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High-frequency circuit layout techniques in PCB Layout

writer: G March 28, 2018

1. Multilayer board layout

High-frequency circuits often have high integration and high wiring density. The use of multi-layer boards is not only a must for wiring, but also an effective means to reduce interference. In the PCB Layout phase, a certain number of printed circuit board dimensions can be reasonably selected, and the intermediate layer can be fully utilized to set the shielding, and the grounding can be better achieved, and the parasitic inductance can be effectively reduced and the signal transmission length can be shortened. Reducing signal crosstalk, etc., all of these methods are beneficial to the reliability of high frequency circuits. According to the data, when the same kind of material is used, the noise of the four-layer board is 20dB lower than that of the double-sided board. However, there is also a problem. The higher the PCB half-layer number, the more complicated the manufacturing process, and the higher the unit cost. This requires us to carry out PCB Layout, in addition to selecting a suitable number of PCB boards, it needs to be performed. Reasonable component layout planning, and use the correct wiring rules to complete the design.

2. The less the lead wire between high-speed electronic device pins is bent, the better

The leads of high-frequency circuit wiring are preferably full-straight and need to be turned. The 45-degree bend line or circular arc can be used. This requirement is only used to improve the fixing strength of copper foil in low-frequency circuits. In high-frequency circuits, this is satisfied. A requirement can reduce the emission of high-frequency signals and the coupling between them.

3. The shorter the lead between high-frequency circuit device pins, the better

The radiant intensity of the signal is proportional to the length of the signal line trace. The longer the high-frequency signal trace, the easier it is to couple to components near it, so for clocks, crystals, and DDR data such as signals, High-frequency signal lines such as LVDS cable, USB cable, and HDMI cable are required to be as short as possible.

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

The so-called "less inter-layer alternation of leads is better" means that fewer vias (Via) are used during element connection. On the other hand, a via can bring about 0.5pF of distributed capacitance. Reducing the number of vias can significantly increase the speed and reduce the possibility of data errors.

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

High-frequency circuit wiring should pay attention to the "cross talk" introduced by the signal line close parallel routing, and 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 functions as an antenna, and the energy of the electromagnetic field is transmitted around the transmission line. Unwanted noise signals generated due to mutual coupling of electromagnetic fields Called Crosstalk. The parameters of the PCB layers, the spacing of the signal lines, the electrical characteristics of the driving and receiving terminals, and the termination of the signal lines have a certain influence on 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:

(1) Inserting a ground line or ground plane between two lines with severe crosstalk under the condition that the wiring space allows, can play a role in isolation and reduce crosstalk.

(2) 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 interference.

(3) Under the premise of permission of the wiring space, increase the spacing between adjacent signal lines and reduce the parallel length of the signal lines. The clock lines should be perpendicular to the key signal lines as far as possible without being parallel.

(4) If the parallel traces in the same layer can hardly be avoided, the directions of the traces must be perpendicular to each other in the two adjacent layers.

(5) In a digital circuit, the normal clock signal is a signal with a rapid edge change, and the external crosstalk is large. Therefore, in the design, the clock line should be surrounded by ground and more ground holes to reduce the distributed capacitance, thereby reducing crosstalk.

(6) As far as possible, the low-frequency differential clock signal is used for the high-frequency signal clock and the ground is packaged. The integrity of the package hole must be observed.

(7) The unused input terminals should not be left unconnected, but should be grounded or connected to the power supply (the power supply is also ground in the high-frequency signal loop). Because the suspended wires may be equivalent to the transmitting antenna, the grounding can suppress the emission. Practice has shown that using this method to eliminate crosstalk can sometimes immediately take effect.

6. The power supply pin of the integrated circuit block increases the high frequency decoupling capacitance

A high-frequency decoupling capacitor is added to the power pin of each IC block. Increasing the high-frequency decoupling capacitor of the power pin can effectively suppress the interference of the high-frequency harmonics on the power pin.

7. High-frequency digital signal ground and analog signal ground isolation

When analog ground lines, digital ground lines, etc. are connected to a common ground line, they must be connected with high-frequency choke beads or isolated directly and choose the appropriate local single-point interconnection. The ground potential of the high-frequency digital signal ground is generally inconsistent, and there is often a certain voltage difference between the two. Furthermore, the high-frequency digital signal ground often has a very rich harmonic component of the high-frequency signal. When the digital signal ground and the analog signal ground are directly connected, the harmonics of the high-frequency signal will interfere with the analog signal through the ground coupling. Therefore, under normal circumstances, the high-frequency digital signal ground and analog signal ground should be isolated, you can use a single point of interconnection in the right place, or use high-frequency choke magnetic beads interconnect.

8. Avoid loops formed by traces

All kinds of high-frequency signal traces should not form a loop as much as possible, if not avoid the loop area should be as small as possible.

9. Must ensure good signal impedance matching

During signal transmission, when the impedance does not match, the signal will be reflected in the transmission channel. The reflection will cause the resultant signal to overshoot, causing the signal to fluctuate near the logic threshold.

The fundamental method of eliminating reflection is to make the impedance of the transmission signal well matched. Since the difference between the load impedance and the characteristic impedance of the transmission line is larger, the reflection is also greater, so the characteristic impedance of the signal transmission line should be equal to the load impedance as much as possible. Also note that the transmission line on the PCB must not show abrupt changes or corners. Try to keep the impedance of the transmission line points contiguous. Otherwise, reflections will occur between the transmission line segments. This requires that the following wiring rules must be observed when performing high-speed PCB layout:

(1) USB wiring rules. Requires USB signal differential routing, line width 10mil, line spacing 6mil, ground and signal line spacing 6mil.

(2) HDMI cabling rules. Requires HDMI signal differential routing, line width 10mil, line spacing 6mil, each pair of HDMI differential signal pair spacing of more than 20mil.

(3) LVDS wiring rules. Requirements LVDS signal differential routing, line width 7mil, line spacing 6mil, the purpose is to control the HDMI differential signal impedance of 100 +-15% ohms

(4) DDR wiring rules. DDR1 traces require the signal not to go through the hole as much as possible, the signal line is equal in width, and the line is equidistant from the line. The trace must meet the 2W principle to reduce the crosstalk between signals. High-speed data is required for high-speed devices with DDR2 and above. The lines are of equal length to ensure that the impedance of the signal matches.

10. To maintain the integrity of the signal transmission and prevent "ground bounce" caused by ground segmentation.

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