The components are moving towards high-speed low consumption of small volume high anti-interference. This development trend has made many new requirements for the design of printed circuit boards.
The PCB design is an important stage of electronic product design. When the electrical schematic is designed, according to the structural requirements, several functional boards are determined according to the functional partition, and determine the external size and installation of each function board, and must also be simultaneously Consider debugging, maintenance convenience, and factors such as shielding, heat dissipation, EMI performance.
Engineers need engineers to determine layout wiring solutions, determine critical circuits and signal lines and wiring methods, and the principle of wiring should be followed.
Several stages of the PCB design process must be checked, analyzed, and modified. After the entire wiring is completed, it can take the processing.
I. Introduction
For a long time, designers often spend energy on the verification of procedures, electric principles, parameter redundancy, etc. Product performance problem.
PCB design principles involve many aspects, including basic principles, anti-interference, electromagnetic compatibility, safety protection, and more. For these aspects, especially in high-frequency circuits (especially in microwave high-frequency circuits), the lack of related concepts often leads to failures of the entire R & D project.
Many people still stay on the basis of "Placing the electrical principle of the conductor" and even think that "PCB design is structural, process and improvement of production efficiency."
Many engineers have not fully recognized that the link should be particularly focused on product design, and mistakenly spend energy in choosing high performance components, the result is a sharp rise in cost, and the performance improvement is minimal.
Second, high speed PCB design
In the product engineering, the PCB design occupies a very important location, especially in high frequency electrical design.
There are some universal rules that will treat as a general guideline. The design principles and techniques of the PCB of the high-frequency circuit can be applied to the design, and the design success rate can be greatly improved.
(1) Cabling design principle of high speed circuit PCB
1. Minimize logical fan, it is best to bring only one load.
2. Avoid using through holes as much as possible between the output of the high speed signal line and the receiving end to avoid the cross cross cross. Especially the clock signal line requires special attention.
3. The two adjacent two-layer signal lines should be perpendicular to each other to avoid corners.
4. The load resistance should be as close as possible to the receiving end as close as possible to the receiving end.
5. To ensure minimum reflection, all open routes (or lines without end matching) must meet the formula:
Lopen - Excise (inches)
Trise - Signal Rising Time (NS)
The propagation delay of the TPD-line (0.188ns / in - pressing line characteristics).
Several high speed logic circuit rise time typical:
6. When the length of the open route exceeds the value required by the above equation, a series damped resistor should be used, and the series-connected electrical resistors should be connected to the output of the output as much as possible.
7. To ensure that analog circuits and digital circuits are separated, AGND and DGND must be connected together via an inductor or magnetic bead and as close as possible to the location of the A / D converter.
8. Ensure the sufficient decoupling of the power supply.
9. It is best to use surface mount resistance and capacitance.
(2) Bypass and decoupling
1. Before selecting a decoupling capacitor, first calculate the resonant frequency requirements required to filter the high frequency current.
2. Big than the self-harmonious frequency, the capacitor will turn into an inductance, thereby losing decoupling capacitance. It should be noted that some logic circuits have spectral energy than commonly used decoupling capacitance itself.
3. The container itself has a resonant frequency called a self-harmonic frequency. If you want to filter out high frequency
4. The desired capacitance value is calculated based on the RF energy, the rise time of the switching circuit, and the frequency range of the particular focus, do not use speculation or in accordance with previous consistent use.
5. Calculate the resonant frequency of the ground and the power plane. The decoupling capacitors that are constructed in this two plane can achieve the maximum benefit.
6. For high-speed components and rich RF bandwidth energy, multiple capacitors should be used in parallel to remove RF energy of largeband width. It should also be noted that when in high frequency, large capacitance changes into inductance, small capacitance maintains capacitive, and at a particular frequency will form an LC resonant circuit, resulting in an infinitely impedance, thus completely lose the bypass effect, if there is This situation occurs, which is more effective using a single capacitor.
7. Set the parallel capacitance on the side of the circuit board all power input connector, and the rise time is faster than the element of the element of the 3 ns.
8. At the diagonal direction of the PCB power input and the wrench, a large-capacity capacitor should be used to ensure the current change generated when the circuit is switched. Decoupling capacitors for other circuits should also be considered. The larger the operating current, the greater the capacitance required. To reduce the pulsation of the voltage and current, improve the stability of the system. Therefore, the decoupling capacitance shoulder to the double effect of retaining and continuation.
9. If too much decoupling capacitors are used, when the power is powered on, a large amount of current is absorbed from the power supply, so a group of large capacitors should be placed in the power output.
(3) Impedance transformation and matching
1. In the low frequency circuit, the concept of matching is quite important (the load impedance is equal to the excitation source). In the high frequency circuit, the matching of the signal line terminal is more important:
On the one hand, ZL = ZC is required to ensure that there is an endless wave along the line; on the other hand, in order to obtain maximum power, the signal line input should be conjugated when the signal is connected to the excitation source. Therefore, the matching of the working performance of the microwave circuit has been directly affected. visible:
If the terminal does not match, reflection and standing waves are generated on the signal line, resulting in a decrease in load power (high power standing wave will generate fire in the belly ").
Due to the presence of reflection waves, the excitation source will have an adverse effect, resulting in a decrease in operating frequency and output power stability.
However, the actual load impedance is not necessarily the same as the signal line characteristic impedance, and the signal line and the excitation source impedance are not constant, and thus the impedance matching technique must be known.
2. λ / 4 impedance converter
When the signal line length L = λ / 4, ie, βL = π / 2,
ZIN = ZC2 / ZL
The above formula indicates that its impedance will have significant changes after the λ / 4PCB transmission line transform. It can be understood that when ZL does not match, the retrofit of the PCB transmission line can be utilized to achieve a matching purpose. For the two-stage characteristic impedances, the PCB transmission lines of Z'c, Z "C, respectively, and the PCB transmission line can be used to achieve the purpose of matching Z'c and Z" C.
It should be noted that the λ / 4 impedance converter matches two-stage impedance different PCB transmission lines that have a narrow operating frequency.
3. Single branch short route match
The PCB transmission line impedance can be used in the form of a PCB transmission line to change the PCB transmission line impedance to change the PCB transmission line impedance.
(4) PCB layering
High-frequency circuits tend to be high integration and large wiring density.
With multi-layer boards, it is not necessary for the wiring to reduce the effective means of reducing interference. The reasonable selection layers can greatly reduce the size of the printed board, which can make full use of the intermediate layer to set the shield, which can better achieve near ground, energy to reduce Parasitic inductance, can effectively shorten the transmission length of the signal, can reduce cross-interference between the signals, etc.
All of this is advantageous for the reliability of the high-frequency circuit, and there is information to show that the four-layer board is 20 dB than the noise of the double panel. However, the number of plate layers is complicated, the higher the cost.
(5) Power isolation and ground division
Different functions or different required circuit wirings often require power isolation and ground division.
Such as analog circuits and digital circuits, weak signal circuits, and strong signal circuits, sensitive circuits (PLL, low jitter triggering, etc.), and other circuits, etc., the interference should be minimized, and the circuit can achieve the expected indicator requirements.
basic requirements:
1. The power supply layer or formation of different regions should be connected together at the power inlet, usually a tree structure or a finger structure, a ground line division method of different functional circuits, and the edge of the segmentation and the plates shall not be less than 2 mm.
2. Different kinds of power regions and regional domains cannot cross each other
3. Trench and bridge. Due to the partition segmentation of the ground plane, the signal transmission between each function circuit returns the discontinuous circuit, in order to ensure signal, power supply, and ground connection, in addition to the transformer isolation (not transmitting DC signals), optocoupler isolation ( It is difficult to transmit high frequency), and there is a common bridging method. "Bridge" is actually a gap on the trench, and there is only one thing, the signal line, power supply, and ground are all over the tunnel as shown. When using this method, if it is a multi-point grounding system (all high speed design is, it is best to pick up the two sides of the bridge to the casing.
Third, conclusion
In product engineering, PCB is designed to occupy a very important location, especially in high frequency design, the same principle design, the same components, and different people have different results.
There are many principles that are difficult to achieve in the project, or others can achieve, so others can't realize, so it is not difficult to do a PCB board, but it is not easy to do a PCB board. Best.
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