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High frequency PCB circuit design essentials.

High frequency PCB circuit design essentials.

high frequency pcb

1. How to choose PCB material for high frequency PCB?
 
For PCB board selection, it is necessary to strike a balance between meeting design requirements and mass production and cost. Design requirements include electrical and mechanical parts. In general, when designing very high-speed PCB board (frequency higher than GHz), this material problem will be more important. For example, in the commonly used FR-4 materials, the dielectric loss at several GHz will have a great impact on the signal attenuation, which may not be applicable. As far as electricity is concerned, we should 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 electromagnetic field of high-frequency signal, also known as crosstalk. Increase the distance between high-speed signal and analog signal, or add ground guard / Mount traces next to analog signal. Attention must also be paid 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 problem of impedance matching. The factors that affect the impedance matching include the structure of signal source and output impedance, the characteristic impedance of the wiring, the characteristics of the load end, and the topology architecture of the wiring. The solution is termination and routing adjustment.

4. How is the difference distribution line realized?
 
There are two points to pay attention to in the differential distribution line. One is that the length of the two lines should be as long as possible; the other is that the distance between the two lines (which is determined by the differential impedance) should always remain unchanged, that is, to keep parallel. There are two ways of parallel, one is that two lines run in the same routing layer, and the other is that two lines run on two adjacent layers (over under). Generally, there are many ways to implement the former side by side.
 
5. For a clock signal line with only one output, how to realize the difference distribution line?
 
When a differential distribution line is used, it must be that the signal source and receiver are also differential signals. Therefore, the difference distribution line cannot be used for clock signal with only one output.
 
6. Can a matching resistance be added between the differential pairs at the receiving end?
 
The matching resistance between differential pairs at the receiving end is usually added, and its value should be equal to the value of differential impedance. So the signal quality will be better.

7. Why should differential impedance lines be close and parallel?
 
The routing of differential pairs should be close and parallel. The so-called appropriate approach is because the distance will affect the differential impedance, which is an important parameter in the design of differential pairs. Parallel is also necessary because of the consistency of differential impedance. If the two lines are far or near, the differential impedance will be inconsistent, which will affect the signal integrity and timing delay.

8. How to deal with some theoretical conflicts in practical wiring
 
Basically, it is right to separate and isolate the A / D ground. It should be noted that the signal wiring should not cross the moat as far as possible, and the returning current path of power supply and signal should not be too large.
 
Crystal oscillator is an analog positive feedback oscillation circuit. In order to have a stable oscillation signal, the specification of loop gain and phase must be met. However, the oscillation specification of the analog signal is easy to be interfered, even if ground guard traces are added, it may not be able to completely isolate the interference. Moreover, if it is too far away, the noise on the ground plane will also affect the positive feedback oscillation circuit. Therefore, the distance between the crystal oscillator and the chip must be close.
 
Indeed, there are many conflicts between high-speed cabling and EMI requirements. But the basic principle is that the resistance and capacitance or ferrite beam added by EMI can not cause some electrical characteristics of the signal not to meet the specification. Therefore, it is better to solve or reduce EMI problems by arranging routing and PCB lamination, such as high-speed signal going through inner layer. Finally, resistance capacitance or ferrite beam is used to reduce the damage to the signal.

9. How to solve the contradiction between manual wiring and automatic wiring of high-speed signals?
 
At present, most of the strong automatic routing software have set constraints to control the winding mode and the number of vias. Sometimes, the setting items of winding engine capacity and constraints of various EDA companies are quite different. For example, whether there are enough constraints to control the serpentine winding way, whether to control the routing distance of differential pairs, etc. This will affect whether the routing mode of automatic routing can meet the designer's idea. In addition, the difficulty of manually adjusting the wiring is absolutely related to the ability of the winding engine. For example, the push ability of the wire, the push ability of the through hole, and even the push ability of the wire to the copper coating, etc. Therefore, choosing a router with strong winding engine is the solution.
 
10. About test coupon.
 
Test coupon is used to measure whether the characteristic impedance of PCB meets the design requirements by TDR (time domain reflector). Generally, there are two kinds of impedance to be controlled: single wire and differential pair. Therefore, the line width and line spacing (with differential pairs) on the test coupon should be the same as the lines to be controlled. The most important thing is the location of the ground point when measuring. In order to reduce the inductance value of the ground lead, the place where the TDR probe is grounded is usually very close to the place where the signal is measured (probe tip). Therefore, the distance and mode between the measuring signal point on the test coupon and the grounding point should conform to the probe used.

11. In the design of high-speed PCB, the blank area of signal layer can be coated with copper. How to distribute the copper coating of multiple signal layers on grounding and power supply?
 
Generally, the copper coating in the blank area is mostly grounded. The distance between the copper coating and the signal line should be paid attention to when copper is deposited beside the high-speed signal line, because the copper coating will reduce the characteristic impedance of the line a little bit. It should also be noted that the characteristic impedance of other layers should not be affected, such as in the structure of dual strip line.
 
12. Can the signal line above the power plane be used to calculate the characteristic impedance? Can the signal between the power supply and the ground plane be calculated with the stripline model?
 
Yes, when calculating the characteristic impedance, both the power plane and the ground plane must be regarded as the reference plane. For example, four layer PCB board: top layer power layer layer layer bottom layer, then the model of characteristic impedance of top layer is 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 meet the test requirements or not depends on whether the specifications of the test points meet the requirements of the test equipment. In addition, if the wiring is too dense and the specification of adding test points is relatively strict, it may not be possible to automatically add test points to each section of the line. Of course, the test points need to be manually supplemented.

14. Will the addition of test points affect the quality of high-speed signals?
 
Whether it will affect the signal quality depends on the way of adding test points and how fast the signal is. Basically, the additional test points (without the existing via or dip pin on-line) may be added online or pulled out of a small line from the line. The former is quite then added with a small capacitance on line, while the latter is a branch. Both of these two cases will have a little influence on the high-speed signal, and the degree of influence is related to the frequency speed and edge rate of the signal. The influence can be obtained by simulation. In principle, the smaller the test point, the shorter the branch, the better.
 
15. How should the ground wire between the boards be connected in a number of PCB components?
 
When the signal or power supply between each PCB board is connected to each other, for example, when a board has power or signal is sent to board B, there must be equal amount of current flowing from the formation to the a board (this is kirchioff current law). The current in this formation will flow back where the impedance is the least. Therefore, at the interface where the power supply or signal are connected, the number of pins allocated to the formation cannot be too small to reduce the impedance, so as to reduce the noise on the formation. In addition, the whole current loop, especially the part with large current, can be analyzed, and the grounding method of formation or ground wire can be adjusted to control the current routing (for example, low impedance is made somewhere to let most of the current go from this place), and the impact on other sensitive signals can be reduced.
 
16. Can you introduce some foreign technical books and data about high speed PCB design?
 
Nowadays, the application of high-speed digital circuits has communication network and calculator. In communication network, PCB board has been working at GHz up and down, and the number of layers is as many as 40 layers as I know. The computer related applications also have been improved by the chip. Whether it is a general PC or server, the maximum operating frequency on the board has reached 400MHz (such as Rambus). In response to the high-speed and high-density routing requirements, the demand of the process process of blind/buried vias, mircrovias and build up is also increasing. These design requirements are available to a large number of manufacturers.

17. Two characteristic impedance formulas are often referred to
 
Microstrip line z = {87 / [sqrt (ER + 1.41)]} ln [5.98h / (0.8W + T)], where W is the linewidth, t is the thickness of copper sheet, h is the distance from the trace to the reference plane, and Er is the dielectric constant of PCB material. This formula can only be applied when 0.1 < (w / h) < 2.0 and 1 < (ER) < 15.
 
Stripline z = [60 / sqrt (ER)] ln {4H / [0.67 π (T + 0.8W)]} where h is the distance between the two reference planes, and the trace is located in the middle of the two reference planes. This formula can only be applied when w / h < 0.35 and T / h < 0.25.

18. Can ground wire be added in the middle of differential signal line?
 
In general, ground wire cannot be added in the middle of differential signal. Because the application principle of differential signal is the most important point is to use the advantages of coupling between differential signals, such as flux cancellation, noise immunity ability and so on. If a ground wire is added in the middle, the coupling effect will be destroyed.
 
19. Does rigid flex board design need special design software and specifications? Where can we undertake this kind of PCB processing in China?
 
Flexible printed circuit can be designed with general PCB design software. The Gerber format is also used for FPC manufacturers. Because the manufacturing process is different from general PCB, each manufacturer will adjust the minimum line width, minimum wire spacing, minimum aperture (via) according to their manufacturing capacity. In addition, some copper sheets can be laid at the turning point of the flexible circuit board for reinforcement.
 
20. The principle of proper selection of PCB and shell grounding point is?
 
The selection principle of PCB and shell grounding point selection is to use chassis ground to provide low impedance path to return current and control the return current path. For example, in the vicinity of high-frequency devices or clock generators, the PCB floor and chassis ground can be connected by fixed screws to minimize the area of the entire current loop, that is, to reduce electromagnetic radiation.
 

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