Microwave Tech - 10 suggestions for high-frequency PCB layout

Multi-layer board wiring of the first step
High-frequency circuit boards often have high integration and high wiring density. The use of multi-layer circuit boards is not only 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 sizes can make full use of the intermediate layer to set up the shield, better achieve near grounding, effectively reduce parasitic inductance, shorten the transmission length and reduce signal crosstalk. All these methods have Conducive to the reliability of high-frequency circuits. According to the data, the noise of the four-layer board is 20dB lower than that of the two-layer board. However, there are also problems. The higher the number of PCB half-layers, the more complicated the manufacturing process and the higher the unit cost. This requires us to select the appropriate number of PCB circuit boards for the PCB layout. Correct component layout planning and correct wiring rules to complete the design.
Second: The smaller the pin bending of high-speed electronic equipment, the better.

The lead wire of the high-frequency circuit wiring is preferably a full line, needs to be rotated, and can be folded in a 45-degree line or an arc. This requirement is only used to improve the fixing strength of copper foil in low-frequency circuits, and to meet the content in high-frequency circuits. One requirement is to reduce the external transmission and mutual coupling of high-frequency signals.
Third: The shorter the lead between the pins of high-frequency circuit equipment, the better.
The radiation intensity of the signal is proportional to the trace length of the signal line. The longer the high-frequency signal lead, the easier it is to couple to the components close to it. Therefore, the data requirements for signal clock, crystal, DDR, and high-frequency signal lines (such as LVDS lines, USB lines and HDMI lines) are as short as possible.
Fourth: The less the transition between the lead layers between the pins of the high-frequency circuit device, the better.
The so-called "minimum alternation between lead layers is better" means that the fewer vias (Via) used in the component connection process, the better. According to this aspect, through holes can bring about 0.5pF distributed capacitance, and reducing the number of through holes can significantly increase the speed and reduce the possibility of data errors.
Fifth: Pay attention to the "crosstalk" signal line introduced by the parallel line.
High-frequency circuit wiring should pay attention to the "crosstalk" introduced by parallel signal lines. Crosstalk refers to the coupling phenomenon between signal lines that are not directly connected. Since high-frequency signals are 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 the electromagnetic field coupling between the undesired noise signal signals generated by each other is called crosstalk. The parameters of the PCB layer, the spacing of the signal lines, the electrical characteristics of the driver and the receiver, and the termination of the signal lines all have a certain impact on the crosstalk. Therefore, in order to reduce the crosstalk of high-frequency signals, it is necessary to do the following as much as possible during the wiring process:
Inserting a ground or ground layer between two grounds that has serious crosstalk can isolate and reduce crosstalk when the wiring space permits.
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 reverse side of the parallel signal line to greatly reduce interference.
On the premise of wiring, space permits, increase the spacing between adjacent signal lines, reduce the parallel length of the signal lines, the clock line should be perpendicular to the key signal line instead of parallel.
If parallel traces in the same layer are almost inevitable, the directions of the traces must be perpendicular to each other in two adjacent layers.
In digital circuits, the usual clock signal is a signal with a fast edge change, and the external crosstalk is very large. Therefore, in the design, the clock line should be surrounded by ground lines and more ground holes to reduce distributed capacitance, thereby reducing crosstalk.
For high-frequency signal clocks, try to use low-voltage differential clock signals and cover the ground. You need to pay attention to the integrity of the packaging.
Do not ground 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 loop). Because the suspension wire can be equivalent to the transmitting antenna, the grounding can suppress the emission. Practice has proved that using this method to eliminate crosstalk can sometimes take effect immediately.
Sixth: Add high-frequency unlocking capacitor to the power supply pin of the integrated circuit module
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 and cause interference.
The ground of the seventh high-frequency digital signal is isolated from the ground of the analog signal.
When the analog ground wire, digital ground wire, etc. are connected to the common ground wire, high-frequency turbulent magnetic beads should be used for connection or direct isolation to select a place suitable for single-point interconnection. The ground potential of the high-frequency digital signal is usually inconsistent, and there is usually a certain voltage difference between the two. Moreover, the ground wire of the high-frequency digital signal usually has very rich harmonic components of the high-frequency signal. When the digital signal ground and the analog signal ground are directly connected, the harmonics of the high-frequency signal interfere with the analog signal through the ground wire coupling. Therefore, generally, the grounding of high-frequency digital signals and the grounding of analog signals are separated, and the single-point interconnection method or the interconnection of high-frequency turbulent magnetic beads at an appropriate position can be passed.
Eighth: Avoid the cycle formed by traces
Do not form loops. All types of high-frequency signal traces should be as many as possible. If it is unavoidable, make the circulation area as small as possible.
Ninth: Must ensure good signal impedance matching
During signal transmission, when the impedance does not match, the signal will be reflected in the transmission channel, and the reflection will exceed the composite signal, causing the signal to fluctuate around the logic threshold.
The basic method to eliminate reflections is to match the impedance of the transmitted signal well. Since the difference between the load impedance and the characteristic impedance of the transmission line is large, the reflection is also large. Therefore, the characteristic impedance of the signal transmission line should be as equal to the load impedance as possible. At the same time, it should be noted that the transmission line on the PCB should not be sudden or cornered, and try to keep the impedance of each point of the transmission line continuous, otherwise there will be reflections between the transmission line segments. This needs to follow the following wiring rules when performing high-speed PCB high-frequency board wiring:
USB wiring rules. Requires differential routing of USB signals. The line width is 10 mils, the line spacing is 6 mils, and the ground and signal lines are 6 mils apart.
HDMI wiring rules. HDMI signal differential wiring is required, the line width is 10 mils, and the line spacing is 6 mils. The spacing between each pair of HDMI differential signal pairs exceeds 20 mils.
LVDS wiring rules. Need LVDS signal differential routing, line width 7mil, line spacing 6mil, the purpose is to control the differential signal impedance of HDMI to 100 + -15% ohm
DDR wiring rules. DDR1 wiring requires that the signal should not pass through the hole as much as possible. The width of the signal lines is equal, and the lines are equidistant from the lines. The line must comply with the 2W principle to reduce crosstalk between signals. For high-speed devices of DDR2 and above, high-frequency data is required. The lengths of these lines are equal to ensure the impedance matching of the signals.
Tenth: Maintain the integrity of signal transmission
Maintain the integrity of signal transmission to prevent "ground bounce" caused by ground segmentation.