High frequency PCB design precautions

HIGH FREQUENCY PCB design precautions

With the rapid development of electronic technology and the wide application of wireless communication technology in various fields, high frequency, high speed and high density have gradually become one of the significant development trends of modern electronic products. High-frequency signal transmission and high-speed digitization force the PCB to move toward micro-holes and buried/blind holes, fine wires, and uniform thinning of dielectric layers. High-frequency high-speed and high-density multilayer PCB design technology has become an important research field. This article first briefly introduces the high-frequency circuit board, and then explains the wiring design of the high-frequency circuit board of the PCB design. Finally, it introduces the precautions for the high-frequency circuit board layout of the PCB design. Let's take a look at the small series.

Introduction to high frequency circuit board

The high-frequency circuit board is a special circuit board with a high electromagnetic frequency. Generally, the high frequency can be defined as a frequency above 1 GHz. Its physical properties, precision, and technical parameters are very high, and it is often used in automotive anti-collision systems, satellite systems, radio systems and other fields.

The high-frequency circuit board provided by the utility model is provided with a rib-blocking rib at the upper opening and the lower opening edge of the hollow groove of the core board, so that the core board and the upper surface and the lower surface are covered. When the copper plate is bonded, the flow glue does not enter the hollow groove, that is, the bonding operation can be completed by one press-fitting, and the high-frequency circuit board which can be completed by the secondary pressing in the prior art, the high-frequency circuit in the utility model The board has a simple structure, low cost and is easy to manufacture.

PCB design high frequency circuit board wiring skills

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

The lead wire of the high-frequency circuit wiring is preferably a full line, which needs to be turned, and can be folded by a 45-degree line or a circular arc. This requirement is only used to improve the fixing strength of the copper foil in the low-frequency circuit, and in the high-frequency circuit, the content is satisfied. One requirement is to reduce the external transmission and mutual coupling of high frequency signals.

Second, the high frequency circuit device between the pin layers alternately less as possible

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. A via can bring about a distributed capacitance of about 0.5pF, and reducing the number of vias can significantly increase speed and reduce the possibility of data errors.

Third, the lead between the high-frequency circuit device pins is as short as possible

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

Fourth, 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 is generated. 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:

(1) 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;

(2) Inserting a ground or ground plane between two lines with severe crosstalk under the condition of the wiring space, can play the role of isolation and reduce crosstalk;

(3) 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 opposite side of the parallel signal line to greatly reduce interference;

(4) 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, thereby reducing crosstalk;

(5) Try to use a low-voltage differential clock signal and package the ground for the high-frequency signal clock, and pay attention to the integrity of the package hole punching;

(6) Under the premise of permitting the wiring space, 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;

(7) 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.

Fifth, 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.

6. Grounding of high-frequency digital signals and analog signal grounding are isolated

When the analog ground wire, digital ground wire, etc. are connected to the common ground wire, the high-frequency turbulent magnetic beads are 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. There is often a certain voltage difference between the two, and the ground of the high-frequency digital signal often has a very rich harmonic component of the high-frequency signal. When the digital signal ground line and the analog signal ground line 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 a single-point interconnection at a suitable position or a high-frequency turbulent magnetic bead interconnection may be employed.

Seven, avoid the loop formed by the trace

Do not form a loop as much as possible for all types of high-frequency signal traces. If it is unavoidable, make the loop area as small as possible.

PCB design high frequency circuit board wiring considerations

1) Reasonable selection of layers

When wiring high-frequency circuit boards in PCB design, the intermediate inner plane can be used as the power and ground layer to shield the antenna, effectively reduce parasitic inductance, shorten signal line length, and reduce cross-talk between signals. Next, the four-layer board is 20 dB lower than the noise of the two-layer board.

2) Routing method

When wiring high-frequency boards in PCB design, the traces must be bent at an angle of 45°, which reduces the emission of high-frequency signals and the coupling between them.

3) Trace length

When wiring high-frequency circuit boards in PCB design, the shorter the trace length, the better. The shorter the parallel distance between the two wires, the better.

4) Number of vias

When wiring high-frequency boards in PCB design, the smaller the number of vias, the better.

5) Interlayer wiring direction

When wiring high-frequency circuit boards in PCB design, the direction of interlayer wiring should be vertical, that is, the top layer is horizontal and the bottom layer is vertical, which can reduce interference between signals.

6) Copper plating

When wiring high-frequency boards in PCB design, adding grounded copper can reduce interference between signals.

7) Packing

When the high-frequency circuit board is wired in the PCB design, the important signal line is subjected to the packet processing, which can significantly improve the anti-interference ability of the signal. Of course, the interference source can be packaged so that it cannot interfere with other signals.

8) Signal line

When wiring high-frequency boards in PCB design, signal traces cannot be looped and need to be daisy-chained.

9) Decoupling capacitor

When wiring a high-frequency circuit board in a PCB design, a decoupling capacitor is connected across the power supply terminal of the integrated circuit.

10) High frequency turbulence

When wiring high-frequency circuit boards in PCB design, digital ground, analog ground, etc. must be connected to a high-frequency choke device when connected to a common ground line, generally a high-frequency ferrite bead with a center hole pierced with a wire.