High frequency PCB layout and wiring 10 tips

1.multi-layer wiring

High frequency circuit often has high integration, high wiring density, using multilayer board is not only necessary for wiring, but also an effective means to reduce interference. In the stage of PCB Layout, reasonable selection of a certain number of layers of printed board size, can make full use of the intermediate layer to set shielding, better realize the nearby grounding, and effectively reduce the parasitic inductance and shorten the transmission length of the signal, but also greatly reduce the signal cross interference, all these methods are beneficial to the reliability of high frequency circuit.

Data shows that the same material, the noise of four layers is 20dB lower than that of double panels. However, there is also a problem, the higher the PCB half layer number, the more complex the manufacturing process, the higher the unit cost, which requires us to carry out the PCB Layout, in addition to the selection of the appropriate number of layers of PCB board, also need to carry out reasonable component layout planning, and adopt the correct wiring rules to complete the design.

2.The less the leads between the pins of high-speed electronic devices bend, the better

It is best to use the lead of high frequency circuit wiring fully straight line, need to turn, can be used 45 degrees fold line or arc turn, this requirement in low frequency circuit is only used to improve the copper foil fixation strength, and in high frequency circuit, meet this requirement can reduce the high-frequency signal external emission and mutual coupling.

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 length of the signal line. The longer the high-frequency signal lead, the easier it is to be coupled to the components close to it. Therefore, for high frequency signal lines such as signal clock, crystal oscillator, DDR data, LVDS wire, USB cable, HDMI cable, etc., the shorter the cable is required as far as possible.

4. High-frequency circuit device pin between the lead layer alternations as little as possible

The phrase "the less interlayer alternations of leads, the better" means that the fewer holes (Via) used in the connection of components are the better. According to one side, a hole can bring about 0.5pF of distributed capacitance. Reducing the number of holes can significantly increase speed and reduce the possibility of data errors.

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

High frequency circuit wiring should pay attention to "crosstalk" caused by short distance parallel routing of signal lines. Crosstalk refers to the coupling phenomenon between signal lines that are not directly connected. Since high-frequency signals are transmitted along the transmission line in the form of electromagnetic waves, signal lines will play the role of antennas, and the energy of electromagnetic fields will be transmitted around the transmission line. The unwanted noise signals generated between signals due to the mutual coupling of electromagnetic fields are called Crosstalk.

The parameters of PCB board layer, the distance between signal wires, the electrical characteristics of the driver and receiver end, and the connection mode of signal wires all have a certain influence on the crosstalk. So in order to reduce the crosstalk of high frequency signals, the following points should be done as far as possible when wiring:

If the wiring space allows, insert a ground or ground plane between the two lines with serious crosstalk, which can act as an isolation and reduce crosstalk.

When time-varying electromagnetic fields exist 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 interference.

On 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 rather than parallel.

If parallel cabling in the same floor is almost unavoidable, the direction of the two adjacent floors must be perpendicular to each other.

In digital circuits, the usual clock signals are those with fast edge changes and great crosstalk. Therefore, in the design, the clock line should be surrounded by ground lines and more ground holes to reduce the distributed capacitance, thus reducing crosstalk.

For the high frequency signal clock, try to use the low voltage differential clock signal and include the ground, and pay attention to the integrity of the ground punching.

The idle input terminal should not be suspended, but grounded or connected to the power supply (which is also ground in the high-frequency signal loop), because the suspended wire may be equivalent to the transmitting antenna, grounding can inhibit the transmission. Eliminating crosstalk in this way has sometimes proved to be effective immediately.

6.Integrated circuit block power pin increase high frequency of lotus root capacitance

Each integrated circuit block power pin to add a high frequency dropout capacitor. Increasing the high frequency decoupling capacitance of the power supply pin can effectively restrain the interference of high frequency harmonics on the power supply pin.

7. High frequency digital signal ground wire and analog signal ground wire to do isolation

When analog ground wire and digital ground wire are connected to the public ground wire, high frequency choke magnetic beads should be used to connect or directly isolate and select the appropriate ground single point interconnection. The ground potential of ground wires of high frequency digital signals is generally inconsistent, and there is often a certain voltage difference between the two directly. Moreover, ground wires of high frequency digital signals are often rich in harmonic components of high frequency signals. When the ground wires of digital signals and analog signal ground wires are directly connected, the harmonics of high frequency signals will interfere with analog signals by means of ground wire coupling.

Therefore, under normal circumstances, the ground wire of the high frequency digital signal and the ground wire of the analog signal should be isolated, which can be used in the way of single point interconnection at the appropriate position, or the way of high frequency choke magnetic bead interconnection.

8. Avoid the loop formed by the cable

All kinds of high-frequency signals should not form a loop. If it cannot be avoided, the loop area 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, the signal will be reflected in the transmission channel, and the reflection will make the synthesized signal form overimpulse, resulting in signal fluctuation near the logical threshold.

The fundamental way to eliminate the reflection is to make the impedance of the transmission signal good match, because the load impedance and the transmission line characteristic impedance difference is larger the reflection is larger, so it should be as far as possible to make the signal transmission line characteristic impedance and load impedance equal. At the same time, it should be noted that the transmission line on the PCB board should not have mutations or corners, and the impedance of each point of the transmission line should be kept continuous as far as possible, otherwise there will be reflections between the sections of the transmission line. This requires that the following wiring rules must be observed when high-speed PCB wiring is carried out:

USB cabling rules: USB signals are required to be routed differently. The cable width is 10mil, the cable distance is 6mil, and the distance between ground and signal cables is 6mil.

HDMI cabling rules: The HDMI signal is required to be routed differently. The line width is 10mil and the line distance is 6mil. The distance between two groups of HDMI differential signal pairs is more than 20mil.

LVDS wiring rules require LVDS signals to be wired differently, with a line width of 7mil and a line distance of 6mil, in order to control HDMI's differential signal pair impedance of 100+-15% ohm DDR wiring rules. Signal cables must be of the same width and equidistant from each other. Cables must meet the 2W principle to reduce crosstalk between signals. For DDR2 high-speed devices or later, cables must be of the same length to ensure signal impedance matching.

10.Maintain the integrity of signal transmission

Maintain the integrity of signal transmission and prevent the "ground projectile phenomenon" caused by the division of ground wire.