High-frequency circuits are often highly integrated, wiring density, the use of multi-layer board is necessary for wiring, but also an effective means to reduce interference. In the PCB Layout stage, a reasonable selection of a certain number of printed circuit board size, can make full use of the middle layer to set the shield, to better achieve the nearest ground, and effectively reduce the parasitic inductance and shorten the signal transmission length, but also large Amplitude reduction of the signal cross-interference, etc., all of these methods are beneficial to the reliability of high-frequency circuits. When the same material, the four-layer board than the double-panel noise 20dB lower. However, there is also a problem, the higher the PCB half-layer, the more complex manufacturing process, the higher the unit cost, which requires the PCB layout, in addition to select the appropriate number of layers of PCB, but also need to be reasonable The layout of the components layout, and the use of the correct routing rules to complete the design.

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

The so-called "lead the interlayer between the less the better" refers to the process of connecting the components used in the hole (Via) the less the better. A vias 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.

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

The radiation intensity of the signal is proportional to the length of the signal line, and the longer the high-frequency signal leads, the easier it is to be coupled to the components near it, so for clocks such as signals, crystal, DDR data, LVDS line, USB cable, HDMI cable and other high-frequency signal lines are required to track as short as possible the better.

3, high-speed electronic device pin between the lead bend less the better

High-frequency circuit wiring lead the best use of all-straight line, the need for a turn, can be used 45 degrees fold or arc turn, this requirement in the low-frequency circuit is only used to improve the fixation strength of copper, and in high-frequency circuit, A requirement can reduce the high-frequency signal from the external launch and the coupling between each other.

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

High-frequency circuit wiring should pay attention to the signal line close parallel parallel to the introduction of the "crosstalk", crosstalk is not directly connected between the signal line coupling phenomenon. Since the high frequency signal is transmitted along the transmission line in the form of electromagnetic waves, the signal line will act as an antenna. The energy of the electromagnetic field will be emitted around the transmission line, and the undesired noise signal between the signals due to the coupling of the electromagnetic fields Called crosstalk (Crosstalk). PCB board parameters, the signal line spacing, drive side and the receiving side of the electrical characteristics and signal line termination of the crosstalk have a certain impact. So in order to reduce the high-frequency signal crosstalk, when wiring requirements as much as possible to do the following:

    (1) in the wiring space allows the conditions, the more serious crosstalk between the two lines into a ground or ground plane, can play a role in isolation and reduce crosstalk;
    (2) When the space around the signal line itself has a time-varying electromagnetic field, if the parallel distribution can not be avoided, a large area "ground" can be arranged on the reverse side of the parallel signal line to greatly reduce the interference;
    (3) in the wiring space permit under the premise of increasing the spacing between adjacent signal lines to reduce the parallel length of the signal line, the clock line as far as possible with the key signal line and not parallel;
    (4) if the same layer of parallel alignment is almost impossible to avoid, in the adjacent two layers, the direction of the line must be perpendicular to each other;
    (5) In the digital circuit, the usual clock signal is a fast edge change signal, external crosstalk. So in the design, the clock line should be surrounded by ground and more to play the ground hole to reduce the distribution of capacitance, thereby reducing crosstalk;
    (6) on the high-frequency signal clock as far as possible the use of low-voltage differential clock signal and package way, need to pay attention to the integrity of the package;
    (7) The unused input is not to be floating, but grounded or connected to the power supply (the power supply is also in the high frequency signal loop), because the floating line may be equivalent to the transmitting antenna, grounding can inhibit the launch. Practice has proved that the use of this approach to eliminate crosstalk sometimes can immediately bear fruit.

5, high-frequency digital signal ground and analog signal ground to do isolation

Analog ground, digital ground, etc. to the public ground when using high-frequency choke magnetic beads or directly isolated and select the appropriate place single point of interconnection. The ground potential of the ground of the high frequency digital signal is generally inconsistent, and there is often a constant voltage difference between the two, and the ground of the high frequency digital signal is often accompanied by a very rich harmonic component of the high frequency signal. Directly connected to the digital signal ground and analog signal ground, the high-frequency signal harmonics will be through the ground coupling of the analog signal interference. Therefore, the ground wire of the high-frequency digital signal and the ground of the analog signal are to be isolated, and it is possible to use a single point of interconnection in a suitable position or a high-frequency choke ball interconnection.

6, the integrated circuit block power pin to increase the high frequency back to the lotus capacitor

The power supply pin of each integrated circuit block is nearly a high frequency decoupling capacitor. Increase the power pin high-frequency back to the lotus capacitor, can effectively suppress the power pin on the formation of high-frequency harmonic interference.

7, to avoid the formation of the loop loop

Various types of high-frequency signal traces 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

Signal in the transmission process, when the impedance does not match, the signal will appear in the transmission channel signal reflection, reflection will make the composite signal formation overshoot, resulting in fluctuations in the signal near the logic threshold.

The fundamental way to eliminate the reflection is to make the impedance of the transmission signal well match, because the load impedance and transmission line characteristic impedance difference is bigger reflection, so should be as much as possible to make the signal transmission line characteristic impedance and load impedance equal. Also note that the transmission line on the PCB can not appear mutations or corners, try to keep the transmission line impedance continuous, otherwise there will be reflection between the sections of the transmission line. This requires that the following wiring rules be followed when performing high-speed PCB routing:

    (1) LVDS routing rules. Requires LVDS signal differential traces, line width 7mil, line distance 6mil, the purpose is to control the HDMI differential signal impedance of 100 + -15% ohm;
    (2) USB cabling rules. Requires USB signal differential traces, line width 10mil, line distance 6mil, ground and signal line distance 6mil;
    (3) HDMI wiring rules. Requires HDMI signal differential traces, line width 10mil, line distance 6mil, each two sets of HDMI differential signal pair spacing of more than 20mil;
    (4) DDR wiring rules. DDR1 alignment requires the signal as far as possible through the hole, the signal line is wide, line and line equidistant, the alignment must meet the 2W principle to reduce the signal crosstalk between the DDR2 and above high-speed devices, also requires high-frequency data to go Line equal length to ensure the impedance matching of the signal.

Maintain the integrity of the signal transmission, to prevent the ground segmentation caused by the "ground phenomenon."