Special Base PCB

1. Routing Method

In high-frequency PCB routing design, one can choose between a 45-degree bend or a curved turn if bending is required. This approach reduces the chances of high-frequency signal emission and coupling.

2. Routing Length

In high-frequency PCB routing during PCB design, shorter routing lengths are preferred, and shorter distances between parallel traces are desirable. The radiation intensity of a signal is directly related to the length of the signal trace. Longer high-frequency signal traces result in increased coupling with other components. That’s why clock signals, DDR, USB, Gigabit Ethernet, and HDMI prefer shorter routing lengths.

3. Number of Vias

In high-frequency PCB routing during PCB design, fewer vias are preferred. Each via introduces approximately 0.5 pF of distributed capacitance. Reducing the number of vias can improve signal speed and reduce the likelihood of data errors. Via stubs introduce impedance discontinuities that can lead to signal integrity issues.

4. Avoiding Crosstalk

When routing high-frequency circuits, it is essential to consider the “crosstalk” introduced by closely parallel signal traces. Crosstalk refers to the coupling phenomenon between signal lines that are not directly connected. As high-frequency signals are transmitted via electromagnetic waves along transmission lines, the signal lines act as antennas, emitting electromagnetic field energy around the transmission line. The undesired noise signals generated due to the mutual coupling of electromagnetic fields between signals are called crosstalk.

The parameters of PCB layer stack-up, spacing between signal lines, electrical characteristics of the driver and receiver ends, and termination methods of signal lines all impact crosstalk. Therefore, to reduce crosstalk in high-frequency signal routing, the following points should be considered:

*If the layout space allows, inserting a ground wire or plane between two lines with significant crosstalk can provide isolation and reduce crosstalk.

*When there is a time-varying electromagnetic field in the space surrounding the signal lines, if parallel distribution cannot be avoided, placing a large area of “ground” on the opposite side of the parallel signal lines can significantly reduce interference.

*Increase the spacing between adjacent signal lines and reduce the parallel length of the lines if the layout space permits. Clock lines should ideally be perpendicular to critical signal lines rather than parallel.

*If parallel traces within the same layer are nearly unavoidable in adjacent layers, the routing directions of the traces must be mutually perpendicular.

*In digital circuits, clock signals typically have fast edge transitions and significant external crosstalk. Therefore, ground lines should surround clock lines in design, and multiple ground via holes should be used to reduce distributed capacitance and crosstalk.

*it is advisable to use low-voltage differential clock signals with a grounded enclosure for high-frequency clock signals. Attention should be paid to the integrity of the ground via holes.

*Unused input terminals should not be left floating but grounded or connected to the power supply (which serves as the ground in high-frequency signal circuits) because floating lines can act as emission antennas. Grounding can suppress emissions. In practice, this method has proven to be effective in eliminating crosstalk.

5. Add High-frequency Decoupling Capacitors to the Power Supply Pins of Integrated Circuit Chips

Each integrated circuit (IC) chip should have a high-frequency decoupling capacitor near its power supply pins. Adding high-frequency decoupling capacitors to the power supply pins effectively suppresses the formation of high-frequency harmonic interference on the power supply pins.

6. Isolate the Ground of High-frequency Digital Signals and the Ground of Analog Signals

Analog ground, digital ground, and other connections to the common ground should be isolated using high-frequency choke beads or direct isolation, and appropriate single-point interconnection should be chosen. The ground potential of high-frequency digital signal lines is generally inconsistent, often resulting in a particular voltage difference between the two. Moreover, the ground of high-frequency digital signals often carries rich harmonic components of high-frequency signals. When the digital signal ground and analog signal ground are directly connected, the harmonic components of the high-frequency signals can interfere with the analog signals through ground coupling.

Therefore, in most cases, isolation is required between the ground of high-frequency digital signals and the ground of analog signals. This can be achieved by single-point interconnection at suitable locations or by using high-frequency choke beads for interconnection.

7. Avoid the Formation of Loops Resulting from Routing

Try to avoid the formation of loops in the routing of various high-frequency signals. If unavoidable, minimize the area of the loop as much as possible.

8. Adopt Fly-by Topology/daisy Chain Routing For DDR4

This wiring technique reduces reflections during high-speed data transmission. It also reduces the number and length of stubs, thereby improving signal integrity and timing of loaded signals.

9. Adopt the 20H Rule to Minimize Plane Coupling

In high-speed PCB designs, there is typically mutual coupling between the power and ground planes, causing RF energy to leak as edge magnetic flux. Moreover, RF energy (RF current) radiates along the PCB edges. To reduce this coupling effect, all power plane physical dimensions should be smaller than the adjacent ground plane dimensions by 20H.

At 10H, significant changes in magnetic flux leakage can occur. At 20H, 70% of magnetic flux leakage can be suppressed. At 100H, 98% of magnetic flux leakage can be suppressed. Although 100H provides better suppression, increasing the edge inset of the power plane and ground plane beyond 20H increases the physical distance between layers without significantly reducing the radiated current.

10. Necessary to Ensure Proper Signal Impedance Matching

During signal transmission, if there is an impedance mismatch, signal reflections occur in the transmission channel. These reflections cause signal overshoot, leading to fluctuations near the logic threshold.

The fundamental solution to eliminate reflections is to ensure proper impedance matching of the transmission signal. The more significant the difference between the load impedance and the characteristic impedance of the transmission line, the greater the reflection. Therefore, it is essential to make the transmission line’s characteristic impedance as close as possible to the load impedance. Additionally, care should be taken to avoid abrupt changes or corners in the transmission lines on the PCB. Maintaining continuous impedance at each point of the transmission line is advisable to prevent reflections between different segments of the transmission line.