1. How to choose PCB board?

PCB board selection must meet the design requirements and mass production and cost of the balance between. The design requirements include electrical and mechanical parts. This is usually important when designing very fast PCB boards (frequencies greater than GHz). For example, the fr-4 material commonly used today may not be suitable because the dielectric loss at several GHz has a great effect on signal attenuation. In the case of electricity, pay attention to the dielectric constant and dielectric loss 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 field, also known as Crosstalk. You can increase the distance between the high speed signal and the analog signal, or add ground guard/shunt traces to the analog signal. Also pay attention to the digital ground to analog ground noise interference.

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

Signal integrity is basically a matter of impedance matching. The factors that affect impedance matching include signal source architecture, output impedance, cable characteristic impedance, load side characteristic, and cable topology architecture. The solution is * terminaTIon and adjust the topology of the cable.

4. How to realize differential wiring?

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 (determined by the difference impedance) should always remain constant, that is, to keep parallel. There are two parallel modes: one is that the two lines run on the same side-by-side layer, and the other is that the two lines run on two adjacent layers of the upper and lower layers. Generally, the former side-by-side implementation is more common.

5. How to realize differential wiring for clock signal line with only one output terminal?

Want to use differential wiring must be signal source and receiving end are also differential signal is meaningful. So it is impossible to use differential wiring for a clock signal with only one output.

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

The matching resistance between the pair of differential lines 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 should the wiring of difference pairs be closest and parallel?

The wiring of difference pairs should be appropriately close and parallel. The proper height is due to the difference impedance, which is an important parameter in designing difference pairs. Parallelism is also required to maintain the consistency of the differential impedance. If the two lines are either far or near, the differential impedance will be inconsistent, which affects signal integrity and TIming delay.

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

(1). Basically, it is right to separate modules/numbers. Care should be taken not to cross the MOAT and not to let the power supply and signal return current path grow too large.

(2). Crystal oscillator is a simulated positive feedback oscillating circuit, and stable oscillating signals must meet the specifications of loop gain and phase, which are prone to interference, even with ground guard traces may not be able to completely isolate interference. And too far away, the noise on the ground plane will also affect the positive feedback oscillation circuit. Therefore, be sure to make the crystal oscillator and chip as close as possible.

(3). Indeed, there are many conflicts between high-speed wiring and EMI requirements. However, the basic principle is that due to the resistance capacitance or Ferrite Bead added by EMI, some electrical characteristics of the signal cannot be caused to fail to meet the specifications. Therefore, it is best to use the technique of arranging wiring and PCB stacking to solve or reduce EMI problems, such as high-speed signal lining. Finally, resistor capacitance or Ferrite Bead method was used to reduce the damage to the signal.

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

Nowadays, most of the automatic cabling devices in strong cabling software have set constraints to control the winding mode and the number of holes. EDA companies sometimes vary widely in setting the capabilities and constraints of winding engines. For example, whether there are enough constraints to control how serpenTIne lines wind, whether there are enough constraints to control the spacing of difference pairs, etc. This will affect whether the automatic wiring out of the wiring can conform to the designer’s idea. In addition, the difficulty of manual wiring adjustment is also absolutely related to the ability of the winding engine. For example, the wire pushing capacity, through the hole pushing capacity, and even the wire on copper coating pushing capacity and so on. So, choose a cabler with strong winding engine ability, it is the way to solve.

10. About Test Coupon.

The Test Coupon is used to measure whether the characteristic impedance of the PRODUCED PCB board meets the design requirements by using the Time Domain Reflectometer (TDR). Generally, the impedance to control is single line and difference pair of two cases. Therefore, the line width and line spacing (if differential) on the Test Coupon should be the same as the line being controlled. The most important thing is the location of the grounding point. In order to reduce the inductance value of ground lead, the ground point of TDR probe is usually very close to the probe tip. Therefore, the distance and method of measuring signal point and grounding point on test Coupon should conform to the used probe.

11. In high-speed PCB design, the blank area of the signal layer can be copper-coated, and how to distribute copper-coated on the grounding and power supply of multiple signal layers?

Generally in the blank area copper coating most of the case is grounded. Just pay attention to the distance between copper and the signal line when copper is applied next to the high-speed signal line, because the copper applied will reduce the characteristic impedance of the line. Also be careful not to affect the characteristic impedance of other layers, as in the dual stripline construction.

12. Can the signal line above the power supply plane be used to calculate the characteristic impedance using the microstrip line model? Can the signal between the power supply and the ground plane be calculated using a ribbon-line model?

Yes, both the power plane and the ground plane must be considered as reference planes when calculating the characteristic impedance. For example, four-layer board: top layer – power layer – stratum – bottom layer. In this case, the model of top layer’s wiring characteristic impedance is a microstrip line model with power plane as reference plane.

13. Can test points automatically generated by software on high density PCB meet the test requirements of mass production in general?

Whether the test points generated automatically by general software can meet the test needs depends on whether the specifications of the added test points meet the requirements of the testing machine. In addition, if the wiring is too dense and the specification of adding test points is strict, it may not be able to automatically add test points to each section of the line, of course, you need to manually complete the test place.

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

Whether it affects the signal quality depends on how the test points are added and how fast the signal is. Basically, additional test points (not via or DIP pin as test points) can be added to the line or pulled out of the line. The former is equivalent to adding a very small capacitor on the line, the latter is an extra branch. Both of these two conditions have more or less influence on high-speed signals, and the degree of influence is related to the frequency speed and edge rate of signal. The influence can be obtained through 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 system, how to connect the ground between the boards?

When the signal or power supply between each PCB board is connected to each other, for example, A board has power supply or signal to B board, there must be an equal amount of current from the floor flow back to A board (this is Kirchoff current law). The current in this layer will find its way back to the lowest impedance. Therefore, the number of pins assigned to the formation should not be too low at each interface, either power or signal connection, to reduce impedance and thus reduce formation noise. It is also possible to analyze the entire current loop, especially the larger part of the current, and adjust the connection of the ground or ground to control the flow of the current (for example, to create a low impedance in one place so that most of the current flows through that place), reducing the impact on other more sensitive signals.