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. The high-frequency and high-speed digitalization of signal transmission has forced PCBs to move towards tiny holes and buried/blind holes, finer wires, and uniform and thin dielectric layers. High-frequency and high-speed PCB design technology has become an important research field. This article first gives a brief introduction to high-frequency circuit boards, and then expounds the wiring skills of high-frequency circuit boards in PCB design. Finally, it introduces the precautions for high-frequency circuit board wiring in PCB design.
High-frequency circuit boards are special circuit boards with high electromagnetic frequencies. Generally speaking, high-frequency can be defined as frequencies above 1GHz. Its physical properties, precision, and technical parameters are very demanding, and are often used in automotive anti-collision systems, satellite systems, radio systems and other fields.
The high-frequency circuit board provided by the utility model is provided with ribs that can block the flow of glue at the edges of the upper opening and the lower opening of the hollow groove of the core board, so that the core board and the covering placed on the upper surface and the lower surface of the core board are arranged. When the copper plate is bonded, the glue will not enter the hollow groove, that is, the bonding operation can be completed by one pressing. The plate structure is simple, the cost is low, and it is easy to manufacture.
1. The less the leads are bent between the pins of high-speed electronic devices, the better
The leads of the high-frequency circuit wiring are preferably all straight lines, and they need to be turned. They can be turned with 45-degree broken lines or arcs. This requirement is only used to improve the fixing strength of the copper foil in the low-frequency circuit, while in the high-frequency circuit, this requirement is met. A requirement can reduce the external emission and mutual coupling of high-frequency signals.
2. The less the alternation between the lead layers between the pins of the high-frequency circuit device, the better
The so-called "the less the interlayer alternation of the leads, the better" means that the fewer vias (Via) used in the component connection process, the better. A via can bring about 0.5pF of distributed capacitance, and reducing the number of vias can significantly improve speed and reduce the possibility of data errors.
3. The shorter the lead between the pins of the high-frequency circuit device, the better
The radiation intensity of the signal is proportional to the trace length of the signal line. The longer the high-frequency signal lead is, the easier it is to couple to the components close to it. Therefore, for signals such as clocks, crystal oscillators, DDR data, High-frequency signal lines such as LVDS lines, USB lines, and HDMI lines are required to be as short as possible.
4. Pay attention to the "crosstalk" introduced by the parallel traces of the signal lines
In high-frequency circuit wiring, attention should be paid to the "crosstalk" introduced by the parallel wiring of signal lines in close proximity. Crosstalk refers to the coupling phenomenon between signal lines that are not directly connected. Since the high-frequency signal is transmitted along the transmission line in the form of electromagnetic waves, the signal line will act as an antenna, and the energy of the electromagnetic field will be emitted around the transmission line. Called crosstalk (Crosstalk). The parameters of the PCB layer, the spacing of the signal lines, the electrical characteristics of the driving end and the receiving end, and the termination method of the signal lines all have a certain influence on the crosstalk. Therefore, in order to reduce the crosstalk of high-frequency signals, it is required to do the following as much as possible when wiring:
a. If parallel traces in the same layer are almost unavoidable, in two adjacent layers, the direction of traces must be perpendicular to each other;
b. Under the condition that the wiring space allows, insert a ground wire or ground plane between the two lines with serious crosstalk, which can play the role of isolation and reduce crosstalk;
c. 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 the interference;
d. In digital circuits, the usual clock signals are signals with fast edge changes, and the external crosstalk is large. Therefore, in the design, the clock line should be surrounded by ground wires and more ground wire holes should be made to reduce distributed capacitance and thus reduce crosstalk;
e. Try to use low-voltage differential clock signals and wrap the ground for the high-frequency signal clock, and pay attention to the integrity of the ground punching;
f. Under the premise of the wiring space permit, increase the spacing between adjacent signal lines, reduce the parallel length of the signal lines, and try to make the clock lines perpendicular to the key signal lines instead of parallel;
g. Do not suspend the unused input terminal, but ground it or connect it to the power supply (the power supply is also grounded in the high-frequency signal loop), because the dangling line may be equivalent to the transmitting antenna, and the grounding can suppress the emission. Practice has proved that eliminating crosstalk in this way can sometimes be effective immediately.
5. Add high-frequency decoupling capacitors to the power pins of the integrated circuit block
A high-frequency decoupling capacitor is added to the power pin of each integrated circuit block. Adding high-frequency decoupling capacitors on the power pins can effectively suppress the interference caused by high-frequency harmonics on the power pins.
6. The ground wire of high frequency digital signal and the ground wire of analog signal are isolated
When the analog ground wire, digital ground wire, etc. are connected to the common ground wire, high-frequency choke magnetic beads should be used for connection or direct isolation and single-point interconnection at a suitable place. The ground potential of the ground wire of high-frequency digital signals is generally inconsistent, and there is often a certain voltage difference between the two. Moreover, the ground wire of high-frequency digital signals often has very rich harmonic components of high-frequency signals. When the digital signal ground wire and the analog signal ground wire are directly connected, the harmonics of the high frequency signal will interfere with the analog signal through the ground wire coupling. Therefore, under normal circumstances, the ground wire of high-frequency digital signals and the ground wire of analog signals need to be isolated, which can be interconnected at a single point at a suitable location, or interconnected by high-frequency choke magnetic beads.
7. Avoid loops formed by traces
All kinds of high-frequency signal traces should not form loops as much as possible. If it cannot be avoided, the loop area should be kept as small as possible.
a. Reasonable selection of layers
When routing high-frequency circuit boards in PCB design, using the middle inner plane as the power supply and ground layer can play a shielding role, effectively reduce parasitic inductance, shorten the length of signal lines, and reduce cross-interference between signals. Generally, The noise of the four-layer board is 20dB lower than that of the two-layer board.
b. Way of wiring
When routing high-frequency circuit boards in PCB design, the traces must be turned at a 45° angle, which can reduce the emission of high-frequency signals and mutual coupling.
c. Trace length
When routing high-frequency circuit boards in PCB design, the shorter the trace length, the better, and the shorter the parallel distance between the two lines, the better.
d. Number of vias
When routing high-frequency boards in PCB design, the fewer the number of vias, the better.
e. Interlayer wiring direction
When wiring high-frequency circuit boards in PCB design, the direction of interlayer wiring should be vertical, that is, the top layer is horizontal and the bottom layer is vertical, which can reduce the interference between signals.
f. Copper plating
When routing high-frequency circuit boards in PCB design, adding grounded copper can reduce interference between signals.
g.
When routing high-frequency circuit boards in PCB design, the important signal lines can be grounded, which can significantly improve the anti-interference ability of the signal. Of course, the interference source can also be grounded so that it cannot interfere with other signals.
h. Signal line
When routing high-frequency circuit boards in PCB design, the signal traces cannot be looped and need to be routed in a daisy-chain manner.
i. Decoupling capacitors
When routing high-frequency circuit boards in PCB design, a decoupling capacitor is connected across the power supply end of the integrated circuit.
j. High frequency choke
When wiring a high-frequency circuit board in PCB design, a high-frequency choke device should be connected to the digital ground, analog ground, etc. when connecting to the common ground wire, generally a high-frequency ferrite bead with a wire in the center hole.
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