuit. Therefore, it is necessary to put the distance between the crystal and the chip as close as possible.

It is true that high-speed cabling has many conflicts with EMI requirements. However, the basic principle is that the resistance capacitor or ferrite bead added by EMI cannot cause some electrical characteristics of the signal to fail to meet the specification. Therefore, it is best to first use the skills of arranging wiring and PCB layering to solve or reduce EMI problems, such as high-speed signal to the inner layer. Finally, resistive capacitors or ferrite bead methods are used to reduce the damage to the signal.

9. how to solve the contradiction between the manual wiring and automatic wiring of high-speed signals?

At present, most of the automatic wiring devices of strong wiring software have set constraints to control the winding mode and the number of holes. The winding engine capabilities and constraint setting projects of each EDA company sometimes differ greatly. For example, whether there are enough constraints to control the way serpentine lines meander, whether you can control the distance between differential pairs, and so on. This will affect whether the automatic wiring out of the way to line up with the designer's ideas. In addition, the difficulty of manually adjusting the wiring is also absolutely related to the ability of the winding engine. For example, the pushing ability of the line, the pushing ability of the hole, and even the pushing ability of the line to the copper coating. Therefore, choosing a wiring device with strong winding engine capability is the solution.

10. About test coupon.

The test coupon is used to measure whether the characteristic impedance of the PCB Reflectometer (TDR) meets the design requirements. Generally, the impedance to be controlled has two cases: single line and differential pair. Therefore, the line width and line distance on the test coupon should be the same as the line to be controlled. The most important thing is to measure the position of the ground point. In order to reduce the inductance value of the ground lead, the ground point of the TDR probe is usually very close to the probe tip, so the distance and manner between the measured signal point and the ground point on the test coupon should be consistent with the probe used.

11. in high-speed PCB design, the blank area of the signal layer can be coated with copper, and how should the copper of multiple signal layers be distributed on the ground and connected power supply?

In general, most of the copper coating in the blank area is grounded. Only when applying copper next to the high-speed signal line, pay attention to the distance between the copper and the signal line, because the applied copper will reduce the characteristic impedance of the line a little. Also be careful not to affect the characteristic impedance of its layer, for example, in the structure of dual strip lines.

12. can the signal line above the power plane use the microstrip line model to calculate the characteristic impedance? Can the signal between the power source and the ground plane be calculated using a strip line 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, then the model of the top line characteristic impedance is a microstrip line model with the power plane as the reference plane.

13. Can the software automatically generate test points on high-density printed boards generally meet the test requirements of mass production?

Whether the test points automatically generated by the general software meet the test requirements must be determined by whether the specifications of the test points meet the requirements of the test machines. In addition, if the line is too dense and the specification of test points is strict, it may not be possible to automatically add test points to each line, of course, you need to manually complete the place to be tested.

14. Will adding test points affect the quality of high-speed signals?

Whether it will affect the signal quality depends on how the test point is added and how fast the signal is. Essentially additional test points (not required to have an existing in-line punch (via or DIP pin) as test points) may be added to or pulled out of the in-line by a small piece of wire. The former is equivalent to adding a small capacitor to the line, and the latter is adding a branch. These two situations will have more or less an impact on the high-speed signal, and the degree of impact is related to the frequency speed of the signal and the edge rate of change of the signal. The size of the effect can be determined by simulation. In principle, the smaller the test point, the better (of course, to meet the requirements of the test machine) the shorter the branch, the better.

15. a number of PCB composition system, how should the ground wire between the boards be connected?

When the signal or power supply between the various PCB boards is connected to each other, for example, when the A board has A power supply or signal to the B board, there must be an equal amount of current flowing from the ground to the A board (this is the Kirchoff current law). The current in this formation will flow back to where it has the least resistance. Therefore, at the interface where each power supply or signal is connected, the number of pins assigned to the formation cannot be too small to reduce the impedance, which can reduce the noise on the formation. In addition, it is also possible to analyze the entire current loop, especially the part of the larger current, and adjust the connection of the formation or ground wire to control the path of the current (for example, create a low impedance in one place, so that most of the current goes from this place), reducing the impact on other more sensitive signals.

16. Can you introduce some foreign technical books and data on high-speed PCB design?

High-speed digital circuits are now used in related fields such as communication networks and calculators. In terms of communication networks, the working frequency of PCB boards has reached up to and below GHz, and the number of layers as far as I know has reached 40 layers. Because of the progress of chips, whether it is a general PC or Server (Server), the highest operating frequency on the board has reached 400MHz (such as Rambus) or more. In response to this demand for high-speed and high-density wiring, the demand for blind/buried vias, mircrovias and build-up processes is increasing. These design requirements have manufacturers that can be mass-produced.

17. two often referred to the characteristic impedance formula:

microstrip Z={87/[sqrt(Er+1.41)]}ln[5.98H/(0.8W+T)], where W is the wire width, T is the copper thickness of the wire, and H is the distance from the wire to the reference plane. Er is the dielectric constant of PCB sheet material. This formula must be used for 0.1<(W/H)<2.0 and 1<(Er)<15.

stripline Z=[60/sqrt(Er)]ln{4H/[0.67π(T+0.8W)]} where H is the distance between the two reference planes, and the line is located in the middle of the two reference planes. This formula must be used for W/H<0.35 and T/H<0.25.

18. difference signal line in the middle can add ground?

Generally, ground wires cannot be added in the middle of differential signals. Because the most important point of the application principle of differential signals is to use the benefits brought by the coupling between differential signals, such as flux cancellation, noise immunity and so on. If a ground line is added in the middle, the coupling effect will be destroyed.

19. Does the design of rigid adagio need special design software and specifications? Where in China can undertake this type of circuit board processing?

A Flexible Printed Circuit board can be designed with the same software as a general PCB design. The same Gerber format for FPC manufacturers to produce. Because the manufacturing process is different from the general PCB, each manufacturer will have its own minimum line width, minimum line distance, and minimum aperture (via) according to their manufacturing capacity. In addition, the flexible circuit board can be strengthened by laying some copper skin around the bend. As for the manufacturer of the production can go online "FPC" when the keyword query should be found.

20. What is the principle of appropriate selection of PCB and shell grounding point?

The principle of selecting PCB and housing ground point selection is to use chassis ground to provide a low-impedance path to the returning current and to control the path of this return current. For example, usually near the high-frequency device or clock generator, the PCB layer can be connected to the chassis ground by fixing screws to minimize the entire current loop area, which reduces electromagnetic radiation.

21. circuit board DEBUG should start from what several aspects?

As far as digital circuits are concerned, first determine three things in sequence: 1. Confirm that the size of all power supply values meet the design requirements. Some multi-power systems may require some specification of the order and speed at which certain power supplies are connected. 2. Verify that all clock signal frequencies are working properly and that there are no non-monotonic issues on the edge of the signal. 3. Check whether the reset signal meets the specifications. If these are normal, the chip should send out the first cycle signal. Next, debug according to the operating principle of the system and bus protocol.

22. in the case of a fixed circuit board size, if the design needs to accommodate more functions, it is often necessary to improve the PCB wire density, but this may lead to enhanced mutual interference with the line, while the line is too thin also so that the impedance can not be reduced, please introduce experts in high-speed (>100MHz) high-density PCB design skills?

In the design of high-speed high-density PCB, crosstalk interference is indeed a special attention, because it has a great impact on timing and signal integrity. Here are a few points to note:

Control the continuity and matching of the characteristic impedance of the track.

The size of the cable spacing. It is commonly seen that the spacing is twice the line width. The influence of track spacing on timing and signal integrity can be known through simulation, and the minimum tolerable distance can be found. Different chip signals may have different results.

Select the appropriate termination method.

Avoid the same direction of the two adjacent layers, or even overlapping the upper and lower layers, because this kind of crosstalk is greater than the case of adjacent cables on the same layer.

The blind/buried via is used to increase the line area. However, the production cost of PCB board will increase. In practice, it is really difficult to achieve complete parallelism and isometric length, but try to do it.

In addition, differential and common-mode terminations can be reserved to mitigate the impact on timing and signal integrity.

23. the filter at the analog power supply is often used LC circuit. But why is LC sometimes worse than RC filtering?

The comparison of LC and RC filtering effect must consider whether the frequency band and inductance value to be filtered are appropriate. Because the reactance of inductance 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, then the filtering effect may not be as good as RC. However, the cost of using RC filtering is that the resistor itself will consume energy, the efficiency is poor, and attention should be paid to the power that the selected resistor can withstand.

24. What is the method of selecting inductance and capacitance value when filtering?

The selection of inductance value not only considers the noise frequency to be filtered, but also considers the reaction ability of instantaneous current. If there is a chance that the output end of the LC will need to output a large current instantaneously, the inductance value is too large to hinder the speed of this large current flowing through the inductor, increasing the ripple noise. The capacitance value is related to the standard value of the ripple noise that can be tolerated. The smaller the ripple noise value, the larger the capacitance value. The ESR/ESL of capacitors also has an impact. In addition, if the LC is placed at the output of a switching regulation power, Also note the effect of the pole/zero generated by this 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 of EMC on PCB boards is usually due to the increase in the number of layers to enhance the shielding effect and the increase of ferrite bead, choke and other high-frequency harmonic suppression devices. In addition, it is usually necessary to match the shielding structure on other institutions to make the entire system pass the EMC requirements. The following is only a few PCB board design tips to reduce the electromagnetic radiation effect generated by the circuit.

When possible, use devices with a slow signal slope (slew rate) to reduce the high frequency components generated by the signal.

Pay attention to the position of the high-frequency components, do not be too close to the external connector.

Pay attention to the impedance matching of high-speed signals, the wiring layer and its return current path to reduce high-frequency reflection and radiation.

Sufficient and appropriate decoupling capacitors are placed in the power pins of each device to mitigate noise at the power layer and ground level. 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 ground, and the ground of the connector can be connected to the chassis ground.

The ground guard/shunt traces can be properly used near some particularly high-speed signals. However, pay attention to the influence of guard/shunt traces on the characteristic impedance of the track.

The power layer shrinks by 20H compared to the inside of the formation. H is the distance between the power layer and the formation.

26. when there are many number/mode function blocks in a PCB board, the conventional practice is to separate the number/mode, why?

The reason for separating the digital/analog ground is that the digital circuit will produce noise in the power supply and ground during the high-low potential switch, and the size of the noise is related to the speed of the signal and the size of the current. If the ground plane is not divided and the noise generated by the digital region circuit is large and the analog region circuit is very close, the analog signal will still be interfered with the ground noise even if the digital-analog signal does not cross. That is to say, the digital-analog undivided method can only be used when the analog circuit area is far away from the digital circuit area that generates large noise.

27. Another method is to ensure that the number/mode separate layout, and the number/mode signal lines do not cross each other, the entire PCB board does not divide, the number/mode are connected to the ground plane. What is the reason?

The requirement that the digital signal can not cross is because the return current path of the digital signal that is slightly faster will flow back to the source of the digital signal near the bottom of the line as far as possible. If the digital signal crosses the line, the noise generated by the return current will appear in the analog circuit area.

28. in the high-speed PCB design schematic design, how to consider the impedance matching problem?

When designing high-speed PCB circuits, impedance matching is one of the design elements. The impedance value has an absolute relationship with the wiring method, such as walking on the surface layer (microstrip) or the inner layer (stripline/double stripline), the distance from the reference layer (power layer or formation), the width of the wire, PCB material, etc., will affect the characteristic impedance value of the wire. That is to say, the impedance value can be determined after wiring. General simulation software will not be able to take into account some impedance discontinuity wiring because of the line model or the mathematical algorithm used, at this time, only some terminators(terminators) can be reserved on the schematic diagram, such as series resistance, to mitigate the effect of line impedance discontinuity. The real fundamental solution to the problem is to try to avoid the occurrence of impedance discontinuity when wiring.

29. Where can I provide a more accurate IBIS model library?

The accuracy of IBIS model directly affects the result of simulation. Basically, IBIS can be regarded as the electrical characteristics data of the actual chip I/O buffer equivalent circuit, which can generally be converted by the SPICE model (can also be measured, but more), and the SPICE data has an absolute relationship with the chip manufacturing, so the same device is provided by different chip manufacturers. The SPICE data is different, and the data in the converted IBIS model will also be different. In other words, if manufacturer A's device is used, only they have the ability to provide accurate model data for their device, because no one else will know better than them what process their device is made from. If the IBIS provided by the manufacturer is not accurate, the only way to continuously ask the manufacturer to improve is the fundamental solution.

30. in the high-speed PCB design, the designer should consider from what aspects of EMC, EMI rules?

General EMI/EMC design needs to consider both radiated and conducted aspects. The former belongs to the higher frequency part (>30MHz) and the latter is the lower frequency part (<30MHz). So you can't just pay attention to the high frequencies and ignore the low frequencies. A good EMI/EMC design must take into account the location of the device at the beginning of the layout, the arrangement of the PCB stack, the important online way, the choice of the device, etc., if these do not have a better arrangement in advance, the solution will be less effective 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 and pay attention to the continuity of the characteristic impedance matching and the reference layer to reduce reflection, and the slope of the signal pushed by the device (slew rate) should be as small as possible to reduce the high-frequency component. When selecting the decoupling/bypass capacitor, pay attention to whether the frequency response meets the requirements to reduce the noise at the power layer. In addition, pay attention to the return path of the high-frequency signal current so that the loop area is as small as possible (that is, the loop impedance is as small as possible) to reduce radiation. The range of high frequency noise can also be controlled by dividing the strata. Finally, the appropriate selection of PCB and shell ground (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 choose, other functions 1.3.4 can choose PADS or Cadence performance price ratio is good. PLD design beginners can use the integrated environment provided by PLD chip manufacturers, and can choose a single point tool when designing more than a million doors.

32. Please recommend an EDA software suitable for high-speed signal processing and transmission.

For conventional circuit design, INNOVEDA PADS are very good, and have simulation software with them, and such designs often account for 70% of applications. In high-speed circuit design, analog and digital hybrid circuit, Cadence solution should belong to the software with relatively good performance and price. Of course, Mentor's performance is still very good, especially its design process management should be the best.

33. Explain the meaning of each layer of PCB board

Topoverlay ---- The name of the top layer device, also called top silkscreen or top component legend, such as R1 C5,

IC10.bottomoverlay---- Similarly multilayer----- If you design a 4-layer board and you place a free pad or via and define it as multilay then its pad will automatically appear on all 4 layers, If you only define it as the top layer, then its pad will only appear on the top layer.

34. 2G above high-frequency PCB design, wiring, typesetting, should pay attention to what aspects?

High frequency PCBS above 2G belong to RF circuit design and are not within the scope of high-speed digital circuit design discussion. The layout and routing of the RF circuit should be considered together with the schematic diagram, because the layout and routing will cause distribution effects. Moreover, the RF circuit design of some passive devices is achieved by parametric definition, special shape copper foil, so it is required that EDA tools can provide parametric devices and can edit special shape copper foil. Mentor's boardstation has dedicated RF design modules to meet these requirements. Moreover, general RF design requires specialized RF circuit analysis tools, the most famous in the industry is agilent's eesoft, and Mentor's tools have a good interface.

35. more than 2G high frequency PCB design, microstrip design should follow what rules?

In RF microstrip line design, the transmission line parameters need to be extracted by 3D field analysis tool. All the rules should be specified in the field extraction tool.

36. for the full digital signal PCB, there is an 80MHz clock source on the board. In addition to the use of wire mesh (grounding), in order to ensure sufficient driving capacity, what kind of circuit should be used for protection?

To ensure the driving ability of the clock, it should not be achieved through protection, generally using a clock driver chip. The general concern about clock drive capability is due to multiple clock loads. The clock driver chip is used to turn one clock signal into several, and point-to-point connection is adopted. When selecting the driver chip, in addition to ensuring the basic match with the load and the signal edge meeting the requirements (the general clock is an effective signal along the clock), the time delay of the clock in the driver chip should be counted when calculating the system timing.

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 length of signal wiring. And the grounding power supply of the board is also a problem. For long distance transmission, differential signals are recommended. The L number will meet the drive capability requirements, but your clock is not too fast to be necessary.

38. 27M,SDRAM clock lines (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 from the receiving end. In addition to shortening the length of the line, what are the 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 harmonic. At this time, the signal duty cycle needs to be modified. In addition, if the clock signal is one-way, the source end series matching is generally used. This suppresses secondary reflections, but does not affect the clock edge rate. The matching value of the source end can be obtained by using the following formula.

39. What is the routing topology architecture?

Topology, also known as routing order. The routing order for a multi-port connected network.

40. how to adjust the topology structure of the cable to improve the integrity of the signal?

This kind of network signal direction is more complex, because the impact of topologies on unidirectional signals, bidirectional signals, and different level types of signals is different, it is difficult to say which topologies are beneficial to signal quality. And for pre-simulation, what kind of topology is used is very demanding for engineers, requiring understanding of circuit principle, signal type, and even wiring difficulty.

41. How to reduce EMI problems by arranging layers?

First of all, EMI should be considered from the system, and PCB alone can not solve the problem. For EMI, I think layering is mainly to provide the shortest return path of the signal, reduce the coupling area, and suppress differential mode interference. In addition, the ground and the power layer are tightly coupled, and the epitaxial of the power layer is better than that of the power layer, which is good for suppressing the common mode interference.

42. why to lay copper?

There are several reasons for generally laying copper. 1, EMC. For a large area of ground or power copper, will play a shielding role, some special, such as PGND plays a protective role. 2, PCB process requirements. In general, in order to ensure the plating effect, or the lamination is not deformed, copper is laid on the PCB board with less wiring. 3, signal integrity requirements, to the high frequency digital signal a complete return path, and reduce the wiring of the DC network. Of course, there are heat dissipation, special device installation requirements for copper and so on.

43. in a system, including dsp and pld, what problems should be paid attention to when wiring?

Look at the ratio of your signal rate to the length of your wiring. If the delay of the signal in the transmission line is comparable to the change of the signal along time, the signal integrity problem must be considered. In addition, for multiple DSPS, time clocks, and data signal routing topology will also affect signal quality and timing, which needs attention.

44. in addition to protel tool wiring, there are 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 has its own strengths.

45. What is the "signal return path"?

Signal return path, that is, return current. When high-speed digital signals are transmitted, the flow of the signal is from the driver along the PCB transmission line to the load, and then from the load along the ground or the power supply through the shortest path back to the driver end. This return signal on the ground or power supply is called the signal return path. In his book, Dr.Johson explains that high-frequency signal transmission is actually the process of charging the dielectric capacitor 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 perform SI analysis with plug-in?

The plug-in model is described in the IBM 3.2 specification. The EBD model is generally used. If it is a special plate, such as a backplane, a SPICE model is required. You can also use multi-board simulation software (HYPERLYNX or IS_multiboard) to set up a multi-board system by entering the distribution parameters of the connector, which are generally obtained from the connector manual. Of course, this method will not be precise, but as long as it is within acceptable limits.

47. what are the ways of terminating?

terminal (terminal), also known as matching. Generally, the matching position is divided into active end matching and terminal matching. The source matching is generally resistance series matching, the terminal matching is generally parallel matching, there are more ways, resistance pull-up, resistance pull-down, Davinan matching, AC matching, Schottky diode matching.

48. the use of terminating (matching) is determined by what factors?

The matching method is generally determined by BUFFER characteristics, toppling conditions, level types and decision modes, and also takes into account signal duty cycle, system power consumption and so on.

49. What are the rules of terminating (matching)?

The most critical problem of digital circuit is timing, and the purpose of matching is to improve the signal quality and get the signal that can be determined at the decision time. For the signal with valid level, the signal quality is stable under the premise of ensuring the establishment and holding time; On the premise of ensuring the monotonicity of the signal delay, the signal change delay speed can meet the requirements. The Mentor ICX product textbook has some information on matching. In addition, "High Speed Digital design a hand book of blackmagic" has a chapter dedicated to terminal, which tells the function of matching on signal integrity from the principle of electromagnetic wave, for reference.

50. Can use the IBIS model of the device to simulate the logic function of the device? If not, how do you simulate the circuit at board and system level?

IBIS models are behavioral models and cannot be used for functional simulation. Functional simulations require SPICE models, or other structure-level models.

51. in the digital and analog coexistence of the system, there are two processing methods, one is digital and analog separate, such as in the formation, digital is an independent piece, analog independent piece, a single point with copper or FB magnetic beads connected, and the power is not separated; The other is that the analog power supply and the digital power supply are connected separately with FB, and the ground is unified. May I ask Mr. Li whether the two methods have the same effect?

It's basically the same thing. Because the power supply and the ground to the high frequency signal are equivalent.

The purpose of the distinction between analog and digital parts is to resist interference, mainly from digital circuits to analog circuits. However, segmentation may cause incomplete signal return path, affect the signal quality of digital signal, and affect the EMC quality of the system. Therefore, no matter which plane is divided, it is necessary to see whether the signal return path is increased in this way, and how much the return signal interferes with the normal working signal. There are also some hybrid designs, regardless of power supply and ground, in the layout, according to the digital part and analog part of the separate layout and wiring, to avoid cross-region signals.

52. safety issues: What is the specific meaning of FCC and EMC?

FCC: federal communication commission

EMC: electro megnetic compatibility. Electro megnetic compatibility

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

53. what is differential wiring?

Differential signals, also known as differential signals, use two identical, opposite polarity signals to transmit one data, relying on the level difference between the two signals for judgment. In order to ensure that the two signals are completely consistent, they should be kept parallel when wiring, and the line width and line spacing should remain unchanged.

54. What is PCB simulation software?

Many different kinds of simulation, the number of words electrical signal integrity (SI) analysis of the simulation analysis of commonly used software have icx, signalvision, hyperlynx, XTK, speectraquest, etc. Some also use Hspice.

55. PCB simulation software is how to perform LAYOUT simulation?

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

56. in the layout, wiring how to deal with to ensure the stability of more than 50M signals

The key of high-speed digital signal routing is to reduce the influence of transmission line on signal quality. Therefore, when the high-speed signal layout is more than 100M, the signal route is required to be as short as possible. In digital circuits, high-speed signals are defined by the signal rise time delay. 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 such a PCB in the material? 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.

Generally, the RF circuit is laid out as an independent single board in the system, and even has a special shielding cavity. Moreover, the RF circuit is generally single-sided or double-panel, and the circuit is relatively simple, all of which is 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, the RF circuit board tends to use a substrate with a high Q value, which has a relatively small dielectric constant, small transmission line distribution capacitance, high impedance, and small signal transmission delay. In the hybrid circuit design, although the RF and digital circuits are done on the same PCB, they are generally divided into the RF circuit area and the digital circuit area, respectively. Between the ground hole belt and shielding box shielding.

58. For the RF part, the IF part and the low frequency circuit part are deployed on the same PCB, what is mentor's solution?

Mentor's board level system design software includes a dedicated RF design module in addition to basic circuit design functions. In RF schematic design module, parametric device model is provided, and bidirectional interface with EESOFT and other RF circuit analysis and simulation tools is provided. In the RF LAYOUT module, it provides the pattern editing function specially used for RF circuit layout and wiring, and also has the bidirectional interface with EESOFT and other RF circuit analysis and simulation tools, so that the schematic diagram and PCB can be reverse-marked back to the analysis and simulation results. At the same time, the design management function of Mentor software can facilitate design reuse, design derivation, and collaborative design. Greatly accelerate the process of hybrid circuit design. Mobile phone board is a typical hybrid circuit design, and many large mobile phone design manufacturers use Mentor and Angel's eesoft as a 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 grid-free, arbitrary Angle cabler. As we all know, for spherical grid arrays, COB devices, grid-free, arbitrary Angle cablers are the key to solving the distribution flux. In the latest autoactive RE, new functions such as push through hole, copper foil, REROUTE are added to make it more convenient to apply. In addition, it supports high-speed cabling, including signal cabling with delay requirements and differential pair cabling.

61. How does Mentor's PCB design software handle differential line teams?

After defining the properties of the difference pairs, the two difference pairs can be routed together to ensure the difference in line width, distance and length of the difference pairs. When encountering obstacles, the two pairs can be separated automatically. When changing layers, the mode of passing holes can be selected.

62. on a 12-layer PCb board, there are three power layers 2.2v, 3.3v,5v, the three power supplies are each in a layer, how to deal with the ground?

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

63. How to check whether the PCB meets the design process requirements at the factory?

Many PCB manufacturers in the PCB processing is completed before the factory, the network connection test must be powered up to ensure that all the connections are correct. At the same time, more and more manufacturers are also using X-ray tests to check for some faults during etching or lamination. For the finished board after patch processing, ICT test inspection is generally used, which requires the addition of ICT test points in the PCB design. If there is a problem, it can also be ruled out by a special X-ray inspection device whether the fault is caused by processing.

64. "the protection of the mechanism" is not the protection of the chassis?

Yes. The casing should be as tight as possible, with little or no conductive materials, and grounded as far as possible.

65. when choosing the chip, does it also need to consider the esd problem of the chip itself?

Whether it is a double-layer board or a multi-layer board, the area of the floor should be increased as much as possible. In the selection of the chip to consider the ESD characteristics of the chip itself, these are generally mentioned in the chip description, and even different manufacturers of the same chip performance will be different. Pay more attention to the design, consider a comprehensive point, and make the performance of the circuit board will be guaranteed. However, ESD problems may still occur, so the protection of the mechanism is also very important for ESD protection.

66. when doing PCB board, in order to reduce interference, should the ground wire form a closed and closed form?

When doing PCB board, generally speaking, it is necessary to reduce the loop area, in order to reduce interference, when the ground wire is distributed, it should not be distributed into a closed form, but into a dendritic shape, and it is better to increase the area as much as possible.

67. If the simulator uses a power supply and the pcb board uses a power supply, should the two power supplies be connected together?

If you can use separate power supplies, of course, it is better, because it is not easy to cause interference between power supplies, but most of the equipment has specific requirements. Since the simulator and the PCB board use two power supplies, according to my idea, it should not be shared.

68. a circuit is composed of several pcb boards, should they be common?

A circuit is composed of several PCBS, most of which require common ground, 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 sources, of course, the interference will be less.

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

Handheld products are metal shell, ESD problems must be more obvious, LCD is also afraid there will be more adverse phenomena. If there is no way to change the existing metal material, it is recommended to add anti-electric materials inside the mechanism, strengthen the PCB ground, and find a way to ground the LCD. Of course, how to operate depends on the specific situation.

70. Design a system containing DSP, PLD, from which aspects to consider ESD?

In terms of the general system, the direct contact of the human body should be considered, and appropriate protection should be carried out on the circuit and the mechanism. As for how much ESD will affect the system, it depends on different circumstances. In a dry environment, ESD phenomenon will be more serious, more sensitive and fine systems, the impact of ESD will be relatively obvious. Although large systems sometimes ESD impact is not obvious, but the design should pay more attention to prevent problems as far as possible.