PCB Blog - Get high frequency circuit PCB board design and wiring

If the frequency of the digital logic PCB board 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. High-frequency circuit design is a very complex design process, and its routing is critical to the entire design.

1. Multilayer board wiring 2

High-frequency circuits tend to 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 PCB Layout stage, a reasonable selection of the size of the printed board with a certain number of layers can make full use of the middle layer to set the shield, better achieve the nearest grounding, and effectively reduce the parasitic inductance and shorten the transmission length of the signal. Significantly reduce signal cross-interference, etc. All of these methods are beneficial to the reliability of high-frequency circuits. Some data show that the noise of the four-layer board is 20dB lower than that of the double-sided board when the same material is used. But at the same time, there is also a problem. The higher the half-layer number of the PCB board, the more complicated the manufacturing process and the higher the unit cost. This requires us to choose the appropriate number of layers when we are doing the PCB board layout. Carry out reasonable component layout planning and use correct routing rules to complete the design.

2. The less lead bends between the pins of high-speed electronic devices, the better

The leads of the high-frequency circuit wiring are all straight lines and 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, but in the high-frequency circuit, this requirement is met. However, it can reduce the external emission and mutual coupling of high-frequency signals.

3. The shorter the lead between the pins of the high-frequency circuit 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.

4. The less the alternation between the lead layers between the pins of the high-frequency circuit device, the better

The so-called "the less the interlayer alternation of the leads, the better" means that the fewer vias used in the component connection process, the better. According to the data, one 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.

5. Pay attention to the "crosstalk" caused by the close 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 the high-frequency signal is transmitted along the transmission line in the form of electromagnetic waves, the signal line will act as an antenna, and the energy of the electromagnetic field will be emitted around the transmission line. called 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: Insert a ground wire or ground plane between the two lines with severe crosstalk under the conditions allowed by the wiring space. It can play the role of 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. The practice has proved that eliminating crosstalk in this way can sometimes be effective immediately.

6. Add high-frequency decoupling capacitors to the power pins of the integrated circuit block

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.

7. The ground wire of high-frequency digital signal and the ground wire of analog signal is isolated

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; 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 the high-frequency digital signal and the ground wire of the analog signal needs to be isolated, which can be interconnected at a single point at a suitable position, or the interconnection of high-frequency choke magnetic beads can be used.

8. Avoid loops formed by traces

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.

9. 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. Therefore, the characteristic impedance of the signal transmission line should be equal to the load impedance as much as possible; at the same time, pay attention to the PCB. The transmission line on the board should not have 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 wiring high-speed PCB boards: 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. The HDMI signal differential trace is required, the line width is 10mil, the line spacing is 6mil, and the spacing between each two HDMI differential signal pairs exceeds 20mil. LVDS wiring rules. The LVDS signal differential trace is required, with a line width of 7 mils and a line spacing of 6 mils. 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.

10. Maintain signal integrity

Maintain the integrity of signal transmission and prevent the "ground bounce phenomenon" caused by ground wire splitting on PCB board.