The design of high frequency circuit PCB is a complex process, involving many factors, which may be directly related to the performance of high frequency circuit. High frequency circuit designer is a very complex design process, and its wiring is very important to the whole design.
Therefore, designers need to constantly research and explore in the actual work, accumulate experience, and combine with new design skills to design high-frequency circuit PCB with excellent performance. This paper collects and sorts out ten skills of high frequency circuit design, hoping to help you get twice the result with half the effort.
1、 Multilayer board wiring
High frequency circuits often have high integration and high wiring density. The use of multilayer board is not only necessary for wiring, but also an effective means to reduce interference. In pcblayout stage, reasonable selection of a certain number of layers of PCB size can make full use of the middle layer to set the shield, better realize the nearby grounding, effectively reduce the parasitic inductance and shorten the signal transmission length, and greatly reduce the signal cross interference. All these methods are beneficial to the reliability of high frequency circuit.
Some data show that the noise of the four-layer board is 20 dB lower than that of the double-sided board with the same material. However, there is also a problem. The higher the number of half layers of PCB, the more complex the manufacturing process, and the higher the unit cost. This requires us to select the appropriate number of layers of PCB, and to make a reasonable layout of components, and use the correct wiring rules to complete the design.
2、 The less lead bends between pins of high-speed electronic devices, the better
In high frequency circuit, it is better to use full straight line, which needs to be turned, and can be turned by 45 degree broken line or arc. This requirement is only used to improve the fixation strength of copper foil in low frequency circuit, but in high frequency circuit, meeting this requirement can reduce the external emission and mutual coupling of high frequency signal.
3、 The shorter the lead between pins of high frequency circuit devices, the better
The radiation intensity of the signal is proportional to the 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 high-frequency signal lines such as signal clock, crystal oscillator, DDR data, LVDS line, USB line, HDMI line, the shorter the line is, the better.
4、 The less lead layer alternation between pins of high frequency circuit devices, the better
The so-called “less interlaminar alternation of leads, the better” means that the fewer vias used in the process of component connection, the better. According to the side, one via can bring about 0.5pf distributed capacitance. Reducing the number of vias can significantly improve the speed and reduce the possibility of data error.
5、 Pay attention to the “crosstalk” caused by the close parallel routing of signal lines
High frequency circuit wiring should pay attention to the “crosstalk” caused by the close parallel routing of signal lines. Crosstalk refers to the coupling phenomenon between signal lines without direct connection.
Because the high-frequency signal is transmitted in the form of electromagnetic wave along the transmission line, the signal line will act as an antenna, and the energy of electromagnetic field will be emitted around the transmission line. The unwanted noise signal between signals due to the mutual coupling of electromagnetic field is called crosstalk.
The parameters of PCB layer, the distance between signal lines, the electrical characteristics of driver and receiver, and the termination mode of signal line all have certain influence on crosstalk.
Therefore, in order to reduce the crosstalk of high-frequency signals, the following points should be done as far as possible when wiring:
If the wiring space allows, inserting a ground wire or ground plane between two lines with serious crosstalk can play the role of isolation and reduce crosstalk. When there is time-varying electromagnetic field in the space around the signal line itself, if parallel distribution cannot be avoided, a large area of “ground” can be arranged on the reverse side of the parallel signal line to greatly reduce the interference.
On the premise of wiring space permission, 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 instead of parallel. If parallel routing in the same layer is almost unavoidable, the directions of routing in two adjacent layers must be perpendicular to each other.
In the digital circuit, the clock signal is usually the signal with fast edge change, which causes great external crosstalk. Therefore, in the design, the clock line should be surrounded by ground wire and more ground wire holes should be made to reduce the distributed capacitance, so as to reduce the crosstalk. For the high frequency signal clock, the low voltage differential clock signal parallel package grounding mode should be used as far as possible, and the integrity of package grounding drilling should be paid attention to.
The idle input terminal should not be suspended, but should be grounded or connected to the power supply (the power supply is also grounded in the high frequency signal circuit), because the suspended line may be equivalent to the transmitting antenna, and the grounding can suppress the transmission. It has been proved that this method can be used to eliminate the crosstalk.
6、 High frequency decoupling capacitor is added to the power supply pin of IC block
A high frequency decoupling capacitor is added to the power pin of each IC block. Increasing the high-frequency decoupling capacitance of the power supply pin can effectively suppress the interference of high-frequency harmonics on the power supply pin.
7、 The ground wire of high frequency digital signal is isolated from the ground wire of analog signal
When the analog ground wire and digital ground wire are connected to the common ground wire, they should be connected with high-frequency choke beads or isolated directly, and single point interconnection should be selected. The ground potential of the ground wire of high-frequency digital signal is generally inconsistent, and there is a certain voltage difference between them. Moreover, the ground wire of high-frequency digital signal often has a very rich harmonic component of high-frequency signal. When the ground wire of digital signal is directly connected with the ground wire of analog signal, the harmonic of high-frequency signal will interfere with the analog signal through the ground wire coupling.
Therefore, in general, the ground wire of high-frequency digital signal and the ground wire of analog signal should be isolated, and the single point interconnection at appropriate position or the high-frequency choke magnetic bead interconnection can be adopted.
8、 Avoid loops formed by routing
All kinds of high-frequency signal wiring should not form a loop as far as possible. If it cannot be avoided, the loop area should be as small as possible.
9、 Good signal impedance matching must be ensured
In the process of signal transmission, when the impedance does not match, the signal will be reflected in the transmission channel. The reflection will cause the composite signal to form overshoot and cause the signal to fluctuate near the logic threshold.
The fundamental way to eliminate the reflection is to make the impedance of the transmission signal match well. Because the greater the difference between the load impedance and the characteristic impedance of the transmission line, the greater the reflection, so the characteristic impedance of the signal transmission line should be equal to the load impedance as far as possible.
At the same time, we should also pay attention to the transmission line on the PCB can not appear mutation or corner, try to keep the impedance of each point of the transmission line continuous, otherwise there will be reflection between each section of the transmission line.
This requires that the following wiring rules must be observed when high-speed PCB wiring is carried out:
USB wiring rules. USB signal differential wiring is required, line width is 10mil, line distance is 6mil, ground wire and signal line distance is 6mil.
HDMI wiring rules. HDMI signal differential routing is required, with a line width of 10mil and a line spacing of 6mil. The distance between each two groups of HDMI differential signal pairs is more than 20MIL.
LVDS routing rules. The LVDS signal is required to be differential wired with a line width of 7mil and a line spacing of 6mil. The purpose is to control the differential signal pair impedance of HDMI to be 100 + – 15% ohm
DDR routing rules. DDR1 routing requires that the signal should not go through the hole as far as possible, and the signal lines should be of equal width and equidistant from each other. The routing must meet the 2W principle to reduce the crosstalk between signals. For DDR2 and above high-speed devices, the high-frequency data lines should also be of equal length to ensure the impedance matching of the signal.
10、 Maintain the integrity of signal transmission
Keep the integrity of signal transmission and prevent the “ground spring phenomenon” caused by the division of ground wire.