If the frequency of the digital logic circuit reaches or exceeds 45MHZ~50MHZ, and the circuit working on this frequency has accounted for a certain amount of the entire electronic system (for example, 1/3), it is usually called a high-frequency circuit (multi-layer circuit board). High-frequency circuit board design is a very complex design process, and its wiring is crucial to the entire design!
1. high multi-layer circuit board wiring
High frequency circuit often has high integration and high wiring density, and the use of multi-layer circuit board is not only necessary for wiring, but also an effective means to reduce interference. In the PCBLayout stage, a reasonable choice of a certain number of layers of printed circuit board size, can make full use of the middle layer to set shielding, better achieve nearby grounding, and effectively reduce the parasitic inductance and shorten the transmission length of the signal, but also greatly reduce the signal cross interference, etc., all these methods are beneficial to the reliability of high-frequency circuits. Some data show that when the same material is used, the noise of the four-layer board (multi-layer circuit board) is 20dB lower than that of the double panel. However, at the same time, there is also a problem, the higher the number of half layers of the circuit board, the more complex the manufacturing process, the higher the unit cost, which requires us to carry out PCBLayout, in addition to choosing the appropriate number of layers of the circuit board, but also need to carry out reasonable component layout planning, and use the correct wiring rules to complete the design.
2. the less the lead bending between the pins of high-speed electronic devices, the better
It is best to use the lead of the high frequency circuit wiring fully straight line, need to turn, can be used 45 degree line or arc turn, this requirement in the low frequency circuit is only used to improve the fixed strength of copper foil, and in the high frequency circuit, meet this requirement can reduce the high-frequency signal external emission and coupling between each other.
3. the less the lead layer alternation between the pins of the high-frequency circuit device, the better
The so-called "the less interlayer alternations of leads, the better" means that the fewer through-holes (Via) used in the connection of components, the better. According to the side, a PCB hole can bring about 0.5pF of distributed capacitance, reducing the number of holes can significantly improve speed and reduce the possibility of data errors.
4. The shorter the lead between the pins of the high-frequency circuit (multi-layer circuit board) 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, so for high frequency signal lines such as the signal clock, crystal oscillator, DDR data, LVDS line, USB line, HDMI line are required as short as possible.
5. pay attention to the "crosstalk" introduced by the signal line near the parallel line
High frequency circuit wiring should pay attention to the "crosstalk" introduced by the short distance parallel line of the signal line, crosstalk refers to the coupling phenomenon between the 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 play the role of an antenna, the energy of the electromagnetic field will be emitted around the transmission line, and the unwanted noise signal generated between the signals due to the mutual coupling of the electromagnetic field is called Crosstalk. The parameters of the PCB board layer, the spacing of the signal line, the electrical characteristics of the driver and receiver end, and the termination mode of the signal line all have a certain influence on the crosstalk. Therefore, in order to reduce the crosstalk of high-frequency signals, the following points are required as far as possible when wiring:
5.1. If the wiring space allows, insert a ground line or ground plane between two lines with more serious crosstalk, which can play the role of isolation and reduce crosstalk.
5.2. when the space around the signal line itself has a time-varying electromagnetic field, if it is impossible to avoid parallel distribution, a large area of "ground" can be arranged on the opposite side of the parallel signal line to greatly reduce interference.
5.3. under the premise of wiring space permit, increase the spacing between adjacent signal lines, reduce the parallel length of the signal line, and try to perpendicular to the key signal line rather than parallel clock line.
5.4. if the parallel line within the same layer is almost unavoidable, in the adjacent two layers, the direction of the line must be perpendicular to each other.
5.5. in digital circuits, the usual clock signal is a fast edge change signal, and the external crosstalk is large. Therefore, in the design, the clock line should be surrounded by the ground line and make more ground holes to reduce the distributed capacitance, thereby reducing crosstalk.
5.6. the high-frequency signal clock as far as possible to use the low voltage differential clock signal and pack the ground, need to pay attention to the integrity of the pack hole.
5.7. Do not suspend the idle input, 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, and the grounding can inhibit the emission. Practice has shown that eliminating crosstalk in this way can sometimes produce immediate results.
6. integrated circuit block (multi-layer circuit board) power supply pin increase high frequency decoupling capacitance
Add a high-frequency decoupling capacitor to the nearest power pin of each integrated circuit 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. avoid the loop formed by the line
All kinds of high-frequency signal wiring should not form a loop, if it cannot be avoided, the loop area should be as small as possible.
8. high-frequency digital signal ground and analog signal ground to do isolation.
When the analog ground wire and digital ground wire are connected to the public ground wire, the high frequency choke magnetic bead should be connected or directly isolated and the appropriate ground single point interconnection should be selected. The ground potential of the ground wire of the high frequency digital signal is generally inconsistent, and there is often a certain voltage difference between the two directly, and the ground wire of the high frequency digital signal is often with a very rich harmonic component of the high frequency signal, when the digital signal ground wire and the analog signal ground wire are directly connected, the harmonic of the high frequency signal will interfere with the analog signal through the ground wire coupling. Therefore, under normal circumstances, the ground wire of the high frequency digital signal and the ground wire of the analog signal are to be isolated, and a single point interconnection can be used in the appropriate position, or a high frequency choke magnetic bead interconnection can be used.
9. must ensure a good signal impedance match
In the process of signal transmission, when the impedance does not match, the signal will be reflected in the transmission channel, and the reflection will cause the synthesized signal to overshoot, resulting in the signal fluctuating near the logical threshold.
The fundamental way to eliminate the reflection is to make the impedance of the transmission signal match well, because the load impedance and the transmission line characteristic impedance difference is larger the reflection is larger, so it should be as far as possible to make the signal transmission line characteristic impedance and load impedance equal. At the same time, note that the transmission line on the circuit board can not appear mutations or corners, and try to keep the impedance of each point of the transmission line continuous, otherwise there will be reflections between the sections of the transmission line. This requires that when conducting high-speed PCB wiring, the following wiring rules must be complied with:
9.1. USB cabling rules. USB signal differential wiring, line width 10mil, line distance 6mil, ground and signal line distance 6mil.
9.2. HDMI wiring rules. HDMI signal differential wiring, line width 10mil, line distance 6mil, each two sets of HDMI differential signal pairs spacing more than 20mil.
9.3. LVDS wiring rules. LVDS signal differential wiring, line width 7mil, line distance 6mil, the purpose is to control HDMI differential signal impedance of 100+-15% ohms
9.4. DDR wiring rules. The signal cables must be of equal width and equidistant from each other. The cables must meet the 2W principle to reduce crosstalk between signals. For high-speed devices of DDR2 or higher, the high-frequency data cables must be of equal length to ensure signal impedance matching.
10. maintain the integrity of signal transmission, to prevent the "ground bounce phenomenon" caused by ground line segmentation
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