Common PCB Design Issues and Solutions for Circuit Board Novice(Part One)

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PCB Design
Keyword
pcb design issue, pcb board, pcb,pcb design tutorial
Article core
Common PCB Design Issues and Solutions for Circuit Board Novice

Catalogs

Catalogs
I.How to select a PCB plate
II.Avoid high frequency interference
III.Implementation of differential distribution lines
IV.The wiring of the difference pair should be close and parallel
V.A clock signal line with only one output terminal can not achieve a differential distribution line
VI.Add a matching resistance between the receiver differential line pairs
VII.signal integrity of high speed PCB design
VIII.Solving the contradiction between manual wiring and automatic wiring of high speed signal
IX.Solution of some theoretical conflicts in actual wiring
X.About test coupon
XI.Relevant information about "Common PCB Design Issues and Solutions for Circuit Board Novice(Part Two)"

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I.How to select a PCB plate

The selection of PCB plate must be balanced between the design requirements and the quality and cost. The design requirements include the two parts of electric and mechanism. This material problem is usually more important when designing a very high speed PCB board (more than GHz). For example, nowadays, the dielectric loss (dielectric loss) of commonly used FR-4 material at a frequency of several GHz will have a great influence on signal attenuation, and may not be used. As far as electric is concerned, it is necessary to pay attention to whether the dielectric constant (dielectric constant) and the dielectric loss are combined with the designed frequency.

II.Avoid high frequency interference

The basic idea of avoiding high frequency interference is to minimize the interference of high frequency electromagnetic fields, that is, the so-called crosstalk (Crosstalk). The distance between the high speed signal and the analog signal can be used, or the ground guard/shunt traces is added to the analog signal. It is also necessary to pay attention to the noise interference of the analog ground.

III.Implementation of differential distribution lines

There are two points to be noticed in differential pair wiring. One is that the lengths of the two lines should be as long as possible, and the other is the distance between the two lines. There are two kinds of parallel ways: one is two lines on the same line layer (side-by-side), and one is two lines in the upper and lower two layers (over-under). Generally, there are more ways to implement the former side-by-side.

IV.The wiring of the difference pair should be close and parallel

The wiring mode of the difference pair should be properly close and parallel.  The so-called appropriate because it is close to the distance affect the differential impedance (differential impedance) value, this value is the important parameters of the differential design. The need to be parallel is also due to the consistency of the differential impedance. If the two line is far and close, the difference impedance will not agree, and the signal integrity (signal integrity) and the time delay (timing delay) will be affected.

V.A clock signal line with only one output terminal can not achieve a differential distribution line

It is meaningful to use a differential distribution line to be a signal source and a receiver or a differential signal. So it is impossible to use a differential distribution line for a clock signal with only one output end.

VI.Add a matching resistance between the receiver differential line pairs

A matching resistance is usually added to the matching resistance between the receiver differential line pair and the value of the differential impedance should be equal to the value of the differential impedance. So the quality of the signal will be better.

VII.signal integrity of high speed PCB design

Signal integrity is basically a problem of impedance matching. The factors that affect the impedance matching include signal source architecture and output impedance (output impedance), the characteristic impedance of the line, the characteristics of the load side, and the topology of topology. The way to solve the problem is to rely on the termination and the topology of adjusting the line.

VIII.Solving the contradiction between manual wiring and automatic wiring of high speed signal

At present, most of the automatic wiring devices of the strong wiring software have set constraints to control the way of winding and the number of holes. Each EDA company's winding engine capability and constraint conditions are sometimes quite different. For example, if there is enough hunting line control constraints (serpentine) winding, the ability to control the differential pair line spacing. Marvell PCB design training classes, training students to complete the Allegro complete medium difficulty PCB design project independently, for PCB design assistant engineer. This will affect whether the automatic wiring way is in line with the designer's idea. In addition, the difficulty of manually adjusting the wiring has an absolute relationship with the ability of the winding engine. For example, the pushing capacity of the walking line, the pushing capacity of the holes, and even the pushing capacity of the copper applying to the line. Therefore, the choice of a wiring engine with a strong ability of wiring is the solution.

IX.Solution of some theoretical conflicts in actual wiring

  • 1) basically, it is right to separate the mode / number of isolation. We should pay attention to signal alignment, try not to stride over the segmentation area (moat), and don't let the returning current path of power and signal become too large.
  • 2) the crystal oscillator is a simulated positive feedback oscillator circuit. If there is a stable oscillation signal, it must meet the specifications of loop gain and phase. The oscillation specification of analog signal is easily disturbed, even if ground guard traces is added, it may not completely isolate the interference. And too far away, the noise on the ground plane will also affect the positive feedback oscillating circuit. Therefore, it is necessary to close the distance between the crystal oscillator and the chip.
  • 3) indeed, there is a lot of conflict between high speed wiring and the requirements of EMI. But the basic principle is that the resistance capacitance or ferrite bead added by EMI can not cause some electrical characteristics of the signal to not conform to the specification. Therefore, it is best to solve or reduce the problem of EMI by arranging the line and PCB stacking techniques, such as the high speed signal in the inner layer. Finally, the resistance capacitance or ferrite bead is used to reduce the damage to the signal.

X.About test coupon

Test coupon is used to measure the characteristic impedance of the PCB board produced by TDR (Time Domain Reflectometer) to meet the design requirements. In general, there are two conditions for the impedance to be controlled by a single line and a differential pair. So, the line width and the line distance on the test coupon are the same as the lines to be controlled. The most important thing is to measure the location of the place at the time of measurement. In order to reduce the grounding wire (ground lead) of the inductance, TDR probe (probe) ground areas are usually very close to the local signal (probe tip), so test coupon measurement signal to ground distance and approach to meet the probe.

The above is the Common PCB Design Issues and Solutions for Circuit Board Novice(Part One), next issue: the Common PCB Design Issues and Solutions for Circuit Board Novice(Part Two) welcome to follow me!

XI.Relevant information about "Common PCB Design Issues and Solutions for Circuit Board Novice(Part One)"
About the article "Common PCB Design Issues and Solutions for Circuit Board Novice(Part Two)", If you have better ideas, don't hesitate to  write your thoughts in the following comment area. You also can find more articles about electronic semiconductor through Google search engine, or refer to the following related articles.

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