PCB Tech - PCB design methods and skills 3

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 will be great after the high frequency enters from the receiving end. In addition to shortening the line length, what other good methods are there?

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, you need to modify the signal duty cycle.
In addition, if it is a unidirectional clock signal, the source terminal series matching is generally used. This can suppress secondary reflections, but will not affect the clock edge rate. The source matching value can be obtained using the formula below.

39. What is the topology of the wiring?
Topology, and some are also called routing order. For the wiring order of a multi-port network.

40. How to adjust the routing topology to improve signal integrity?
This kind of network signal direction is more complicated, because for unidirectional, bidirectional signals, and different level types of signals, the topology influences are different, and it is difficult to say which topology is beneficial to the signal quality. And when doing pre-simulation, which topology to use is very demanding on engineers, requiring understanding of circuit principles, signal types, and even wiring difficulty.

41. How to reduce EMI problems by arranging stacks?
First of all, EMI must be considered from the system. PCB alone cannot solve the problem.
For EMI, stacking is mainly to provide the shortest return path for the signal, reduce the coupling area, and suppress differential mode interference. In addition, the ground layer is tightly coupled with the power layer, which is more epitaxial than the power layer, which is good for suppressing common mode interference.

42. Why should copper be laid?
There are generally several reasons for copper paving.
1. EMC. For large-area ground or power supply copper, it will play a shielding role, and some special grounds, such as PGND, play a protective role.
2. PCB process requirements. Generally, in order to ensure the electroplating effect, or the lamination is not deformed, copper is laid on the PCB layer with less wiring.
3. The signal integrity is required to provide a complete return path for high-frequency digital signals and reduce the wiring of the DC network. Of course, there are also reasons such as heat dissipation, special device installation requires copper 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 time delay of the signal on the transmission line is comparable to the signal change edge time, the signal integrity problem must be considered. In addition, for multiple DSPs, clocks, and data signal routing will also affect the signal quality and timing, which requires 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 has its own strengths.

45. What is the "signal return path"?
Signal return path, namely return current. When high-speed digital signals are being transmitted, the signal flows from the driver to the load along the PCB transmission line, and then from the load back to the driver through the shortest path along the ground or power supply. This return signal on the ground or power supply is called the signal return path. Dr. Johson explained in his book that high-frequency signal transmission is actually a process of charging the dielectric capacitor sandwiched between the transmission line and the DC layer. SI analyzes the electromagnetic characteristics of the enclosure and the coupling between them.

46. How to perform SI analysis on connectors?
In the IBIS3.2 specification, there is a description of the connector model. The EBD model is generally used. 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, enter 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 termination methods?
Termination (terminal), also known as matching. Generally, there are active end matching and terminal matching according to the matching position. The source terminal matching is generally resistance series matching, and the terminal matching is generally parallel matching. There are many ways, including resistance pull-up, resistance 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, topological conditions, level types and judgment methods, and the signal duty cycle, system power consumption, etc. should also be considered.

49. What are the rules for using termination (matching)?
The most critical aspect of the digital circuit is the timing issue. The purpose of adding matching is to improve the signal quality and obtain a determinable signal at the moment of decision. For level-valid signals, the signal quality is stable under the premise of ensuring the setup and hold time; for the valid signals, the signal change delay speed meets the requirements under the premise of ensuring the monotonicity of the signal delay. Mentor ICX product teaching materials have some information about matching. In addition, "High Speed Digital design a hand book of blackmagic" has a chapter dedicated to the terminal, telling the effect of matching on signal integrity from the principle of electromagnetic waves, which can be used for reference.

50. Can the IBIS model of the device be used to simulate the logic function of the device? If not, how to perform board-level and system-level simulation of the circuit?
The IBIS model is a behavioral model and cannot be used for functional simulation. Functional simulation requires SPICE models or other structural level models.

51. In a system where both digital and analog coexist, there are two processing methods. One is to separate the digital ground from the analog ground. For example, in the stratum, the digital ground is an independent piece, the analog ground is an independent piece, and a copper sheet or FB magnet is used for a single point. Bead connection, but the power supply is not separated; the other is that the analog power supply and the digital power supply are separated by FB connection, and the ground is a unified ground. May I ask Mr. Li, are these two methods the same effect?
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 analog and digital parts is to resist interference, mainly the interference of digital circuits to analog circuits. However, the segmentation may cause incomplete signal return paths, affect the signal quality of the digital signal, and affect the EMC quality of the system. Therefore, no matter which plane is divided, it depends on whether this is done, whether the signal return path is enlarged, and how much the return signal interferes with the normal working signal.
There are also some mixed designs, regardless of power supply and ground. In the layout, the digital part and the analog part are placed and routed separately to avoid cross-zone signals.

52. Safety issues: What is the specific meaning of FCC and EMC?
FCC: federal communication commission
EMC: electro megnetic compatibility
FCC is a standards organization, and EMC is a standard. The promulgation of standards has corresponding reasons, standards and test methods.

53. What is differential wiring?
Differential signals, some are also called differential signals, use two exactly the same signals with opposite polarities to transmit one data, and make a decision based on the difference between the two signal levels. In order to ensure that the two signals are exactly the same, they must be kept parallel when wiring, and the line width and line spacing remain unchanged.

54. What are the PCB simulation software?
There are many types of simulation. Commonly used software for high-speed digital circuit signal integrity analysis (SI) includes icx, signalvision, hyperlynx, XTK, speectraquest, etc. Some also use Hspice.

55. How does PCB simulation software 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 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 the signal traces to 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 radio frequency part, 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 material requirements for such a PCB board? How to prevent radio frequency, intermediate frequency 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 arranged and wired as an independent single board in the system, and there is even a special shielded cavity. Moreover, the radio frequency circuit is generally single-sided or double-sided, and the circuit is relatively simple, all of which are used to reduce the influence on the distribution parameters of the radio frequency circuit and improve the consistency of the radio frequency system. Compared with general FR4 materials, RF circuit boards tend to use high-Q substrates. This material has a relatively small dielectric constant, a small transmission line distributed capacitance, high impedance, and small signal transmission delay.
In hybrid circuit design, although the radio frequency and digital circuits are built on the same PCB, they are generally divided into radio frequency circuit areas and digital circuit areas, and they are laid out and routed separately. Use grounding via tape and shielding box to shield between them.

58. For the radio frequency part, the intermediate frequency part and the low frequency circuit board part are deployed on the same PCB, what solution does mentor have?
Mentor's board-level system design software has specialized RF design modules in addition to basic circuit design functions. In the RF schematic design module, provide parameterized device models, and provide bidirectional interfaces with radio frequency circuit analysis simulation tools such as EESOFT; in the RF LAYOUT module, provide pattern editing functions dedicated to the layout of radio frequency circuits, and The bidirectional interface of EESOFT and other radio frequency circuit analysis simulation tools can back-annotate the schematic diagram and PCB for the analysis and simulation results. At the same time, using the design management function of Mentor software, design reuse, design derivation, and collaborative design can be easily realized. Greatly accelerate the process of hybrid circuit design.
The mobile phone board is a typical hybrid circuit design, and many large mobile phone design manufacturers use Mentor and Angelen'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.