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Microwave Tech

Microwave Tech

Microwave Tech

Microwave Tech

High-Frequency PCB Circuit Design of Common Problems(2)
2021-08-02
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Author:Fanny

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 digitization of signal transmission force PCB to micro-hole and buried/blind hole, fine conductor, medium layer uniform thin, high frequency, high-density multilayer PCB design technology have become an important research field.  Based on years of experience in hardware design, the author summarizes some design skills and matters needing the attention of high-frequency PCB for your reference.

High-frequency PCB

13. Does software that automatically generates test points on a high-density printed board generally meet the testing requirements for mass production?

Whether the test points automatically generated by general software meet the testing requirements must be determined by whether the specification of the test points meets 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 possible to automatically add test points to each segment of the line, of course, you need to manually fill in the areas to be tested.


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

Whether it will affect the quality of the signal depends on how you add the test points and how fast the signal is. Essentially additional test points (not via or DIP pins existing on the line as test points) may be added to the line or pulled a short line from the line. The former is equivalent to adding a small capacitor to the line, while the latter is adding a branch. Both of these situations will have some influence on the high-speed signal, the degree of influence is related to the frequency speed and the signal edge rate. The impact size can be learned through simulation. In principle, the smaller the test points, the better (but also to meet the requirements of the test machine) the shorter the branch.


15. Some PCB systems, how to connect the ground between the boards?

When the signal or power supply between each PCB board is in action, for example, there is a power supply onboard A or the signal is sent to board B, there must be an equal amount of current flowing back to board A from the ground layer (Kirchoff current law). The current flowing through this formation will find its way back to where the impedance is lowest. Therefore, the number of pins assigned to the formation should not be too small to reduce impedance at each interface, whether it is a power supply or a signal connection, to reduce the noise on the formation. Alternatively, it is possible to analyze the entire current loop, especially the large part of the current, and adjust the ground or ground connection to control the current movement (for example, to create a low impedance at a point from which most of the current will flow), reducing the impact on other more sensitive signals.


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

High-speed digital circuits are now used in communication networks and calculators and other related fields. In terms of communication network, the working frequency of the PCB board is up to GHz, and the number of layers as far as I know is up to 40 layers. Calculator-related applications also because of the progress of chips, whether the general PC or Server (Server), the highest working frequency on the board has reached 400MHz (such as Rambus) above. The demand for blind/ Buried vias, Mircrovias, and build-up processes is increasing due to the high speed and high-density wiring demand. These design requirements can be mass-produced by manufacturers.


17. Two commonly referred characteristic impedance formulas:

Z={87/[SQRT (Er+1.41)]}ln[5.98h /(0.8W+T)] where W is the cable width, T is the copper skin thickness of the cable, H is the distance between the cable and the reference plane, Er is the dielectric constant of PCB board. This formula must be used within 0.1< (W/H)< 2.0 and 1 & lt; (Er)< 15 is the case.

Stripline Z=[60/ SQRT (Er)]ln{4H/[0.67π(T+0.8W)]} where H is the distance between the two reference planes and the stripline is located in the middle of the two reference planes. This formula must be used in W/H< And 0.35 T/H< The value is 0.25.


18. Can ground wire be added in the middle of the differential signal line?

The middle of the differential signal is generally not to add the ground. The most important application principle of the differential signal is to take advantage of the benefits of coupling between differential signals, such as flux Cancellation and noise immunity. If you put a ground wire in the middle, you break the coupling effect.


19. Does the rigid adagio design need special design software and specifications? Where can domestic undertake this kind of circuit board processing?

Flexible Printed circuits can be designed using the same software that is used to design PCBs. Same for FPC manufacturers in Gerber format. Because the manufacturing process is different from the common PCB, each manufacturer will have its limits on the minimum line width, minimum line spacing, and minimum aperture (VIA) based on their manufacturing capacity. In addition, copper can be laid at the end of the flexible circuit board to reinforce. As for the manufacturer can go online "FPC" when the keyword query should be found.


20. What are the principles for the proper selection of PCB and shell grounding points?

The principle of PCB and enclosure ground point selection is to use chassis ground to provide a low impedance path to the returning current and a path to control the returning current. For example, the PCB ground can be connected to the chassis ground using fixing screws, usually near a high-frequency device or clock generator, minimize the overall current loop area and thus reduce electromagnetic radiation.


21. Circuit board debugging should be started from what aspects?

In the case of digital circuits, the first three things are determined in sequence: 1. Make sure that all the power values meet the design requirements. Some multi-power systems may require a specification for the sequence and speed of connections between certain power supplies. 2. Ensure that all clock signal frequencies are working properly and there is no non-monotonic problem on the edge of the signal. 3. Check whether the reset signal meets the specifications. If this is all right, the chip should send a signal for the first cycle. Next, debug the system based on the operating principle and bus protocol.


22. In the case of the circuit board size, is fixed, if the design needs to accommodate more functions, it is often necessary to improve the PCB wiring density, but this may lead to mutual interference enhancement of wiring, wiring is too thin at the same time impedance can not be reduced, please expert introduction in high speed (> 100MHz) high-density PCB design techniques?

Crosstalk interference is of particular concern when designing high-speed, high-density PCB, as it has a significant impact on timing and signal integrity. Here are a few points to note: Control the continuity and matching of the characteristic impedance of the route. The size of the spacing between lines. The spacing usually seen is twice the line width. Through simulation, we can know the influence of line spacing on timing and signal integrity and find out the tolerable minimum spacing. Different chip signals can have different results.


23.The filter at the analog power supply is often an LC circuit. But why is LC sometimes worse than RC filtering?

The comparison of LC and RC filtering effects must consider whether the frequency band to be filtered and the inductance value is appropriate. The value of reactance is related to the value and frequency of the inductance. If the noise frequency of the power supply is low and the inductance value is not large enough, then the filtering effect may not be as good as the RC. However, the cost of using RC filtering is that the resistance itself will consume energy, and the efficiency is poor, and attention should be paid to the power that the selected resistance can withstand.


24.What is the method of selecting inductance and capacitance value when filtering?

In addition to the noise frequency to be filtered out, the inductance value should also be considered in the instantaneous current response capacity. If there is a chance that the LC output will need to output a large current instantaneously, then the inductance value is too large to block the speed of the current flowing through the inductor, increasing the ripple noise. The capacitance value is related to the allowable ripple noise specification value. The smaller the ripple noise, the larger the capacitance. The ESR/ESL of the capacitor also has an impact. In addition, if the LC is placed on the output of a switching regulator power supply, note the effect of the pole/zero generated by the LC on the stability of the negative feedback control loop. iPcb is a High-Tech Manufacturing Enterprises focusing on the development and production of high precision PCB.