The less alternating the lead layers between the pins of high-frequency circuit devices, the better

The so-called 'less interlayer alternation of leads, better' refers to the use of fewer vias during the component connection process. One via can bring a distributed capacitance of about 0.5pF, reducing the number of vias can significantly improve speed and reduce the possibility of data errors.

The shorter the leads between the pins of high-frequency circuit devices, the better

The radiation intensity of a signal is proportional to the length of the signal line. The longer the high-frequency signal lead, the easier it is to couple to components close to it. Therefore, for high-frequency signal lines such as clock, crystal oscillator, DDR data, LVDS lines, USB lines, HDMI lines, etc., it is required to have as short a wiring as possible.

The fewer lead bends between pins of high-speed electronic devices, the better

The leads of high-frequency circuit wiring are all straight and require turning, which can be done with a 45 degree zigzag or circular arc turning. This requirement is only used to improve the adhesion strength of copper foil in low-frequency circuits, while in high-frequency circuits, meeting this requirement can reduce the external emission and mutual coupling of high-frequency signals.

Pay attention to the "crosstalk" introduced by parallel routing of signal lines at close intervals

High frequency circuit wiring should pay attention to the "crosstalk" introduced by parallel routing of signal lines in close proximity. Crosstalk refers to the coupling phenomenon between signal lines that are not directly connected. Because high-frequency signals are transmitted in the form of electromagnetic waves along transmission lines, signal lines act as antennas, and the energy of electromagnetic fields is emitted near the transmission lines. The unwanted noise signals generated between signals due to the mutual coupling of electromagnetic fields are called crosstalk. The parameters of PCB board layers, spacing of signal lines, electrical characteristics of driver and receiver terminals, and signal line termination methods all have an impact on crosstalk. To reduce the crosstalk of high-frequency signals, it is required to achieve the following points as much as possible during wiring:

(1) Inserting a ground wire or plane between two lines with severe crosstalk, provided that the wiring space allows for it, can serve as isolation and reduce crosstalk;

(2) When there is a time-varying electromagnetic field in the space near 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 significantly reduce interference;

(3) If 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 hub signal line rather than parallel;

(4) If parallel routing within a unified layer is almost unavoidable, the direction of routing in adjacent layers must be perpendicular to each other;

(5) In digital circuits, the usual clock signals are those with fast edge changes and high external crosstalk. Therefore, in the design, the clock line should be wrapped with a ground wire and more ground holes should be drilled to reduce distributed capacitance and thus reduce crosstalk;

(6) For high-frequency signal clocks, it is recommended to use low-voltage differential clock signals and ground wrapping methods, and attention should be paid to the integrity of the ground punching holes;

(7) Do not suspend unused input terminals, but ground or connect them to a power source (which is also grounded in the high-frequency signal circuit). Since suspended wires may be equivalent to transmitting antennas, grounding can suppress transmission. Practice has proven that using this method to remove crosstalk can sometimes yield immediate results.

Isolate the ground wire of high-frequency digital signals from the ground wire of analog signals

When connecting analog ground wires, digital ground wires, etc. to the common ground wire, high-frequency choke magnetic beads should be used for connection or direct isolation, and a suitable single point interconnection should be selected. The ground potential of the ground wire of high-frequency digital signals is generally inconsistent, and there is often a voltage difference between the two. In addition, the ground wire of high-frequency digital signals often carries a very rich harmonic component of the high-frequency signal. When directly connecting the digital signal ground wire and the analog signal ground wire, the harmonic of the high-frequency signal will interfere with the analog signal through ground wire coupling. Therefore, in general, the ground wires of high-frequency digital signals and analog signals need to be isolated, which can be achieved through single point interconnection at appropriate locations or high-frequency choke magnetic bead interconnection.

Adding high-frequency decoupling capacitors to the power pins of integrated circuit blocks

Add a high-frequency decoupling capacitor to the power pin of each integrated circuit block nearby. Adding high-frequency decoupling capacitors to the power pins can suppress the interference caused by high-frequency harmonics on the power pins.

Avoid the formation of loops caused by wiring

Various high-frequency signal routing should avoid forming loops as much as possible. If unavoidable, the loop area should be kept as small as possible.

It is necessary to ensure good signal impedance matching

During the transmission of signals, when the impedance does not match, the signal will undergo reflection in the transmission channel, which will cause the synthesized signal to form overshoot and cause the signal to fluctuate around the logic threshold.

Article source: PCB manufacturerhttp://www.yonghongpcb.com/