CESGATE's suggestions: 10 ways to do high-frequency PCB layout and wiring

[1] Multi-layer wiring

High frequency circuit is often high integration, wiring density, the use of multilayer is not only necessary for wiring, but also an effective means to reduce interference. In PCB Layout stage, reasonable choice of a certain layer printed circuit board size, the middle tier can be fully used to set the shielding, better implement nearby ground, and effectively reduce the parasitic inductance and shorten the length of the transmission of signals, but also can greatly reduce the cross interference of signals etc., all of these methods are good for the reliability of the high frequency circuit.

Data shows, when consort material, 4 board should compare the noise of double face plate low 20dB. However, there is a problem at the same time. The higher the PCB half layer is, the more complex the manufacturing process is, and the higher the unit cost will be. Therefore, when conducting PCB Layout, we should not only select the PCB with appropriate layers, but also carry out reasonable component Layout planning and adopt correct wiring rules to complete the design.

 

[2] The less lead bending between pins of high-speed electronic devices, the better

It is best to use a full straight line for high-frequency circuit wiring, which can be turned by 45 degree broken line or arc turning. This requirement is only used to improve the fixed strength of copper foil in low-frequency circuits, but in high-frequency circuits, meeting this requirement can reduce the external emission and mutual coupling of high-frequency signals.

 

[3] The shorter the lead between pins of high-frequency circuit devices, the better

The radiation intensity of the signal is directly proportional to the length of the signal line, the longer the high-frequency signal lead, the easier it is to coupling to the components close to it, so for such as the signal clock, crystal oscillator, DDR data, LVDS line, USB line, HDMI line and other high-frequency signal lines are required as far as possible the shorter the better.

 

[4] The lead layer between pins of high-frequency circuit devices should be alternated as little as possible

"As few layers of leads as possible" means that as few vias are used during component connections. On the data side, a single hole can deliver about 0.5pF of distributed capacitance, and reducing the number of holes can significantly improve speed and reduce the possibility of data errors.

 

[5] Pay attention to the "crosstalk" introduced by the signal line near the parallel line.

When wiring high frequency circuits, pay attention to the "crosstalk" caused by the close parallel routing of signal cables. Crosstalk refers to the coupling phenomenon between signal cables that are not directly connected. As high frequency signals are transmitted in the form of electromagnetic waves along the transmission line, the signal line will act as an antenna, and the energy of electromagnetic fields will be transmitted around the transmission line. The unwanted noise signals generated by the coupling of electromagnetic fields between signals are called Crosstalk.

The parameters of PCB board layer, the distance between signal lines, the electrical characteristics of driving end and receiving end, and the connection mode of signal line all have certain influence on crosstalk. Therefore, in order to reduce the crosstalk of high frequency signals, the following points should be done as much as possible during wiring:

If the wiring space permits, a ground wire or ground plane is inserted between the two lines with serious crosstalk, which can play a role of isolation and reduce crosstalk.

When the space around the signal line itself has time-varying electromagnetic field, 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 of allowable wiring space, increase the distance between adjacent signal lines, reduce the parallel length of the signal lines, and try to make the clock line perpendicular to the key signal line rather than parallel.

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

In the digital circuit, the clock signal is usually the signal with fast edge change and high external crosstalk. Therefore, in the design, the clock line should be surrounded by ground lines and drilled more ground holes to reduce the distributed capacitance, thus reducing crosstalk.

For high frequency signal clock, use low voltage differential clock signal and ground encapsulation mode as far as possible, and pay attention to the integrity of ground encapsulation drilling.

Idle input terminals should not be suspended, but should be grounded or connected to a power source (the power source is also ground in the hf signal loop), because the suspended line may be equivalent to a transmitting antenna, and grounding can inhibit transmission. In practice, eliminating crosstalk in this way is sometimes effective immediately.

 

[6] The power pin of integrated circuit block increases the high frequency decoupling capacitance

The power pin of each IC block is equipped with a high-frequency decoupling capacitor nearby. Increasing the high frequency decoupling capacitance of the power supply pin can effectively suppress the interference of high frequency harmonic formation.

 

[7] High frequency digital signal grounding and analog signal grounding do isolation

When analog ground wire, digital ground wire and so on are connected to public ground wire, high frequency choke magnetic bead should be used to connect or directly isolated and suitable single point interconnection should be selected. High frequency digital signal of ground wire ground potential is not consistent, both directly there is often a voltage difference, and the high frequency digital signal ground wire is often with very rich harmonic component of high frequency signal ground when connected directly to a digital signal and analog signal ground, the high frequency signal harmonic will through the way of the ground coupled to the analog signal interference.

Therefore, in general, 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 a suitable position of the way of single point interconnection, or the high frequency choke magnetic bead interconnection.

 

[8] Avoid the loop formed by wiring

All kinds of high frequency signal wiring should not form loops as far as possible, if unavoidable, should make the loop area as small as possible.

 

[9] Good signal impedance matching must be guaranteed

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 make the synthetic signal overshoot, resulting in the signal fluctuation near the logical threshold.

The fundamental way to eliminate reflection is to make the impedance of the transmission signal match well. 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 and the load impedance should be equal as far as possible. At the same time, it should also be noted that the transmission line on PCB should not appear mutations or corners. Try to keep the impedance of each point of the transmission line continuous, otherwise there will be reflections between each section of the transmission line. This requires that when high-speed PCB wiring, the following wiring rules must be followed:

USB wiring rules: USB signal differential wiring is required, with a line width of 10mil and a line distance of 6mil, and a distance between ground and signal lines of 6mil.

HDMI wiring rules: HDMI signal differential wiring, line width of 10mil, line spacing of 6mil, each two groups of HDMI differential signal spacing exceeds 20mil.

LVDS wiring rules require LVDS signal differential wiring, line width 7mil, line spacing 6mil, the purpose is to control HDMI differential signal pair impedance of 100+-15% ohm DDR wiring rules. DDR1 cabling requires that signals do not pass through holes. The signal cables are equally wide and the cables are equally spaced from each other. The cabling must meet the 2W principle to reduce crosstalk between signals.

 

[10] Maintain the integrity of signal transmission

Maintain the integrity of signal transmission to prevent the "ground bounce phenomenon" caused by ground wire segmentation.