PCB Tech - Ten rules of high-frequency circuit design and wiring skills

[First trick] Multi-layer board wiring High-frequency circuits tend to have high integration and high wiring density. The use of multi-layer boards is not only necessary for wiring, but also an effective means to reduce interference.

In the PCB Layout stage, a reasonable selection of the printed board size with a certain number of layers can make full use of the intermediate layer to set up the shield, better realize the nearest grounding, and effectively reduce the parasitic inductance and shorten the signal transmission length, while still maintaining a large All of these methods are beneficial to the reliability of high-frequency circuits, such as reducing signal cross-interference.

Some data show that when the same 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 number of PCB half-layers, the more complicated the manufacturing process and the higher the unit cost. This requires us to select PCBs with appropriate layers in addition to selecting the appropriate number of layers. Reasonable component layout planning, and use correct wiring rules to complete the design.

[Second trick] The lead between the pins of high-speed electronic devices should be bent as little as possible. The lead of high-frequency circuit wiring is best to use a full straight line, which needs to be turned. It can be turned by a 45-degree broken line or a circular arc. It is only used to improve the fixing strength of copper foil, but in high-frequency circuits, meeting this requirement can reduce the external emission and mutual coupling of high-frequency signals.

[Third trick] 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 lead of the high-frequency signal, the easier it is to couple to it. For high-frequency signal lines such as signal clock, crystal oscillator, DDR data, LVDS line, USB line, HDMI line, etc., it is required to keep the trace as short as possible.

[Fourth measure] The less the lead-layer alternation between the pins of the high-frequency circuit device, the better. The so-called "the less the lead-layer alternation, the better" refers to the fewer vias (via) used in the component connection process, the better . On the data side, one 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.

[Fifth trick] Pay attention to the "crosstalk" introduced by signal lines in close parallel routing. High-frequency circuit wiring should pay attention to the "crosstalk" introduced by signal lines in close parallel routing. Crosstalk refers to the signal lines that are not directly connected. Coupling phenomenon.

Because 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 the electromagnetic field will be emitted around the transmission line. Undesired noise signals are generated due to the mutual coupling of electromagnetic fields between the signals. Called crosstalk (Crosstalk).

The parameters of the PCB layer, the spacing of the signal lines, the electrical characteristics of the driving end and the receiving end, and the signal line termination method all have a certain impact on the 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: When the wiring space allows, insert a ground wire or ground plane between the two wires with more serious crosstalk. Play the role of isolation and reduce crosstalk.

When there is a time-varying electromagnetic field in the space surrounding 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.

If the wiring space permits, increase the spacing 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 instead of parallel. If parallel traces in the same layer are almost unavoidable, the directions of traces must be perpendicular to each other in two adjacent layers.

In digital circuits, the usual clock signals are signals with fast edge changes, which have high external crosstalk. Therefore, in the design, it is advisable to surround the clock line with a ground wire and punch more ground wire holes to reduce distributed capacitance, thereby reducing crosstalk. For the high-frequency signal clock, try to use the low-voltage differential clock signal and the ground mode, and pay attention to the integrity of the ground hole.

The unused input terminal should not be suspended, but 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 the grounding can inhibit the transmission. Practice has proved that using this method to eliminate crosstalk can sometimes yield immediate results.

[Sixth trick] Add a high-frequency decoupling capacitor to the power supply pin of the integrated circuit block. Add a high-frequency decoupling capacitor to the power supply pin of each integrated circuit block. Increasing the high-frequency decoupling capacitor of the power supply pin can effectively suppress the interference of high-frequency harmonics on the power supply pin.

[Seventh trick] The ground wire of high-frequency digital signal and the ground wire of analog signal should be isolated. When connecting analog ground wire, digital ground wire, etc. to the public ground wire, use high-frequency choke magnetic beads to connect or directly isolate and choose a suitable place Single point interconnection.

The ground potential of the ground wire of the high-frequency digital signal is generally inconsistent, and there is often a certain voltage difference between the two directly. Moreover, the ground wire of the high-frequency digital signal often contains very rich harmonic components of the high-frequency signal. When the digital signal ground wire and the analog signal ground wire are directly connected, the harmonics of the high frequency signal will interfere with the analog signal through the ground wire coupling.

Therefore, under normal circumstances, the ground wire of the high-frequency digital signal and the ground wire of the analog signal are to be isolated, and the method of single-point interconnection at a suitable position or the method of interconnection by high-frequency choke magnetic beads can be adopted.

[Eighth Tip] Avoid loops formed by traces. Try not to form loops in various high-frequency signal traces. If it is unavoidable, make the loop area as small as possible.

[Ninth trick] It is necessary to ensure good signal impedance matching. During the transmission of the signal, when the impedance does not match, the signal will reflect in the transmission channel. The reflection will cause the synthesized signal to form an overshoot, resulting in the signal Fluctuating around the logic threshold.

The fundamental way to eliminate reflection is to match the impedance of the transmission signal well. Since 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 should be made equal to the load impedance as much as possible.

At the same time, note that the transmission line on the PCB must not have sudden changes or corners, and try to keep the impedance of each point of the transmission line continuous, otherwise there will be reflections between the segments of the transmission line.

This requires that during high-speed PCB wiring, the following wiring rules must be observed: USB wiring rules.

Requires USB signal differential routing, line width 10mil, line spacing 6mil, ground line and signal line spacing 6mil.

HDMI wiring rules.

The HDMI signal differential routing is required, the line width is 10mil, the line spacing is 6mil, and the spacing between each two sets of HDMI differential signal pairs exceeds 20mil.

LVDS wiring rules.

Requires LVDS signal differential routing, line width 7mil, line spacing 6mil, the purpose is to control the differential signal pair impedance of HDMI to 100+-15% ohm DDR wiring rules.

DDR1 wiring requires signals not to go through holes as much as possible. The signal lines are of equal width and the distance between lines. The wiring must meet the 2W principle to reduce crosstalk between signals. For high-speed devices of DDR2 and above, high-frequency data is also required. The lines are equal in length to ensure the impedance matching of the signal.

[Tenth trick] Maintain the integrity of signal transmission. Maintain the integrity of signal transmission and prevent the "ground bounce phenomenon" caused by ground wire splitting.