High frequency PCB circuit design common 66 problems

With the rapid development of electronic technology and the wide application of wireless communication technology in various fields, high frequency, high speed and high density have gradually become one of the significant development trends of modern electronic products. High frequency and high speed digitalization of signal transmission force PCB to go towards micro hole and buried/blind hole, fine wire, uniform and thin dielectric layer. High frequency, high speed and high density multilayer PCB design technology has become an important research field.

High frequency, high speed and high density multilayer PCB design technology

1. How to choose PCB board?

The choice of PCB panel must meet the design requirements and the production and cost of the balance. Design requirements include electrical and mechanical parts. This material issue is usually important when designing very high speed PCB boards (frequencies greater than GHz). For example, the current commonly used fr-4 material, dielectric loss at several GHz frequencies, will have a great impact on signal attenuation and may not be applicable. In the case of electricity, attention should be paid to whether the dielectric constant and dielectric loss are appropriate at the designed frequency.

2. How to avoid high-frequency interference?

The basic idea to avoid high frequency interference is to minimize the interference of high frequency signal electromagnetic field, which is called Crosstalk. This may be to widen the distance between the high-speed signal and the analogue, or to add ground guard/shunt traces to the analogue. Also note the digital ground to analog ground noise interference.

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

Signal integrity is basically a matter of impedance matching. And the influencing factors of impedance matching with the construction of the signal source and the output impedance (the output impedance), line characteristic impedance, the load side, walk line topology architecture (topology), etc. The solution is based on termination and the topology of the alignment.

4. How is the differential wiring realized?

There are two points to note in the wiring of a difference pair. 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 (which is determined by the difference impedance) should remain constant, that is, should be parallel. There are two ways to do this. One is for two lines to be side-by-side, and the other is for two lines to be over-under. In general, the former is side-by-side.

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

It makes sense to use differential wiring if the signal source and receiver are also differential signals. Therefore, differential wiring cannot be used for clock signals with only one output.

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

The matching resistance between the difference pairs at the receiving end is usually added and should be equal to the value of the difference impedance. So the signal quality is better.

7. Why should the wiring of difference pairs be close and parallel?

The wiring of differential pairs should be suitably close and parallel. So-called proper near because the spacing will affect the differential impedance (differential impedance) value, this value is to design the important parameters of a differential pair. 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 will affect signal integrity and timing delay.

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

Basically, it’s good to separate modules/Numbers. Returning to the returning current path of the supply and signal is important to keep the returning current path as close to the returning moat as possible.

Crystal oscillations are analog positive feedback oscillating circuits. In order to have stable oscillating signals, we must meet the specifications of loop gain and phase. However, these oscillating specifications of analog signals are very likely to be disturbed, and even with ground guard traces, we may not be able to completely isolate these disturbances. And too far away, the noise on the ground plane will affect the positive feedback oscillation circuit. Therefore, it is necessary to bring the distance between the crystal oscillator and the chip closer.

It is true that high-speed wiring has many conflicts with EMI requirements. But the basic principle is added by EMI resistance or ferrite bead, do not make some electrical characteristics of the signal is not in conformity with the specification. Therefore, it is better to solve or reduce EMI problems by arranging wiring and PCB lamination, such as high-speed signal lining. The last to use resistance or ferrite bead, in order to reduce the harm of the signal.

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

Nowadays, most of the strong wiring software’s automatic wiring devices have set constraints to control the winding mode and the number of holes. The winding engine capabilities and constraints set by EDA companies sometimes vary widely. For example, whether there are enough constraints to control the serpentine winding mode, whether the line spacing of difference pairs can be controlled, etc. This will affect the automatic wiring out of the wiring can be in line with the designer’s idea. In addition, the difficulty of manually adjusting the wiring is also directly related to the ability of the winding engine. For example, the ability to push the wire, the ability to push through the hole, and even the ability to push the wire to copper, and so on. Therefore, the choice of a winding engine with strong wiring, is the solution.

10. Test coupon.

The test coupon is used for TDR (Time Domain Reflectometer) production by measuring the characteristic impedance of PCB board meets the design requirements. The impedance to be controlled generally has a single line and a differential pair. Therefore, the line width and spacing on the test coupon (with difference pairs) should be the same as the line you want to control. The most important thing is the location of the ground at the time of measurement. In order to reduce the inductance of ground lead, the ground of TDR probe is usually very close to the probe tip. Therefore, the distance and manner of the measured signal on the test coupon should match the used probe.

11. In high-speed PCB design, the blank area of the signal layer can be copper coated, and how should the copper coating of multiple signal layers be distributed on the ground and the power supply?

In general, the copper coating in the blank area is mostly grounded. However, when applying copper to the high-speed signal line, pay attention to the distance between the copper and the signal line, because the copper will reduce the characteristic impedance of the line a little. It is also important not to affect the characteristic impedance of its layer, such as in the dual strip line structure.

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 stripline model?

Yes, both the power plane and the ground plane must be considered as reference planes when calculating the characteristic impedance. For example, the four-layer plate: top layer – power layer – stratum – bottom layer. At this time, the model of characteristic impedance of top layer routing is a microstrip line model with the power plane as the reference plane.

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

Whether the test points automatically generated by general software meet the test requirements must be determined by whether the specifications for adding test points meet the requirements of test machines and tools. In addition, if the wiring is too dense and the specification of adding test points is relatively strict, it may be impossible to add test points to each segment of the line automatically. Of course, the place to be tested needs to be completed manually.

14. Will adding test points affect the quality of high-speed signal?

Whether or not this affects signal quality depends on how the test points are added and how fast the signal is. Basically additional test points (without the via or DIP pin) may be added to the line or pulled out of the line. The former is the equivalent of adding a tiny capacitor to the wire, while the latter has an extra branch. Both of these conditions will have some influence on the high-speed signal to some extent, which is related to the frequency speed of the signal and the edge rate of the signal. The impact size can be determined by simulation. In principle, the smaller the test points, the better (and, of course, meet the requirements of the test machine), and the shorter the branch, the better.

15. Several PCBS make up the system. How should the ground wires between the boards be connected?

When the signal or power supply of each PCB board is connected with each other, for example, when the power supply or signal of board A is sent to board B, there must be an equal amount of current flowing from the ground layer back to board A (Kirchoff current law). The current in this formation will find its way back to where the impedance is least. Therefore, the number of pins assigned to the formation at all interfaces, whether power supply or signal interconnection, should not be too small to reduce impedance and thus reduce noise at the formation. In addition, it is possible to analyze the entire current loop, especially the part with high current, and adjust the connection method of the formation or ground wire to control the current travel (for example, to create a low impedance somewhere and let most of the current go from there) to reduce the impact on other more sensitive signals.

16. Can you introduce some foreign technical books and data about high-speed PCB design?

Applications of high – speed digital circuits include communication networks and calculators. In terms of communication network, the working frequency of PCB board has reached up to GHz, with as many as 40 layers as far as I know. Calculator related applications also because of the progress of chip, whether it is ordinary PC or Server (Server), the highest frequency of work on the board has reached more than 400MHz (such as Rambus). With the high-speed and high-density wiring demand, the demands for blind/buried vias, mircrovias and build-up manufacturing processes are also increasing. These design requirements can be mass produced by the manufacturer.

17. Two commonly referred characteristic impedance formulas:

Microstrip Z={87/

[SQRT (Er+1.41)]}ln[5.98h /(0.8w +T)], where W is the line width, T is the copper thickness of the line, H is the distance from the line to the reference plane, and Er is the dielectric constant of PCB sheet. This formula can only be applied when 0.1<(W/H)<2.0 and 1<(Er)<15.

Stripline Z=[60/ SQRT (Er)]ln{4H/[0.67pi (T+ 0.8w)]} where H is the distance between the two reference planes and the alignment is 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 a ground wire be added in the middle of the differential signal line?

The difference signal generally cannot add ground wire in the middle. Because of the difference signal the application principle of the most important thing is using the difference signal between the benefits of mutual coupling (coupling), such as the flux cancellation, anti noise (noise immunity) ability, etc. If a ground wire is added in the middle, the coupling effect will be destroyed.

19. Does rigid and flexible board design need special design software and specifications? Domestic where can undertake this kind of circuit board processing?

We can use software for general PCB designs to design Flexible Printed Circuit. Also in Gerber format for FPC manufacturers. Since the manufacturing process is different from that of a normal PCB, manufacturers will have restrictions on the minimum wire-width, minimum wire-spacing, and minimum aperture (via) depending on their manufacturing capability. In addition, the flexible circuit board in the turning point spread some copper to be reinforced. As for the manufacturer can be online “FPC” when the keyword query should be able to be found.

20. What is the principle of selecting the proper grounding point of PCB and enclosure?

The principle of selecting the junction site of PCB and casing is to provide a low-impedance path for returning current and to control the return current. For example, a PCB’s ground can be connected to a scaffold ground by a stationary screw, typically near a high-frequency device or clock generator, to minimize the entire current loop area and thereby reduce electromagnetic radiation.

21, circuit board DEBUG should start from what aspects?

In the case of digital circuits, three things are first identified in sequence: 1. Make sure that all power supply values are the size required for the design. Some systems with multiple power sources may require some specification of the order and speed of connections between certain power sources. 2. Verify that all clock tones are working properly and that there is no non-mono tones at the edge of the signal. 3. Confirm whether the reset signal meets the requirements of the specification. If all is well, the chip should signal the first cycle. Next, debug according to the system operation principle and bus protocol.

22. If the size of the circuit board is fixed, it is often necessary to improve the PCB routing density if more functions are to be included in the design. However, this may lead to the enhancement of mutual interference of the wiring, and at the same time, the wiring is too thin and the impedance cannot be reduced.

When designing high-speed and high-density PCB, special attention should be paid to crosstalk interference because it greatly affects timing and signal integrity. Here are a few things to note:

Control the continuity and matching of line characteristic impedance.

The size of the line spacing. The spacing commonly seen is twice the line width. The effect of routing spacing on timing and signal integrity can be known through simulation to find the minimum tolerable spacing. Different chip signals may have different results.

Select the appropriate end-to-end mode.

Avoid the upper and lower adjacent two layers of the same direction of the line, and even the upper and lower lines overlap, because this crosstalk than the same layer adjacent to the line is bigger.

Use blind/buried via to increase line area. However, the production cost of PCB board will increase. In practice, it is difficult to achieve full parallel and equal length, but try to do it.

In addition, differential and common-mode terminations can be reserved to mitigate the impact on timing and signal integrity.

23. Frequent filtering at the analog power supply