The third is the EMI generated by the right-angle tip.
Differential Signal is more and more widely used in high-speed circuit design, and the most critical signal in the circuit often adopts a differential structure design. Definition: In layman's terms, it means that the driver sends two equal-value, anti-phase Signal, the receiving end judges the logic state "0" or "1" by comparing the difference between these two voltages. The pair of traces that carry differential signals is called a differential trace.
Compared with ordinary single-ended signal routing, the most obvious advantages of differential signals are reflected in the following three aspects:
a. Strong anti-interference ability, because the coupling between the two differential lines is very good, when there is noise interference from the outside world, they are coupled to the two lines almost at the same time, and the receiving end only cares about the difference between the two signals , so the external common mode noise can be completely canceled.
b. It can effectively suppress EMI. For the same reason, because the polarities of the two signals are opposite, the electromagnetic fields radiated by them can cancel each other out. The tighter the coupling, the less the electromagnetic energy released to the outside world.
c. Accurate timing positioning, because the switching change of the differential signal is located at the intersection of the two signals, unlike ordinary single-ended signals that rely on two high and low threshold voltages to judge, so it is less affected by the process and temperature, and can partially reduce the timing. error, but also more suitable for circuits with low amplitude signals. The currently popular LVDS (low voltage differential signaling) refers to this small-amplitude differential signaling technology.
For PCB engineers, the most concerned thing is how to ensure that these advantages of differential routing can be fully utilized in actual routing. Perhaps anyone who has been in touch with Layout will understand the general requirements of differential routing, which is "equal length and equal distance".
The equal length is to ensure that the two differential signals keep opposite polarities at all times and reduce the common mode component; the equal distance is mainly to ensure that the differential impedances of the two are consistent and reduce reflections. "The principle of as close as possible to the stars" is sometimes one of the requirements for differential routing.
4. Serpentine line:
Serpentine lines are a type of wiring method often used in Layout. Its main purpose is to adjust the delay to meet the system timing design requirements. Designers must first understand that serpentine lines will destroy signal quality and change transmission delay, and should be avoided as much as possible when wiring. However, in actual design, in order to ensure that the signals have sufficient hold time, or to reduce the time offset between the same group of signals, it is often necessary to deliberately perform winding.
be careful:
The differential signal lines that appear in pairs are generally routed in parallel, and as few holes as possible are drilled. When holes must be punched, the two lines should be punched together to achieve impedance matching. A group of buses with the same attribute should be routed side by side as much as possible to achieve the same length as possible. The vias drawn from the patch pad should be as far away from the pad as possible.
(3) Common rules for wiring
1. Direction control rules for wiring:
That is, the wiring directions of adjacent layers form an orthogonal structure. Avoid running different signal lines in the same direction on adjacent layers to reduce unnecessary crosstalk between layers; when it is difficult to avoid this situation due to board structure limitations (such as some backplanes), especially when the signal rate is high, It should be considered to isolate each wiring layer with a ground plane, and isolate each signal line with a ground signal line.
2. Open-loop inspection rules for wiring:
Generally, the wiring (Dangling Line) floating at one end is not allowed, mainly to avoid the "antenna effect" and reduce unnecessary interference radiation and reception, otherwise it may bring unpredictable results.
3. Yang resistance matching check rules:
The wiring width of the same network should be consistent. The change of line width will cause uneven line characteristic impedance. When the transmission speed is high, reflection will occur. This situation should be avoided as much as possible in the design. Under certain conditions, such as connector lead-out wires, BGA package lead-out wires with similar structures may not be able to avoid changes in line width, and the effective length of the inconsistent part in the middle should be minimized.
4. Trace length control rules:
That is, the short-line rule, the wiring length should be kept as short as possible during design to reduce the interference caused by too long wiring, especially some important signal lines, such as clock lines, must place their oscillators very close to the device The place. In the case of driving multiple devices, the network topology should be decided according to the specific situation.
5. Chamfering rules:
Acute angles and right angles should be avoided in PCB design, resulting in unnecessary radiation and poor process performance.
6. Device decoupling rules:
A. Add the necessary decoupling capacitor on the printed board to filter out the interference signal on the power supply and stabilize the power signal. In multi-layer boards, the position of the decoupling capacitor is generally not very demanding, but for double-layer boards, the layout of the decoupling capacitor and the wiring method of the power supply will directly affect the stability of the entire system, and sometimes even affect the design. success or failure.
B. In the double-layer board design, the current should generally be filtered by the filter capacitor before being used by the device.
C. In high-speed circuit design, the correct use of decoupling capacitors is related to the stability of the entire board.
7. Device layout partition 1 layering rules:
A. The main purpose is to prevent mutual interference between modules with different operating frequencies, and at the same time shorten the wiring length of the high-frequency part as much as possible.
B. For hybrid circuits, there is also a method of arranging analog and digital circuits on both sides of the printed board, using different layers of wiring, and using ground isolation in the middle.
8. Ground loop rules:
The minimum rule of the loop is that the area of the loop formed by the signal line and its loop should be as small as possible. The smaller the loop area, the less external radiation and the smaller the interference from the outside world.
9. Integrity rules for power and ground layers:
For areas with dense via holes, care should be taken to prevent the holes from being connected to each other in the hollowed-out area of the power supply and the formation, forming a division of the plane layer, thereby destroying the integrity of the plane layer, and thus increasing the loop area of the signal line in the formation .
10. 3w rules:
In order to reduce the crosstalk between the lines, the distance between the lines should be ensured to be large enough. When the line center spacing is not less than 3 times the line width, 70% of the electric field can be kept without interfering with each other, which is called the 3W rule. If you want to achieve 98% of the electric field does not interfere with each other, you can use a 10W spacing.
11. Protection of Pingwei:
Corresponding to the ground loop rules, in fact, it is also to minimize the loop area of the signal, which is more common in some important signals, such as clock signals and synchronization signals; for some particularly important and high-frequency signals, copper-axis cables should be considered The design of the shielding structure is to isolate the upper, lower, left, and right sides of the distributed lines with ground wires, and it is also necessary to consider how to effectively combine the shielding ground with the actual ground plane.
12. Wire termination network rules:
In high-speed digital circuits, when the delay time of PCB wiring is greater than 1/4 of the signal rise time (or fall time), the wiring can be regarded as a transmission line. In order to ensure that the input and output impedance of the signal and the impedance of the transmission line are correctly matched, Various forms of matching methods can be used, and the selected matching method is related to the connection mode of the network and the topology of the wiring.
A. For point-to-point (one output corresponds to one input) connection, you can choose series matching at the beginning or parallel matching at the end. The former has a simple structure and low cost, but has a large delay. The latter has a good matching effect, but the structure is complex and the cost is high.
B. For point-to-multipoint (one output corresponds to multiple outputs) connection, when the topology of the network is a daisy chain, terminal parallel matching should be selected. When the network is a star structure, a point-to-point structure can be referred to. Star and daisy chain are two basic topological structures, and other structures can be regarded as the deformation of the basic structure, and some flexible measures can be taken to match. In actual operation, factors such as cost, power consumption, and performance must be taken into account. Generally, complete matching is not pursued, as long as interference such as reflection caused by mismatch is limited to an acceptable range.
13. Routing i closed-loop inspection rules:
Prevent signal lines from forming self-loops between different layers. Such problems are prone to occur in multi-layer board design, and self-loop will cause radiation interference.
14. The branch length control rules of wiring:
Try to control the length of the branch, the general requirement is Tdelay<=Trise/20.
15. The resonance rule of the wiring:
Mainly for the design of high-frequency signals, that is, the length of the wiring must not be in integer multiples of its wavelength to avoid resonance.
16. Control rules for isolated copper areas:
The appearance of the isolated copper area will bring some unpredictable problems, so connecting the isolated copper area to other signals is helpful to improve the signal quality, usually the isolated copper area is grounded or deleted. In actual production, PCB manufacturers add some copper foil to the vacant parts of some boards. This is mainly to facilitate the processing of printed boards, and it also has a certain effect on preventing printed boards from warping.
17. The power and ground layer rules of Chongsang:
Different power layers should avoid overlapping in space. The main purpose is to reduce the interference between different power supplies, especially between some power supplies with large voltage differences, the overlapping problem of power supply planes must be avoided. If it is difficult to avoid, the middle interval ground layer can be considered.
18. 20H rules:
Since the electric field between the power plane and the ground plane changes, electromagnetic interference will be radiated outward at the edge of the board. called the edge effect.
The solution is to retract the power plane so that the electric field is only conducted within the range of the ground plane. Taking one H (the thickness of the medium between the power supply and the ground) as a unit, if the shrinkage is 20H, 70% of the electric field can be confined to the edge of the ground layer; if the shrinkage is 100H, 98% of the electric field can be confined.
(4) Others
For single-layer and double-layer boards, the power cord should be as thick and short as possible. The width requirements of the power line and the ground line can be calculated based on the line width of 1mm corresponding to a maximum current of 1A, and the loop formed by the power supply line and the ground line should be as small as possible.
