With the rapid development of electronic technology and the widespread 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. The increasing frequency and high-speed digitization of signal transmission are forcing PCBs towards miniaturized vias and buried/blind vias, finer conductors, and thinner, more uniform dielectric layers. High-frequency, high-speed, and high-density multilayer PCB design technology has become an important research area. This article first provides a brief introduction to high-frequency circuit boards, then elaborates on the routing techniques for high-frequency circuit boards in PCB design, and finally introduces the precautions for routing high-frequency circuit boards in PCB design. Let's take a closer look.
Introduction to High Frequency Circuit Boards
High-frequency circuit boards are special circuit boards with high electromagnetic frequencies. Generally, high frequency can be defined as a frequency above 1 GHz. They have very high requirements for various physical properties, precision, and technical parameters, and are often used in automotive collision avoidance systems, satellite systems, radio systems, and other fields.
The high-frequency circuit board provided by this utility model has baffles at the edges of the upper and lower openings of the hollow groove in the core board to prevent adhesive from flowing into the hollow groove when the core board is bonded to the copper-clad laminate placed on its upper and lower surfaces. That is, the bonding operation can be completed in one press. Compared with the high-frequency circuit boards of the prior art that require two presses to complete the bonding, the high-frequency circuit board of this utility model has a simple structure, low cost and is easy to manufacture.
PCB design high-frequency circuit board routing techniques
1
The fewer bends in the leads between the pins of high-speed electronic devices, the better.
For high-frequency circuit wiring, the leads should ideally be straight. If a bend is necessary, a 45-degree bend or an arc bend can be used. In low-frequency circuits, this requirement is only used to improve the adhesion strength of the copper foil, but in high-frequency circuits, meeting this requirement can reduce the external transmission of high-frequency signals and the coupling between them.
2
The fewer the interlayer overlaps of leads between the pins of high-frequency circuit components, the better.
The saying "the fewer the interlayer alternations of leads, the better" refers to using as few vias as possible during component connection . A single via can introduce approximately 0.5pF of distributed capacitance; reducing the number of vias can significantly improve speed and reduce the likelihood of data errors.
3
The shorter the leads between the pins of high-frequency circuit components, the better.
The radiation intensity of a signal is directly proportional to the length of the signal line. The longer the high-frequency signal lead, the easier it is to couple to the components nearby. Therefore, for high-frequency signal lines such as clock signals, crystal oscillators, DDR data lines, LVDS lines, USB lines, and HDMI lines, it is required that the traces be as short as possible.
4
Note the crosstalk introduced by closely spaced parallel signal lines.
When routing high-frequency circuits, it's crucial to be mindful of crosstalk introduced by closely spaced parallel signal lines. Crosstalk refers to the coupling phenomenon between signal lines that are not directly connected. Since high-frequency signals are transmitted as electromagnetic waves along transmission lines, the signal lines act as antennas, emitting electromagnetic energy around them. The unwanted noise generated by the mutual coupling of these electromagnetic fields between signals is called crosstalk. PCB layer parameters, signal line spacing, the electrical characteristics of the driver and receiver terminals, and the signal line termination method all have a certain impact on crosstalk. Therefore, to reduce crosstalk in high-frequency signals, the following points should be considered as much as possible during routing:
(1) If parallel routing within the same floor is almost unavoidable, the routing directions of the routing lines in two adjacent floors must be perpendicular to each other;
(2) If the wiring space allows, inserting a ground wire or ground plane between two lines with severe crosstalk can serve as an isolation and reduce crosstalk.
(3) When there is a time-varying electromagnetic field in the space around the signal line, if parallel distribution cannot be avoided, a large area of "ground" can be arranged on the opposite side of the parallel signal line to greatly reduce interference.
(4) In digital circuits, clock signals are usually signals with fast edge changes, resulting in large crosstalk. Therefore, in the design, the clock line should be surrounded by ground lines and multiple ground vias should be drilled to reduce distributed capacitance, thereby reducing crosstalk;
(5) Use low-voltage differential clock signals and grounding for high-frequency clock signals as much as possible, and pay attention to the integrity of the grounding vias;
(6) If the wiring space permits, increase the spacing between adjacent signal lines, reduce the parallel length of signal lines, and make the clock line as perpendicular to the critical signal line as possible rather than parallel to it;
(7) Do not leave unused input terminals floating; instead, ground them or connect them to a power supply (the power supply is also ground in high-frequency signal circuits). This is because a floating wire may be equivalent to a transmitting antenna, and grounding can suppress transmission. Practice has shown that this method can sometimes be effective in eliminating crosstalk immediately.
5
Add high-frequency decoupling capacitors to the power supply pins of the integrated circuit.
Add a high-frequency decoupling capacitor near the power supply pin of each integrated circuit block. Adding a high-frequency decoupling capacitor to the power supply pin can effectively suppress high-frequency harmonic interference on the power supply pin.
6
Isolate the ground lines of high-frequency digital signals and analog signals.
When connecting analog ground lines and digital ground lines to a common ground, a high-frequency choke bead should be used for connection, or they should be directly isolated and interconnected at a suitable single point. The ground potentials of high-frequency digital signal ground lines are generally inconsistent, often resulting in a voltage difference. Furthermore, high-frequency digital signal ground lines often carry abundant harmonic components. When directly connecting digital signal ground lines and analog signal ground lines, these harmonics will interfere with the analog signal through ground coupling. Therefore, it is generally necessary to isolate high-frequency digital signal ground lines from analog signal ground lines. This can be achieved through single-point interconnection at a suitable location or by using a high-frequency choke bead for interconnection.
7
Avoid loops formed by wiring
Avoid forming loops in the routing of various high-frequency signals as much as possible. If it is unavoidable, the loop area should be minimized.
PCB Design High-Frequency Circuit Board Routing Considerations
1
Choose the number of floors appropriately
In PCB design, when routing high-frequency circuit boards, using the middle inner plane as the power and ground plane can serve as a shield, effectively reducing parasitic inductance, shortening signal line length, and reducing cross-interference between signals. Generally, a four-layer board has 20dB lower noise than a two-layer board.
2
wiring method
When routing high-frequency circuit boards in PCB design, the traces must bend at a 45° angle to reduce the emission of high-frequency signals and the coupling between them.
3
Trace length
When routing high-frequency circuit boards in PCB design, the shorter the trace length, the better, and the shorter the distance between two parallel lines, the better.
4
Number of vias
When routing high-frequency circuit boards in PCB design, the fewer vias, the better.
5
Interlayer wiring direction
When routing high-frequency circuit boards in PCB design, the routing direction between layers should be vertical, that is, the top layer is horizontal and the bottom layer is vertical. This can reduce interference between signals.
6
Copper plating
In PCB design, adding grounded copper pours can reduce interference between signals when routing high-frequency circuit boards.
7
Land lease
In PCB design, when routing high-frequency circuit boards, grounding important signal lines can significantly improve their anti-interference capability. Of course, grounding can also be used to prevent interference sources from interfering with other signals.
8
signal line
When routing high-frequency circuit boards in PCB design, signal traces must not be looped and must be routed in a daisy chain manner.
9
Decoupling capacitors
When routing high-frequency circuit boards in PCB design, decoupling capacitors are connected across the power supply terminals of integrated circuits.
10
High-frequency choke
When routing high-frequency circuit boards in PCB design, high-frequency choke devices should be connected when connecting digital ground, analog ground, etc. to the common ground line. These are usually high-frequency ferrite beads with a wire through the center hole.
