Obtain PCB design and wiring of high-frequency circuit

Obtain PCB design and wiring of high-frequency circuit
If the frequency PCB of digital logic reaches or exceeds 45MHZ~50MHZ, and the circuit working above this frequency has accounted for a certain amount of the entire electronic system (for example, 1./3), it is usually called a high-frequency circuit High frequency circuit design is a very complex design process, and its wiring is very important to the whole design
1. Multilayer board wiring 2
High frequency circuits tend to have high integration and high wire density. The use of multilayer boards is not only a necessary condition for wiring, but also an effective means to reduce interference. In the PCB layout stage, reasonable selection of PCB size with a certain number of layers can make full use of the middle layer to set the mask, better realize the nearest grounding, effectively reduce parasitic inductance, and shorten the signal transmission length. All these methods are beneficial to the reliability of high-frequency circuits. Some data show that when the same data is used, the noise of the four layer board is 20 dB lower than that of the double-sided board. But at the same time, there is also a problem. The higher the number of half layers of PCB, the more complex the manufacturing process, and the higher the organizational cost. This requires us to select the appropriate number of layers for PCB layout. Make reasonable component layout planning, and use the correct routing rules to complete the design.
PCB board

2. The less lead wire bending between pins of high-speed electronic equipment, the better
The leads of high-frequency circuit wiring are straight lines and need to be rotated. They can be rotated with a 45 degree dotted line or arc. This requirement is only used to improve the fixed strength of copper foil in low-frequency circuits, but it is met in high-frequency circuits. However, it can reduce the external emission and mutual coupling of high-frequency signals.
3. The shorter the lead between the pins of high-frequency circuit components, the better
The radiation intensity of the signal is proportional to the trace length of the signal line. The longer the HF signal lead, the easier it is to couple to the component near it. This requires that the LVDS cable, USB cable, HDMI cable and other high-frequency signal cables should be as short as possible for the clock, crystal oscillator, DDR data and other signals.
4. The less alternating between lead layers between pins of high-frequency circuit components, the better
The so-called "less interlaminar alternating of the leads, the better" means that the fewer through holes used in the component connection process, the better. According to the data, a through-hole can bring 0.5pF distributed capacitance. Reducing the number of through-hole can significantly improve the speed and reduce the possibility of data errors.
5. Pay attention to the "crosstalk" caused by closely parallel traces of signal lines
In the high frequency circuit wiring, attention should be paid to the "crosstalk" introduced by the parallel wiring of adjacent signal lines. Crosstalk refers to the coupling between signal lines that are not directly connected. Since the high-frequency signal is transmitted along the transmission line in the form of electromagnetic wave, the signal line will act as an antenna, and the energy of the electromagnetic field will be transmitted around the transmission line. This is called crosstalk. The parameters of PCB layer, the spacing of signal lines, the electrical characteristics of driver and receiver, and the termination method of signal lines all have certain effects on crosstalk. In order to reduce the crosstalk of high frequency signals, the following work should be done as far as possible during wiring: insert a ground wire or ground plane between two lines with serious crosstalk when the wiring space allows. It can isolate and reduce crosstalk. When there is time-varying electromagnetic field around the signal line, if the parallel distribution cannot be avoided, a large area of "grounding" can be arranged on the opposite side of the parallel signal line to greatly reduce interference. If the wiring space allows, increase the spacing between adjacent signal lines, reduce the parallel length of signal lines, and try to make the clock lines perpendicular to the key signal lines instead of parallel. If parallel traces in the same layer are almost unavoidable, the traces must be perpendicular to each other on two adjacent layers. In digital circuits, the usual clock signal is a signal with fast edge changes, and the external crosstalk is large. This, in the design, the clock wire should be surrounded by the ground wire, and more ground wire holes should be formed to reduce the distributed capacitance, thereby reducing crosstalk. For high frequency signal clock, try to use low voltage differential clock signal and ground it. Pay attention to the integrity of floor punching. Do not hang up the unused input terminal, but ground it or connect it to the power supply (the power supply is also grounded in the high-frequency signal circuit), because the suspension wire may be equivalent to the transmission antenna, and grounding can inhibit transmission. Practice has proved that crosstalk elimination with such pipes can sometimes be effective immediately.
6. Add a high-frequency decoupling capacitor to the power supply pin of the integrated circuit block
High frequency decoupling capacitors are added to the power supply pins of each integrated circuit block. Adding high-frequency decoupling capacitor to the power supply pin can effectively suppress the interference of high-frequency harmonics to the power supply pin.
7. High frequency digital signal ground wire is isolated from analog signal ground wire
When analog grounding wire, digital grounding wire, etc. are connected to the common grounding wire, high-frequency choke magnetic beads shall be used for connection or direct isolation, and single point interconnection shall be conducted at appropriate locations. The grounding potential of high-frequency digital signal ground wire is generally inconsistent, and there is often a certain voltage difference between them; When the digital signal ground wire is directly connected with the analog signal ground wire, the harmonics of the high-frequency signal will couple and interfere with the analog signal through the ground wire. This, under normal circumstances, the ground wire of high-frequency digital signal and the ground wire of analog signal need to be isolated, which can be connected at a single point at a suitable location, or connected by high-frequency choke magnetic beads.
8. Avoid trace forming loop
The tracks of various high-frequency signals shall not form a loop as far as possible. If it is unavoidable, keep the loop area as small as possible.
9. Good signal impedance matching must be ensured
In the signal transmission process, when the impedance does not match, the signal will be reflected in the transmission channel, and the reflection will cause the composite signal to form an overshoot, causing the signal to fluctuate near the logic threshold. The basic method to eliminate reflection is to make the impedance of the transmitted signal match well. Because the greater the difference between the load impedance and the characteristic impedance of the transmission line, the greater the reflection. Recall that the characteristic impedance of the signal transmission line should be equal to the load impedance as much as possible; At the same time, pay attention to the PCB. The transmission line on the circuit board shall not have sudden changes or corners. Keep the impedance of each point of the transmission line as continuous as possible, otherwise, reflection will occur between each part of the transmission line. This requires that the following wiring rules must be observed when wiring high-speed PCB boards: USB wiring rules. The differential trace of USB signal is required. The line width is 10 mils, the line spacing is 6 mils, and the spacing between the grounding wire and the signal wire is 6 mils. HDMI cabling rules. HDMI signal differential trace is required, the line width is 10mil, the line spacing is 6mil, and the spacing between two HDMI differential signal pairs exceeds 20mil. LVDS cabling rules. The LVDS signal differential trace is required, with a line width of 7 mils and a line spacing of 6 mils. The purpose is to control the impedance of HDMI differential signal pair to 100 ± 15 ohm. DDR routing rules. DDR1 wiring requires that the signal shall not pass through the hole as much as possible, the width of the signal line shall be equal, and the distance between the lines shall be equal. Wiring must comply with the 2W principle to reduce crosstalk between signals. High frequency data is also required for high-speed devices with DDR2 and above. The line lengths are equal to ensure that the signal impedance matches.
10. Maintain signal integrity
Keep the integrity of signal transmission and prevent the "ground bounce" phenomenon caused by the splitting of the grounding wire