High Frequency Circuit Wiring techniques in PCB Layout

High Frequency Circuit Wiring techniques in PCB Layout

1. Multi-board wiring

High-frequency circuits often have a high degree of integration and a high wiring density. Using a multi-layer board is not only required for wiring, but also an effective means to reduce interference. In the PCB Layout stage, a proper selection of printing board sizes of a certain number of layers can make full use of the middle layer to set shielding to better achieve nearby grounding, it can effectively reduce the parasitic inductance, shorten the signal transmission length, and greatly reduce the cross interference of signals. All these methods are favorable for the reliability of high-frequency circuits. Data shows that when the same material is used, the layer 4 is 20 dB lower than the noise of the dual panel. However, there is also a problem. The higher the half-layers of PCB, the more complicated the manufacturing process, and the higher the unit cost. This requires us to conduct PCB Layout, in addition to selecting suitable PCB layers, you also need to make reasonable layout of components and use correct wiring rules to complete the design.

2. The less bending the leads between high-speed electronic device pins, the better

It is best to use a straight line for the leads of High-Frequency Circuit Wiring, which requires a turning point. It can be a 45-degree line or arc turning point. This requirement is only used to increase the solid strength of copper foil in the low-frequency circuit, however, in high-frequency circuits, meeting this requirement can reduce the transmission and coupling of high-frequency signals.

3. The shorter the lead between the pins of the high-frequency circuit device, the better.

The signal radiation intensity is proportional to the line length of the signal line. The longer the high-frequency signal leads, the easier it will be coupled to components close to it, therefore, high-frequency signal lines, such as clock, crystal oscillator, DDR data, LVDS, USB, and HDMI cables, must be kept as short as possible.

4. The less inter-channel switching between pins of high-frequency circuit devices, the better

The so-called "less inter-layer alternation of leads, the better" means that the less the pass through (Via) used in the component connection process, the better. On the data side, a passing hole can bring about PF of distribution capacitance, reducing the passing hole number can significantly increase the speed and reduce the possibility of data errors.

5. Pay attention to the "crosstalk" introduced by the close-range parallel cabling of the signal line"

For high-frequency circuit wiring, pay attention to the "crosstalk" introduced by the near-distance parallel cabling of signal lines. crosstalk refers to the coupling between signal lines that are not directly connected. Because the high-frequency signal is transmitted along the transmission line in the form of electromagnetic waves, the signal line will act as an antenna, and the energy of the electromagnetic field will be transmitted around the transmission line, the undesirable noise signals produced by the coupling of electromagnetic fields between signals are called Crosstalk ). PCB layer parameters, signal line spacing, electrical characteristics of the driving end and the receiving end, and signal line connection methods have certain effects on crosstalk. Therefore, in order to reduce the crosstalk of high-frequency signals, the following requirements should be met during cabling:

(1) Insert a ground line or ground plane between two lines with severe crosstalk in the conditions permitted by the wiring space, which can be used for isolation to reduce crosstalk.

(2) When the space around the signal line itself has a time-varying electromagnetic field, if parallel distribution cannot be avoided, a large area of "Ground" can be arranged on the opposite side of the parallel signal line to greatly reduce interference.

(3) When the wiring space permits, increase the spacing between adjacent signal lines and reduce the parallel length of the signal line. The clock line should be perpendicular to the key signal line rather than parallel.

(4) If parallel cabling within the same layer is almost unavoidable, in the adjacent two layers, the cabling must be perpendicular to each other.

(5) In digital circuits, clock signals are usually signals with fast edge changes, causing high external crosstalk. Therefore, in the design, the clock line should be surrounded by a ground wire and more ground hole to reduce the distribution capacitance, thus reducing crosstalk.

(6) Low-voltage differential clock signals should be used for high-frequency signal clock, and the integrity of the overlay drilling should be paid attention.

(7) do not pin unused inputs to the ground or power supply (the power supply is also the ground in the high-frequency signal loop), because the suspended cables may be equivalent to the transmitting antenna, grounding can suppress the launch. Practice has proved that using this method to eliminate crosstalk can sometimes take effect immediately.

6. The power supply pin of the integrated circuit block increases the high-frequency backcoupling capacitor.

The power supply pin of each integrated circuit block adds a high-frequency return capacitor nearby. The high-frequency coupling capacitor added to the power supply pin can effectively suppress interference caused by high-frequency harmonic waves on the power supply pin.

7. Ground Wires of high-frequency digital signals are isolated from analog signal ground wires.

When the analog Ground Wire and digital ground wire are connected to the public ground wire, connect them with high-frequency blocking magnetic beads or directly isolate them and select a suitable place for Single Point interconnection. The ground potential of a high-frequency digital signal is generally inconsistent, and there is a certain voltage difference between the two, the ground line of a high-frequency digital signal often carries a rich set of harmonic components of a high-frequency signal. When the ground line of a digital signal is directly connected to the ground line of a analog signal, the harmonic of the high-frequency signal will interfere with the analog signal through ground-ground coupling. Therefore, the ground line of the high-frequency digital signal and the ground line of the analog signal are usually isolated. You can use the single point interconnection method in the appropriate position, or the high-frequency blocking magnetic beads interconnection method.

8. Avoid loop formed by cabling

Do not form a loop if possible for high-frequency signal cabling. Otherwise, make the loop area as small as possible.

9. Good signal impedance matching must be ensured.

During signal transmission, when the impedance does not match, the signal will be reflected in the transmission channel, and reflection will lead to the formation of the synthetic signal, this causes the signal to fluctuate near the logical threshold.

The fundamental way to eliminate reflection is to make the impedance of the transmission signal well match, because the difference between the load impedance and the characteristic impedance of the transmission line is greater than that of the reflection, so try to make the characteristic impedance of the signal transmission line equal to the load impedance. At the same time, it should be noted that the transmission line on the PCB should not have a sudden change or corner, and the impedance at each point of the transmission line should be kept continuous as far as possible. Otherwise, reflection may also occur between each segment of the transmission line. This requires that the following wiring rules be observed during high-speed PCB cabling:

(1) USB wiring rules. USB signal differential cabling is required. The line width is 10mil, the line distance is 6mil, and the line and line distance are 6mil.

(2) HDMI cabling rules. It is required that the HDMI signal be deprecated separately, with a line width of 10 mil and a line distance of 6 mil. The gap between each two sets of HDMI differential signal pairs exceeds 20 mil.

(3) LVDS cabling rules. LVDS signal differential cabling is required. The line width is 7 mil and the line distance is 6 mil. The purpose is to control the differential signal impedance of HDMI to 100 +-15% ohm.

(4) DDR cabling rules. DDR1 cabling requires that the signal should not pass through the hole as much as possible, the signal line is equal to width, and the line and line are equally distance. The cabling must meet the 2W principle to reduce the crosstalk between signals. for high-speed devices of DDR2 and above, high-frequency data is also required to take a long line to ensure signal impedance matching.

10. Maintain the integrity of signal transmission to prevent the ground bullet phenomenon caused by ground line division ".