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. High frequency and high speed digitalization of signal transmission force PCB to move towards micro hole and buried/blind hole, wire fine, medium layer uniform thin shape, high frequency and high speed PCB design technology has become an important research field. This paper first makes a brief introduction to the high frequency circuit board, then expounds the PCB design of high frequency circuit board wiring skills, and finally introduces the PCB design of high frequency circuit board wiring precautions, specific follow Xiaobian together to understand.
High frequency circuit board is a special circuit board with high electromagnetic frequency. Generally speaking, high frequency can be defined as a frequency above 1GHz. Its physical performance, precision, technical parameters requirements are very high, often used in automotive collision prevention system, satellite system, radio system and other fields.
The high frequency printed circuit board provided by the utility model is provided with a guard that can block the flow glue at the edge of the upper opening and the lower opening of the hollow groove of the core plate, so that the flow glue will not enter the hollow groove when the core plate is glued to the copper coated plate placed on the upper surface and the lower surface, that is, the bonding operation can be completed once pressing. Compared with the existing technology, the high frequency circuit board can be completed only after secondary pressing. The high frequency circuit board in the utility model has the advantages of simple structure, low cost and easy manufacture.
First, the less the lead bending between the pins of high-speed electronic devices, the better
It is best to use the lead of high frequency circuit wiring fully straight line, need to turn, can be used 45 degrees fold line or arc turn, this requirement in low frequency circuit is only used to improve the copper foil fixation strength, and in high frequency circuit, meet this requirement can reduce the high-frequency signal external emission and mutual coupling.
Two, high-frequency circuit device pin between the lead layer alternations less, the better
The phrase "the less interlayer alternations of leads, the better" means that the fewer holes (Via) used in the connection of components are the better. A single pass hole brings about 0.5pF of distributed capacitance, and reducing the number of pass holes significantly increases speed and reduces the possibility of data errors.
Three, the shorter the lead between the pin of the high-frequency circuit device, the better
The radiation intensity of the signal is proportional to the length of the signal line. The longer the high-frequency signal lead, the easier it is to be coupled to the components close to it. Therefore, for high frequency signal lines such as signal clock, crystal oscillator, DDR data, LVDS wire, USB cable, HDMI cable, etc., the shorter the cable is required as far as possible.
Iv. Pay attention to "crosstalk" introduced by short distance parallel running of signal line
High frequency circuit wiring should pay attention to "crosstalk" caused by short distance parallel routing of signal lines. Crosstalk refers to the coupling phenomenon between signal lines that are not directly connected. Since high-frequency signals are transmitted along the transmission line in the form of electromagnetic waves, signal lines will play the role of antennas, and the energy of electromagnetic fields will be transmitted around the transmission line. The unwanted noise signals generated between signals due to the mutual coupling of electromagnetic fields are called Crosstalk. The parameters of the PCB circuit board layer, the distance between signal wires, the electrical characteristics of the driver and receiver end, and the connection mode of signal wires all have some influence on the crosstalk. So in order to reduce the crosstalk of high frequency signals, the following points should be done as far as possible when wiring:
(1) If parallel cabling in the same floor is almost unavoidable, the direction of the two adjacent floors must be perpendicular to each other;
(2) If the wiring space allows, insert a ground or ground plane between the two lines with serious crosstalk, which can play the role of isolation and reduce crosstalk;
(3) When time-varying electromagnetic fields exist 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, the usual clock signals are those with fast edge changes and large external crosstalk. Therefore, in the design, the clock line should be surrounded by the ground line and more ground holes to reduce the distributed capacitance, so as to reduce crosstalk;
(5) For high frequency signal clock, try to use the low voltage differential clock signal and include the ground mode, it is necessary to pay attention to the integrity of the ground punching;
(6) Increase the spacing between adjacent signal lines, reduce the parallel length of signal lines, and try to make the clock line perpendicular to the key signal line rather than parallel;
(7) Do not suspend the idle input terminal, but ground it or connect it to the power supply (the power supply is also ground in the high-frequency signal loop), because the suspended line may be equivalent to the transmitting antenna, grounding can inhibit the transmission. Eliminating crosstalk in this way has sometimes proved to be effective immediately.
Five, the integrated circuit block of the power pin to increase the high frequency of lotus root capacitance
Each integrated circuit block power pin to add a high frequency dropout capacitor. Increasing the high frequency decoupling capacitance of the power supply pin can effectively restrain the interference of high frequency harmonics on the power supply pin.
Six, high frequency digital signal ground and analog signal ground to do isolation
When analog ground wire and digital ground wire are connected to the public ground wire, high frequency choke magnetic beads should be used to connect or directly isolate and select the appropriate ground single point interconnection. The ground potential of ground wires of high frequency digital signals is generally inconsistent, and there is often a certain voltage difference between the two directly. Moreover, ground wires of high frequency digital signals are often rich in harmonic components of high frequency signals. When the ground wires of digital signals and analog signal ground wires are directly connected, the harmonics of high frequency signals will interfere with analog signals by means of ground wire coupling. Therefore, under normal circumstances, the ground wire of the high frequency digital signal and the ground wire of the analog signal should be isolated, which can be used in the way of single point interconnection at the appropriate position, or the way of high frequency choke magnetic bead interconnection.
Seven, avoid the loop formed by the cable
All kinds of high-frequency signals should not form a loop. If it cannot be avoided, the loop area should be as small as possible.
(1) Reasonable selection of layers
When wiring high-frequency circuit boards in PCB design, the inner plane in the middle is used as the power and ground layer, which can play a shielding role, effectively reduce the parasitic inductance, shorten the length of signal lines, and reduce the cross interference between signals. Generally, the noise of four-layer board is 20dB lower than that of two-layer board.
(2) Cabling mode
In the PCB design of high frequency circuit board wiring, the line must be in accordance with the 45° Angle, so as to reduce the high-frequency signal transmission and mutual coupling.
(3) Cable length
When wiring high-frequency circuit boards in PCB design, the shorter the length of the wires, the shorter the parallel distance between the two wires, the better.
(4) The number of holes
When wiring high frequency circuit boards in PCB design, the fewer holes the better.
(5) Direction of interlayer wiring
When wiring high-frequency circuit boards in PCB design, the direction of wiring between layers should be vertical, that is, the top layer is horizontal and the bottom layer is vertical, which can reduce the interference between signals.
(6) Apply copper
When wiring high frequency circuit board in PCB design, adding grounded copper can reduce the interference between signals.
(7) Parcel the land
In the PCB design of high frequency circuit board wiring, the important signal line for ground processing, can significantly improve the anti-interference ability of the signal, of course, can also be the interference source for ground processing, so that it can not interfere with other signals.
(8) Signal cable
When routing high frequency circuit boards in PCB design, do not route signals in loops. Route signals in Daisy chain mode.
(9) Decoupling capacitance
When wiring the high frequency circuit board in PCB design, the decoupling capacitor is crossed at the power end of the integrated circuit.
(10) High frequency choke
When wiring high frequency circuit boards in PCB design, high frequency choke devices should be connected when connecting common ground wires digitally and analog, which are generally high frequency ferrite magnetic beads with wires through the center hole.