1, how to choose PCB circuit board?
Select the PCB circuit board must meet the design requirements and can achieve the balance between the cost of production and cost. Design requirements include electrical and institutional two parts. Usually in the design of very high-speed PCB board (greater than GHz frequency) when the material problem will be more important. For example, now commonly used FR-4 material, in the case of a few GHz frequency of dielectric loss (dielectric loss) will have a significant impact on signal attenuation, may not apply. For electrical purposes, it is necessary to pay attention to the dielectric constant (dielectric constant) and dielectric loss in the design of the frequency is practical.
2, how to avoid high frequency interference?
The basic idea of avoiding high frequency interference is to reduce the interference of electromagnetic field of high frequency signal, which is called crosstalk (Crosstalk). Can be used to pull the high speed signal and the distance between analog signals, or add ground guard/shunt traces next to the analog signal. Also pay attention to the digital simulation of the noise interference.
3, in the high-speed design, how to solve the problem of signal integrity?
Signal integrity is essentially the problem of impedance matching. The factors that affect the impedance matching are the structure of the signal source and the output impedance (output impedance), the characteristic impedance of the line, the characteristics of the load side, the topology of the topology. The way to solve the problem is to connect the terminal (termination) and adjust the topology of the line.
4, how to achieve the difference distribution line?
Pairs of differential wiring should pay attention to two points, first, the length of the two lines to be as long as possible, and the other is the distance between the two lines (distance from the differential impedance) to remain unchanged, that is, to keep the parallel. There are two parallel ways, one for the two lines in the same line layer (side-by-side), one for the two lines in the upper and lower adjacent to the two layer (over-under). Generally before the side-by-side to achieve more.
5, for only one output clock signal line, how to achieve the difference distribution line?
To use the difference between the distribution line must be the source and the receiver is also a difference signal meaningful. So the clock signal with only one output is unable to use the difference distribution.
6, the receiver can be added to the difference between the lines of a matching resistor?
The matching resistance between the receiving terminals is usually added, and the value is equal to the value of the differential impedance. This signal quality will be better.
7, why the differential pair of wiring to close and parallel?
The wiring of the differential pair should be 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. It is also necessary to maintain the consistency of the differential impedance. If the two lines are far and near, the differential impedance will be inconsistent, it will affect the signal integrity (signal integrity) and time delay (timing).
8, how to deal with the actual wiring of some of the theory of conflict
(1) it is true that the module / number partition is right. It is important to note that the signal line as far as possible not to cross the path (moat), as well as do not let the power supply and signal return current path (returning current) becomes too large.
(2) crystal is positive feedback oscillation circuit simulation, have stable oscillation signal, gain and loop must meet the phase specification, and the specification of this oscillation analog signal is vulnerable to interference, even with the ground guard traces may not be completely isolated interference. And far away from the ground plane noise will also affect the positive feedback oscillation circuit. Therefore, we must make the crystal and the distance into the chip may be close.
(3) there is indeed a lot of conflict between high-speed wiring and EMI requirements. But the basic principle is due to the addition of EMI resistance capacitance or ferrite bead, can not cause some of the electrical characteristics of the signal does not meet the specification. So, it is best to use the arrangement of the line and PCB stack skills to solve or reduce the problem of EMI, such as high-speed signal to go inside. Finally, the resistance capacitance or ferrite bead is used to reduce the damage to the signal.
9. How to solve the contradiction between manual wiring and automatic wiring of high - speed signal
At present, the majority of the automatic routing software has a set of constraints to control the winding and the number of vias. The company's ability to bypass the EDA engine and the constraints of the project is sometimes very far apart. For example, if there is enough hunting line control constraints (serpentine) winding, the ability to control the differential pair line spacing. This will affect the way the wiring can automatically meet the designer's ideas. In addition, the difficulty of manually adjusting the wiring is also associated with the ability of the winding engine is absolutely. Such as the ability to push the line, the ability to push through the hole, and even the line of copper pushing capacity, etc.. Therefore, the choice of a winding engine capacity of the router, is the solution.
10, about test coupon.
Test coupon is used to TDR (Time Domain Reflectometer) to measure the characteristics of the production of PCB board to meet the design requirements of the characteristics of the. Generally, there are two kinds of impedance: single line and differential pair. Therefore, the test coupon on the line width and line spacing (with differential time) and the line to be controlled. The most important thing is to measure the location of the ground. 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.
11, in the high-speed PCB design, the signal layer can be deposited in the blank area of copper, and a number of signal layer deposited on the ground and the power supply should be how to allocate?
In the blank area