If the frequency of the digital logic circuit reaches or exceeds 45MHZ~50MHZ, and the circuit working above 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 (multilayer circuit). plate). High frequency circuit board design is a very complex design process, and its wiring is critical to the entire design!
It can also greatly reduce signal cross-talk, etc. All these methods are beneficial to the reliability of high-frequency circuits. Some data show that the noise of the four-layer board (multi-layer circuit board) is 20dB lower than that of the double-sided board when the same material is used. However, at the same time, there is also a problem. The higher the half-layer number of the circuit board, the more complex the manufacturing process and the higher the unit cost. This requires us to choose a circuit board with the appropriate number of layers when conducting PCB Layout. Reasonable component layout planning, and use the correct wiring rules to complete the design.
The second measure, the less lead bending 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.
The third measure, the less the alternation between the lead layers between the pins of the high-frequency circuit device, the better
The so-called "the interlayer alternation of the leads is as few as possible" means that the fewer vias (Via) used in the component connection process, the better. According to the side, a PCB via can bring about 0.5pF of distributed capacitance, and reducing the number of vias can significantly improve the speed and reduce the possibility of data errors.
The fourth measure, the shorter the lead between the pins of the high-frequency circuit (multilayer circuit board) 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.
The fifth measure, 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 high-frequency signals are transmitted in the form of electromagnetic waves along the transmission line, the signal line will act as an antenna, the energy of the electromagnetic field will be emitted around the transmission line, and the signals will be generated due to the mutual coupling of the electromagnetic fields. Undesirable noise signals 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:
If the wiring space allows, inserting a ground wire or a ground plane between the two lines with serious crosstalk can play a role in isolation and reduce crosstalk.
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. Under the premise of 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.
If parallel traces in the same layer are almost unavoidable, on two adjacent layers, the traces must be oriented perpendicular to each other.
In a digital circuit, the usual clock signal is a signal with fast edge change, and the external crosstalk is large. Therefore, in the design, the clock line should be surrounded by a ground wire and more ground wire holes should be made to reduce the distributed capacitance and thus reduce the crosstalk.
For high-frequency signal clocks, try to use low-voltage differential clock signals and wrap the ground. Pay attention to the integrity of the ground punching.
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.
The sixth measure, increase the high-frequency decoupling capacitor on the power supply pin of the integrated circuit block (multilayer circuit board)
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.
The seventh measure, avoid the loop formed by the wiring
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.
The eighth trick is to isolate the ground wire of the high-frequency digital signal and the ground wire of the analog signal.
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.
The ninth measure, must ensure good signal impedance matching
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 composite signal to form an overshoot, causing the signal to fluctuate near the logic threshold.
The fundamental way to eliminate reflection is to make the impedance of the transmission signal well matched. 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 as equal as the load impedance as much as possible. At the same time, it should be noted that the transmission line on the circuit board cannot have sudden changes or corners, and try to keep the impedance of each point of the transmission line continuous, otherwise there will be reflections between the segments of the transmission line. This requires that the following wiring rules must be followed when conducting high-speed PCB wiring;
USB wiring rules. Differential traces for USB signals are required, with a line width of 10 mils, a line spacing of 6 mils, and a 6 mil spacing between ground and signal lines.
HDMI wiring rules. Differential traces for HDMI signals are required, with a line width of 10 mil and a line spacing of 6 mil. The spacing between each pair of HDMI differential signal pairs exceeds 20 mil.
LVDS wiring rules. LVDS signal differential traces are required, 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 wiring requires that signals do not pass through holes as much as possible, the signal lines are of equal width, and the lines are equidistant. The wiring must meet the 2W principle to reduce crosstalk between signals. For high-speed devices of DDR2 and above, high-frequency data is also required. The length of the line is equal to ensure the impedance matching of the signal.
The tenth trick is to maintain the integrity of signal transmission and prevent the "ground bounce phenomenon" caused by the division of the ground wire