How to layout high frequency pcb

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High-frequency PCB refers to the electromagnetic frequency of the higher special circuit boards for high-frequency (frequency greater than 300MHZ or wavelength less than 1 meter) and microwave (frequency greater than 3GHZ or wavelength less than 0.1 meters) in the field of PCB, is in the microwave substrate copper-clad laminate boards on the use of ordinary rigid circuit boards manufactured using some of the process or the use of special treatment methods and the production of circuit boards. High-frequency circuit board circuit design is a very complex process, the layout of each line must be in place, the next will focus on high-frequency circuit board layout methods.

How to layout high frequency pcb？

1. Multilayer board wiring

High-frequency circuits are often highly integrated, wiring density, the use of multi-layer board is both necessary for wiring, but also an effective means of reducing interference. In the PCB Layout stage, a reasonable choice of a certain number of layers of printed circuit board size, can make full use of the intermediate layer to set up shielding, a better realization of the proximity of grounding, and effectively reduce the parasitic inductance and shorten the length of the signal transmission, but also significantly reduce the signal cross-interference, etc., all of which are beneficial to the reliability of high-frequency circuitry.

2. High-speed electronic devices between the pins of the lead bends the less the better

High-frequency circuit wiring of the lead is best to use a straight line, the need to turn, available 45-degree fold or arc turn, this requirement in the low-frequency circuits are only used to improve the strength of the copper foil adhesion, while in the high-frequency circuits to meet this requirement can reduce the high-frequency signals to the outside of the launch and coupling between each other.

3. High-frequency circuit device pins between the lead the shorter the better

Signal radiation intensity is proportional to the length of the signal line, high-frequency signal leads the longer, the easier it is coupled to the components close to it, so for such signals as the clock, crystal, DDR data, LVDS lines, USB lines, HDMI lines, and other high-frequency signal lines are required as much as possible, the shorter the line, the better.

4. The fourth trick: high-frequency circuit device pins between the lead layer alternation the less the better!

The so-called “less alternation between the layers of the lead, the better” means that the components used in the connection process of the hole (Via) the less the better. According to the side, a hole can bring about 0.5pF of distributed capacitance, reduce the number of holes can significantly improve speed and reduce the possibility of data errors.

5. Pay attention to the signal line close to the parallel alignment of the introduction of “crosstalk”

High-frequency circuit wiring should pay attention to the signal lines in close proximity to the parallel alignment of the introduction of “crosstalk”, crosstalk refers to the coupling phenomenon between the signal lines are not directly connected. As high-frequency signals along the transmission line is transmitted in the form of electromagnetic waves, the signal line will play the role of the antenna, the energy of the electromagnetic field will be emitted around the transmission line, the signal due to the mutual coupling of the electromagnetic field and the resulting undesired noise signal is called crosstalk. parameters of the PCB board layer, the spacing of the signal line, the driving end and the receiving end of the electrical characteristics, as well as the signal line termination method of the crosstalk have a certain impact. So, in order to reduce the crosstalk of high-frequency signals 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 allows, insert a ground line or ground plane between two lines with serious crosstalk, which can play the role of isolation and reduce crosstalk. When the space around the signal line itself there is a time-varying electromagnetic field, if you can not avoid parallel distribution, parallel signal lines can be arranged on the opposite side of a large area of “ground” to significantly reduce the interference.

Under the premise of wiring space permits, increase the spacing between adjacent signal lines, reduce the parallel length of the signal lines, clock lines as perpendicular as possible with the key signal lines rather than parallel. If the parallel alignment within the same layer is almost unavoidable, in two adjacent layers, the direction of the alignment must be perpendicular to each other.

In digital circuits, the clock signal is usually a fast-edge signal, external crosstalk. Therefore, in the design, the clock line should be surrounded by ground wires and more ground holes to reduce the distribution capacitance, thereby reducing crosstalk. Clock on high-frequency signals try to use low-voltage differential clock signals and packet ground way, need to pay attention to the integrity of the packet ground hole punching.

Idle not used inputs do not hang, but will be grounded or connected to the power supply (power in the high-frequency signal loop is also the ground), because the suspended line may be equivalent to the transmitting antenna, grounding will be able to inhibit the emission. Practice has shown that this approach to eliminate crosstalk can sometimes be immediately effective.

6. IC block power supply pins to increase the high-frequency decoupling capacitance

Each integrated circuit block power supply pin near the increase of a high-frequency decoupling capacitance. Increase the power supply pin high-frequency decoupling capacitance, can effectively inhibit the power supply pin on the high-frequency harmonics form interference.

7. Isolation of high-frequency digital and analog signal ground lines

Analog ground, digital ground line to the public ground with high-frequency choke bead connection or direct isolation and choose a suitable place for single-point interconnection. High-frequency digital signal ground potential is generally inconsistent, there is often a certain voltage difference between the two directly; and, high-frequency digital signal ground is often with a very rich high-frequency signal harmonic components, when directly connected to the digital signal ground and analog signal ground, high-frequency signal harmonics will be coupled through the ground to the analog signal to interfere with the way.
In general, the high-frequency digital signal ground and analog signal ground is to do the isolation, can be used in the appropriate location of a single point of interconnection, or the use of high-frequency choke bead interconnection.

8. Avoid the formation of the loop alignment

Various types of high-frequency signal alignment try not to form a loop, if you can not avoid the loop area should be as small as possible.

9. Must ensure good signal impedance matching

Signal in the transmission process, when the impedance mismatch, the signal will occur in the transmission channel signal reflection, reflection will make the synthesized signal overshoot, resulting in the signal fluctuations in the vicinity of the logic threshold.
Eliminate the reflection of the fundamental approach is to make the transmission signal impedance is well-matched, due to the load impedance and the transmission line of the characteristic impedance of the larger the difference between the reflection of the larger, so should be as much as possible to make the signal transmission line of the characteristic impedance of the load impedance and load impedance is equal; at the same time, but also to pay attention to the PCB on the transmission line can not be a sudden change or corners, as far as possible to maintain the transmission line at all points of the impedance continuity, or in the transmission line between the various segments will be a reflection.

10. Maintain the integrity of the signal transmission

Maintain the integrity of the signal transmission, to prevent the “ground bounce phenomenon” caused by the division of the ground.