The first step - multi-layer board wiring
High-frequency circuits often have high integration and high wiring density. The use of multi-layer boards is not only necessary for wiring, but also an effective means to reduce interference. In the PCBLayout stage, a reasonable selection of the printed board size with a certain number of layers can make full use of the middle layer to set up shielding, better achieve the nearest grounding, and effectively reduce the parasitic inductance and shorten the transmission length of the signal. All of 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 is 20dB lower than that of the double-layer board. However, there is also a problem at the same time. The higher the number of PCB half-layers, the more complex the manufacturing process, and the higher the unit cost. This requires us to select a PCB with a suitable number of layers when performing PCBLayout. Reasonable The layout planning of components and components, and the use of correct routing rules to complete the design.
The second trick is to bend the leads between the pins of multiple high-speed electronic devices as little as possible
The lead wires of high-frequency circuit wiring are preferably all straight lines, and they need to be turned. They can be bent at 45 degrees or arcs. This requirement is only used to improve the fixation strength of copper foil in low-frequency circuits. —Requirements can reduce the external emission and mutual coupling of high-frequency signals.
The third measure is the lead between the pins of high-frequency circuit devices
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 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.
The fourth trick is to reduce the alternation between the lead layers between the pins of high-frequency circuit devices.
The so-called "the less the interlayer alternation of leads, the better" means that the fewer vias (Via) used in the component connection process, the better. According to the side, a via can bring a distributed capacitance of about 0.5pF, and reducing via coupons can significantly improve speed and reduce the possibility of data errors.
The fifth measure is to pay attention to the "crosstalk" introduced by the close parallel routing of signal lines
When wiring high-frequency circuits, attention should be paid to the "crosstalk" caused by the close parallel routing of signal lines. 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, the energy of the electromagnetic field will be emitted around the transmission line, and the undesired noise signal will be generated between the signals due to the mutual coupling of the electromagnetic field Called crosstalk (Crosstalk). The parameters of the PCB board 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:
Under the condition that the wiring space permits, inserting a ground wire or ground plane between the two lines with serious crosstalk can play a role of isolation and reduce crosstalk.
When there is a time-varying electromagnetic field in the space around the signal lines, if parallel distribution cannot be avoided, a large-area "ground" can be arranged on the opposite side of the parallel signal lines to greatly reduce interference.
Under the premise that the wiring space permits, 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 traces in the same layer are almost unavoidable, the directions of the traces must be perpendicular to each other on two adjacent layers.
In digital circuits, the usual clock signals are signals with fast edge changes, and the external crosstalk is large. Therefore, in the design, it is advisable to surround the clock line with ground wires and drill more ground wire holes to reduce distributed capacitance, thereby reducing crosstalk.
For the high-frequency signal clock, try to use the low-voltage differential clock signal and wrap the ground, and pay attention to the integrity of the ground punching.
The unused input terminal should not be empty, but grounded or connected to the power supply (the power supply is also grounded in the high-frequency signal loop), because the empty line may be equivalent to the transmitting antenna, and the grounding will suppress the emission. Practice has proved that using this method to eliminate crosstalk can sometimes be effective immediately.
The sixth trick is to add a high-frequency decoupling capacitor to the power supply pin of the integrated circuit block
A high-frequency decoupling capacitor is added nearby to the power supply pin of each integrated circuit block. Increasing the high-frequency decoupling capacitor of the power supply pin can effectively suppress the high-frequency harmonics on the power supply pin to form interference.
The seventh measure is to isolate the ground wire of the high-frequency digital signal from the ground wire of the analog signal
When the analog ground wire and digital ground wire are connected to the public ground wire, they should be connected with high-frequency choke magnetic beads or directly isolated and selected for single-point interconnection in a suitable place. 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. Moreover, the ground wire of the high-frequency digital signal often has a very rich harmonic component of the high-frequency signal. When the digital signal ground and the analog signal ground are directly connected, the harmonics of the high-frequency signal will interfere with the analog signal through ground coupling. Therefore, under normal circumstances, the ground wire of the high-frequency digital signal and the ground wire of the analog signal need to be isolated, and the method of single-point interconnection at a suitable position can be adopted, or the method of interconnection of high-frequency choke beads can be used.
The eighth trick to avoid the loop formed by the wiring
All kinds of high-frequency signal traces should try not to form a loop. If it cannot be avoided, the loop area should be kept as small as possible.
The ninth measure must ensure good signal impedance matching
During the signal transmission process, when the impedance does not match, the signal will be reflected in the transmission channel, and the reflection will cause the composite signal to be formed, causing the signal to fluctuate near the logic threshold.
The fundamental way to eliminate reflection is to match the impedance of the transmission signal well. Since 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 much as possible. At the same time, it is also necessary to pay attention to the transmission line on the PCB without sudden changes or corners. 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 the following wiring rules must be followed when performing high-speed PCB wiring:
USB wiring rules. The USB signal differential wiring is required, the line width is 10mil, the line distance is 6mil, and the ground wire and signal line distance are 6mil.
HDMI wiring rules. HDMI signal differential routing is required, the line width is 10mil, the line spacing is 6mil, and the distance between each two HDMI differential signal pairs exceeds 20mil.
LVDS wiring rules. LVDS signal differential routing is required, the line width is 7mil, and the line spacing is 6mil. The purpose is to control the impedance of the differential signal pair of HDMI to 100+-15% ohms
DDR routing rules. DDR1 wiring requires that the signal should not go through the hole as much as possible, the signal line should be equal in width, and the lines should be equidistant. The wiring must meet the 2W principle to reduce the crosstalk between signals. The lines are equal in length to ensure signal impedance matching.
The tenth measure to maintain the integrity of signal transmission
Maintain the integrity of signal transmission and prevent the "single ground phenomenon" caused by ground wire segmentation.
