Ten PCB Routing Rules for High Frequency Circuits

If the frequency of digital logic circuit reaches or exceeds 45MHZ~50MHZ, and the circuit working above this frequency has taken up a certain part of the whole electronic system (for example, 1/3), it is usually called high frequency circuit. High frequency circuit design is a very complex design process, and its wiring is very important to the whole design!

1. Multilayer board wiring

High frequency circuits are often highly integrated and wiring density is high. The use of multi-layer board is not only necessary for wiring, but also an effective means to reduce interference.

In PCB Layout stage, reasonable selection of the size of PCB with certain layers can make full use of the middle layer to set up shield, better realize near grounding, effectively reduce parasitic inductance and shorten the transmission length of signals, and also greatly reduce the cross-interference of signals.

All these methods are beneficial to the reliability of high frequency circuits.

Data show that the noise of 4-layer PCB is 20 dB lower than that of double-sided PCB when the same material is used. However, there is also a problem. The more layers of PCB, the more complex the manufacturing process, and the higher the unit cost. This requires us to select the appropriate layers of PCB board in PCB Layout, but also need to make reasonable layout planning of components, and adopt the correct wiring rules to complete the design.

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

High-frequency circuit wiring leads should be straight line, need turning point, can use 45 degree break line or arc turning point.

This requirement is only used to improve the fixed strength of copper foil in low frequency circuit, but in high frequency circuit, meeting this requirement can reduce the external transmission of high frequency signal and mutual coupling.

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 components close to it. Therefore, for high frequency signal lines such as clock, crystal oscillator, DDR data, LVDS line, USB line, HDMI line, the shorter the route is, the better.

4. The fewer alternations of lead layers between pins of high frequency circuit devices, the better

The so-called "less interlayer alternation of lead is better" refers to the fewer holes (Via) used in the process of component connection. According to the side, a through hole can bring about about about 0.5pF distributed capacitance. Reducing the number of through holes can significantly improve the speed and reduce the possibility of data errors.

5. Pay attention to the "crosstalk" introduced by the close parallel line of the signal line

In high frequency circuit wiring, attention should be paid to the "crosstalk" introduced by the close parallel routes of signal lines. Crosstalk refers to the coupling phenomenon between signal lines without direct connection. Because the high frequency signal is transmitted along the transmission line in the form of electromagnetic wave, the signal line will play an antenna role, and the energy of the electromagnetic field will be transmitted around the transmission line. The undesirable noise signal caused by the mutual coupling of the electromagnetic field between the signals is called Crosstalk. The parameters of PCB board, the spacing of signal lines, the electrical characteristics of driver and receiver, and the connection mode of signal lines have certain effects on crosstalk. So in order to reduce the crosstalk of high frequency signal, the following points should be done as far as possible when wiring:

• Under the permissible condition of wiring space, inserting a ground line or ground plane between two lines with serious crosstalk can isolate and reduce crosstalk. When the time-varying electromagnetic field exists in the space around the signal line itself, if parallel distribution cannot be avoided, large area "ground" can be arranged on the opposite side of the parallel signal line to greatly reduce interference.

• Under the premise of permission of wiring space, the distance between adjacent signal lines should be increased, the parallel length of signal lines should be reduced, and the clock lines should be perpendicular to and not parallel to the key signal lines. If parallel lines in the same layer are almost unavoidable, in the adjacent two layers, the directions of the lines must be perpendicular to each other.

• In digital circuits, the usual clock signal is the signal with fast edge change, which has large external crosstalk. Therefore, in the design, the clockline should be surrounded by wires and more grounding holes to reduce the distributed capacitance, thereby reducing crosstalk. For high-frequency signal clocks, the low-voltage differential clock signal and encapsulation method should be used as far as possible, and the integrity of encapsulation and perforation should be paid attention to.

• The unused input terminal should not be suspended, but be grounded or connected to the power supply (the power supply is also grounded in the high frequency signal loop), because the suspended line may be equivalent to the transmitting antenna, and grounding can suppress the transmission. Practice has proved that this method can sometimes achieve immediate results in eliminating crosstalk.

6. Increase the high frequency decoupling capacitance of the power pin of the integrated circuit 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 pin can effectively suppress the interference of high frequency harmonics on the power pin.

7. Isolation of Ground Wire of High Frequency Digital Signal and Analog Signal

When analog and digital ground wires are connected to common ground wires, they should be connected by high frequency choke beads or isolated directly and selected suitable places for single point interconnection. Ground potential of high frequency digital signal is generally inconsistent. There is a voltage difference between them. Moreover, the ground wire of high frequency digital signal often has abundant harmonic components of high frequency signal. When the ground wire of digital signal and analog signal is connected directly, the harmonic of high frequency signal will pass through. Ground-wire coupling interferes with analog signals. So in general, the ground wire of high frequency digital signal and analog signal should be isolated. It can be connected by a single point at an appropriate location or by high frequency choke beads.

8. Avoid loops formed by routing

All kinds of high frequency signal routes should not form a loop as far as possible. If it is unavoidable, the area of the loop should be as small as possible.

9. Good signal impedance matching must be ensured

In the process of signal transmission, when the impedance is not matched, signal reflection will occur in the transmission channel, which will cause the synthetic signal to overshoot and cause the signal to fluctuate near the logical threshold.

The fundamental way to eliminate 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, the more equal the characteristic impedance of the signal transmission line and the load impedance should be made as far as possible. At the same time, we should also pay attention to the transmission line on PCB can not appear sudden change or corner, try to keep the impedance of each point of the transmission line continuous, otherwise there will be reflection between the transmission line segments. This requires that the following routing rules must be followed in high-speed PCB routing:

USB wiring rules

USB signal differential routing is required, with a line width of 10 mil, a line spacing of 6 mil, a ground line and a signal line spacing of 6 mil.

HDMI routing rules

Differential routing of HDMI signals is required, with a line width of 10 mil and a line spacing of 6 mil. The spacing between two groups of HDMI differential signal pairs exceeds 20 mil.

LVDS routing rules

The differential routing of LVDS signal is required, with a width of 7 mil and a distance of 6 mil. The purpose is to control the differential impedance of HDMI signal to 100 +15% ohm.

DDR routing rules

DDR1 routing requires that the signal should not pass through holes as far as possible, the signal line should be equal in width and the line should be equal in distance. The routing must satisfy the 2W principle to reduce the crosstalk between signals. For high-speed devices with DDR2 or more, the high-frequency data routing should be equal in length to ensure the impedance matching of signals.

10. Maintaining the integrity of signal transmission

Maintain the integrity of signal transmission and prevent the "ground bullet phenomenon" caused by ground line segmentation.