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With the rapid development of electronic technology, as well as the extensive application of wireless communication technology in various fields, high-frequency, high-speed, high-density has gradually become one of the significant development trends of modern electronic products. High-frequency signal transmission and high-speed digitization, forcing the PCB to go to the micro-small holes and buried/blind holes, conductor refinement, dielectric layer uniformity and thinning, high-frequency high-speed PCB design technology has become an important area of research.
High-frequency PCB is a special circuit board with higher electromagnetic frequency, generally speaking, high-frequency can be defined as the frequency above 1GHz. Its physical properties, precision, and technical parameters are very high, and it is commonly used in automotive collision avoidance systems, satellite systems, radio systems and other fields.
In the actual engineering operation, the selection of high frequency PCB seems simple, but there are still many factors to consider. Today, we would like to introduce to you what factors should be considered when designing high frequency PCBs.
High Frequency PCB
1. High-speed electronic device pin-to-pin lead wire bends the less the better
High-frequency circuit drawing line of the lead wire is best to use a straight line, need to turn, can be used to 45-degree folding line or arc turn, this requirement in low-frequency circuits is only used to improve the adhesive strength of the copper foil, but in high-frequency circuits to meet the requirements of this can reduce the high-frequency signal to the outside of the emission and coupling between each other.
2. The fewer alternating layers of lead wires between the pins of high-frequency circuit devices, the better.
The so-called “fewer alternating layers of lead wires” refers to the fewer via holes 4.Via) used in the connection process of the components, the better. A via can bring about 0.5pF of distribution capacitance, reduce the number of vias can significantly increase the speed and reduce the possibility of data errors.
3. The shorter the leads between the pins of high frequency circuit devices, the better.
Signal radiation intensity is proportional to the length of the signal line, the longer the high-frequency signal leads, the easier it is coupled to the components close to it, so for such signals as clocks, crystals, DDR data, LVDS lines, USB cables, HDMI cables, and other high-frequency signal lines are required to try to shorten the line as much as possible, the better.
4. Pay attention to the “crosstalk” introduced by the parallel alignment of signal lines in close proximity.
High-frequency circuit drawing should pay attention to the signal lines in close proximity to the parallel alignment of the introduction of “crosstalk”, crosstalk refers to the coupling phenomenon between the signal lines that are not directly connected. Since the high-frequency signal along the transmission line is transmitted in the form of electromagnetic waves, the signal line will play the role of the antenna, the electromagnetic field of energy will be emitted around the transmission line, the signal due to the mutual coupling of the electromagnetic field generated by the undesired noise signal is known as crosstalk 4.Crosstalk). Crosstalk is caused by the parameters of the PCB circuit board layers, the spacing of the signal lines, the electrical characteristics of the driver and receiver, and the receiver channels at the end of the signal lines. Therefore, in order to minimize the crosstalk of high-frequency signals, it is required to do the following when drawing the line as much as possible:
4.1) If parallel alignments within the same layer are almost unavoidable, the direction of the alignment must be perpendicular to each other in two adjacent layers.
4.2) If space permits, inserting a ground line or ground plane between two lines with more serious crosstalk can reduce crosstalk by providing isolation.
4.3) When the space around the signal line itself exists a time-varying electromagnetic field, if it is not possible to avoid parallel distribution, you can lay a large area of “ground” on the opposite side of the parallel signal line to significantly reduce interference.
4.4) In digital circuits, the usual clock signals are fast edge change signals, which cause a lot of interference. Therefore, in the design, the clock line should be surrounded by ground wires and more ground holes to reduce the distribution capacitance, thereby reducing crosstalk.
4.5) For high-frequency signal clock as much as possible to use low-voltage differential clock signal and pack the ground pipeline, need to pay attention to pack the integrity of the ground hole.
4.6) Under the premise of drawing space, increase the distance between neighboring signal lines, reduce the parallel length of the signal lines, and try to use perpendicular rather than parallel clock lines and key signal lines.
4.7) Do not leave unused inputs unused, but ground them or connect them to a power source (the power source is also the ground in the high frequency signal loop), because an unused line may be equivalent to a transmitting antenna, and grounding suppresses transmissions. Practice has shown that this method of eliminating crosstalk can sometimes be immediately effective.
5. Add high-frequency decoupling capacitors to the power pins of the IC blocks.
A high-frequency decoupling capacitor is added to the power pin of each IC block. Adding a new high-frequency decoupling capacitor on the power supply pin can effectively inhibit high-frequency harmonics on the power supply pin from forming interference.
6. Separate the high-frequency digital signal ground from the analog signal ground.
When connecting the analog and digital ground wires to the common ground,use a high-frequency choke bead to connect them or isolate them directly and select a suitable place for single-point interconnection. The ground potentials of high frequency digital signals are generally inconsistent, and there is often a voltage difference between the two directly. Moreover, high frequency digital signals often carry very rich harmonic components of high frequency signals, and when connecting the digital and analog grounds directly, the harmonics of the high frequency signals will interfere with the analog signals through the ground-coupled pipeline. Therefore, usually, the high-frequency digital signal ground and analog signal ground is to be isolated, you can use a single point of interconnection in the appropriate location of the pipeline, or the use of high-frequency choke bead interconnection of the pipeline.
7. Avoid the formation of loops in the alignment.
All kinds of high-frequency signal alignment as far as possible not to form a loop, if it can not be avoided, then the loop area should be as small as possible.
8. Reasonable choice of layers
In the PCB design of high-frequency circuit boards drawing lines, the use of the middle plane as the power and ground layer, can play the role of the mask,effectively reduce parasitic inductance, shorten the length of the signal line, reduce the cross-interference between the signals, in general, four-layer board than the two-layer board noise is 20dB lower.
9. Alignment Piping
In the PCB design of high-frequency PCB drawing line, the alignment must be in accordance with the 45 ° angle of the corner, which can reduce the high-frequency signal emission and coupling between each other.
10. Alignment length
In the PCB design of high-frequency circuit board line drawing, the shorter the line length, the better, the shorter the distance between two lines in parallel, the better.
11. Number of vias
In PCB design for high-frequency circuit board line drawing, the fewer the number of holes the better.
12. Interlayer drawing direction
In the PCB design of high-frequency PCB line drawing, the direction of the interlayer line should be taken in the vertical direction, that is, the top layer for the horizontal direction, the bottom layer for the vertical direction, which can reduce the interference between the signals.
13. Copper Laying
In the PCB design of high-frequency PCB drawing, the new grounded copper can reduce the interference between the signals.
14. Ground wrapping
In the PCB design of high-frequency circuit board line drawing,the important signal lines to pack ground processing, can significantly improve the signal's ability to resist interference, of course, can also be the source of interference packet ground processing,so that it can not interfere with other signals.
15. Signal lines
In the PCB design of high-frequency PCB drawing line, the signal alignment can not be looped, you need to draw the line in accordance with the daisy chain pipeline.
16. Decoupling capacitors
In the PCB design of high-frequency printed circuit board line drawing, the power supply side of the integrated circuit across the decoupling capacitor.
17.High Frequency Choke
In the PCB design of high-frequency circuit boards line drawing, digital ground, analog ground and other connections to the public ground to be connected to a high-frequency choke device, generally the center hole wearing a wire high-frequency ferrite beads.
Today we introduced the PCB design of high frequency PCB drawing skills, as well as the high-frequency PCB Note.