Ten "tricks" for high-frequency PCB layout (II)

[Fourth trick] The less the transition between the lead layers between the pins of the high-frequency circuit device, the better.

The so-called "the least alternating between the layers of the leads is better" means that the fewer vias (Via) used in the component connection process, the better. According to the side, a via can bring about a distributed capacitance of about 0.5pF, and 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 the parallel lines of the signal lines.

High-frequency circuit wiring should pay attention to the "crosstalk" introduced by the parallel lines of the signal lines. 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 acts as an antenna, and the energy of the electromagnetic field is emitted around the transmission line, and an undesired noise signal generated by the mutual coupling of the electromagnetic fields between the signals Called Crosstalk.

The parameters of the PCB layer, the spacing of the signal lines, the electrical characteristics of the driver and receiver, and the termination of the signal line all have a certain impact on crosstalk. Therefore, in order to reduce the crosstalk of high-frequency signals, it is required to do the following as much as possible during wiring:

Inserting a ground or ground plane between two lines with severe crosstalk can allow isolation and reduce crosstalk under the conditions allowed by the wiring space.

When there is a time-varying electromagnetic field in the space around the signal line, if parallel distribution cannot be avoided, a large area "ground" can be placed on the reverse side of the parallel signal line to greatly reduce the interference.

Under the premise of wiring space permission, increase the spacing between adjacent signal lines, reduce the parallel length of the signal lines, and the clock lines should be perpendicular to the key signal lines and not parallel.

If the parallel traces in the same layer are almost unavoidable, in the adjacent two layers, the direction of the traces must be perpendicular to each other.

In digital circuits, the usual clock signals are signals with fast edge changes, and the external crosstalk is large. 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 cover the ground. You need to pay attention to the integrity of the package.

Do not leave the unused input terminal, but ground it or connect it to the power supply (the power supply is also ground in the high-frequency signal loop). Because the suspended line may be equivalent to the transmitting antenna, grounding can suppress the emission. Practice has proved that using this method to eliminate crosstalk can sometimes be effective immediately.

[Sixth measure] The power supply pin of the integrated circuit block increases the high frequency untwisting capacitance

A high frequency untwisting capacitor is added 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 high frequency harmonics on the power supply pin to form interference.

[Seventh trick] The ground of the high-frequency digital signal is isolated from the analog signal ground.

When analog ground lines, digital ground lines, etc. are connected to the common ground line, high-frequency turbulent magnetic beads should be used to connect or directly isolate and select a suitable place for single-point interconnection. The ground potential of the ground of the high-frequency digital signal is generally inconsistent, and there is often a certain voltage difference between the two directly. Moreover, the ground line of the high-frequency digital signal 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 interfere with the analog signal by means of ground-line coupling.

Therefore, in general, the ground of the high-frequency digital signal and the ground of the analog signal are to be isolated, and the method of single-point interconnection at a suitable position or the interconnection of high-frequency turbulent magnetic beads can be adopted.