High frequency circuit wiring techniques in PCB design

- Apr 22, 2017-

High frequency circuit is often a high degree of integration, the wiring density is large, the use of multilayer board is necessary for wiring, but also an effective means to reduce interference. In the PCB Layout stage, printed board dimensions reasonable choice of certain layers, can make full use of the intermediate layer shielding, better realize the nearest ground, and effectively reduce the parasitic inductance and shorten the length of the signal transmission, but also greatly reduce the cross interference signal, all of these methods are reliability of high frequency the favorable circuit.

When the same material is used, the noise of the four plates is lower than that of the double deck 20dB. However, there is also a problem, PCB half of the higher layers, the manufacturing process is more complex, the unit cost will be higher, which requires the PCB Layout, in addition to the PCB board to select the appropriate number of components, but also the need for a reasonable layout, and adopt the correct routing rules to complete the design.

1, the high frequency circuit device pin between the lead layer the less the better

The so-called "lead between the less the better" refers to the process of connecting components used in the less the better. A hole can bring about 0.5pF of the distributed capacitance, reducing the number of holes can significantly improve the speed and reduce the possibility of data error.

2, high frequency circuit device pin between the shorter the better

The radiation intensity signal and the signal line is the line length is proportional to the high frequency signal lead longer, the more likely it is coupled to the components near it up, so the clock oscillator, DDR, such as signal data, LVDS line, USB line, HDMI line high frequency signal line are required to do may go line as short as possible.

3, high speed electronic devices between the lead wire bending the less the better

Lead high frequency circuit wiring the best use of the whole line, turning to 45 degrees, the available line or arc turning, the requirements in the low-frequency circuit only used to improve the fixation strength of copper foil, and in high frequency circuit, meet this requirement can reduce the coupling emission of high frequency signals and the foreign.

4, pay attention to the signal line close to the line parallel to the introduction of crosstalk"

High frequency circuit wiring should pay attention to the signal line near the parallel line of the introduction of "crosstalk", crosstalk is not directly connected between the signal line coupling phenomenon. Due to the high frequency signal along the transmission line is transmitted in the form of electromagnetic waves, the signal line will play the role of an antenna electromagnetic energy around the transmission line emission, noise signal is not expected due to the mutual coupling between the electromagnetic signal generated is called crosstalk (Crosstalk). The parameters of the PCB board, the distance between the signal lines, the electrical characteristics of the driver and receiver, and the termination of the signal line have some influence on the crosstalk.

5, high frequency digital signal ground and analog signal ground wire isolation

Analog ground wire, digital ground wire, etc. to connect to a common ground wire to use high frequency magnetic beads to connect or direct isolation and select the appropriate place a single point of interconnection. The high frequency digital signal to the ground potential is generally not consistent, they often have the direct voltage difference, and ground harmonic components of high frequency digital signal with high frequency signal is often very rich, when directly connected to digital signal and analog signal to the ground line, the harmonic will high frequency signal interference simulation the signal through the wire coupling way. So under normal circumstances, the high frequency digital signal of the ground wire and analog signal ground wire is to be isolated, you can use a single point of the Internet in the appropriate location, or the use of high-frequency magnetic network method of choking.

6. The power supply pin of the integrated circuit block is increased by the high frequency decoupling capacitor

The power supply pin of each integrated circuit block is provided with a high frequency decoupling capacitor. Increase the frequency of the power supply pins, can effectively suppress the high frequency harmonic interference on the power supply pin.

7, to avoid the formation of the loop line

All kinds of high-frequency signal line as far as possible not to form a loop, if you can not avoid the loop area should be as small as possible.

8, must ensure good signal impedance matching

In the process of signal transmission, when the impedance is not matched, the signal will be reflected in the transmission channel, and the reflection will make the composite signal overshoot.

The fundamental way is to eliminate the reflection impedance transmission signal good matching, because of the characteristics of impedance load impedance of the transmission line from the reflection is also bigger, so should as soon as possible to the characteristic impedance of the signal transmission line and load impedance. At the same time, we should also pay attention to the transmission line on the PCB can not appear mutation or corner, as far as possible to maintain the continuity of the transmission line impedance points, otherwise there will be reflected in each segment of the transmission line.

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