If the frequency of digital logic circuits reaches or exceeds 45MHz~50MHz, and the circuits operating at this frequency account for a certain proportion of the entire electronic system (for example, 1/3), they are usually called high-frequency circuits. High-frequency circuit design is a very complex process, and its wiring is crucial to the entire design! Mastering the following ten methods will help you avoid many pitfalls in high-frequency circuit design.
[First Technique] Multilayer Board Wiring
High-frequency circuits often have high integration and high wiring density. Using multilayer boards is not only necessary for wiring but also an effective means of reducing interference. During the PCB layout stage, the appropriate selection of the number of layers and the size of the printed circuit board can make full use of the intermediate layers to set up shielding, better achieve grounding nearby, effectively reduce parasitic inductance and shorten signal transmission length, and also significantly reduce signal crosstalk. All of these methods are beneficial to the reliability of high-frequency circuits.
Data shows that, using the same materials, four-layer boards have 20dB lower noise than double-sided boards. However, there is also a problem: the higher the number of half-layers in a PCB, the more complex the manufacturing process and the higher the unit cost. This requires that when performing PCB layout, in addition to selecting a PCB with an appropriate number of layers, we also need to plan the layout of components reasonably and use the correct routing rules to complete the design.
[Second Tip] Minimize the bends in the leads between pins of high-speed electronic devices.
For high-frequency circuit wiring, the leads should ideally be straight. If a bend is necessary, a 45-degree bend or an arc bend can be used. In low-frequency circuits, this requirement is only used to improve the adhesion strength of the copper foil, but in high-frequency circuits, meeting this requirement can reduce the external transmission of high-frequency signals and the coupling between them.
[Third Tip] The shorter the leads between the pins of high-frequency circuit components, the better.
The radiation intensity of a signal is directly proportional to the length of the signal line. The longer the high-frequency signal lead, the easier it is to couple to the components nearby. Therefore, for high-frequency signal lines such as clock signals, crystal oscillators, DDR data lines, LVDS lines, USB lines, and HDMI lines, it is required that the traces be as short as possible.
[Fourth tip] Minimize the number of interlayer overlaps in the leads between high-frequency circuit components.
The saying "the fewer the interlayer alternations of leads, the better" refers to using as few vias as possible during component connection. It is understood that a single via can introduce approximately 0.5pF of distributed capacitance; reducing the number of vias can significantly improve speed and reduce the likelihood of data errors.
[Fifth Tip] Be aware of crosstalk introduced by closely spaced parallel signal lines.
When routing high-frequency circuits, it is important to be aware of the "crosstalk" introduced by closely spaced parallel signal lines. Crosstalk refers to the coupling phenomenon between signal lines that are not directly connected. Since high-frequency signals are transmitted along transmission lines in the form of electromagnetic waves, the signal lines act as antennas, and the energy of the electromagnetic field is emitted around the transmission lines. The unwanted noise signals generated by the mutual coupling of electromagnetic fields between signals are called crosstalk.
The parameters of PCB layers, the spacing of signal lines, the electrical characteristics of the driver and receiver terminals, and the signal line termination method all have a certain impact on crosstalk. Therefore, in order to reduce crosstalk of high-frequency signals, the following points should be observed as much as possible during routing:
If the wiring space allows, inserting a ground wire or ground plane between two lines with severe crosstalk can serve as an isolation measure and reduce crosstalk.
When the space around the signal line itself contains 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.
If the wiring space allows, increase the spacing between adjacent signal lines, reduce the parallel length of signal lines, and make sure that clock lines are perpendicular to critical signal lines rather than parallel to them.
If parallel routing within the same layer is almost unavoidable, then the routing directions of two adjacent layers must be perpendicular to each other.
In digital circuits, clock signals typically have rapidly changing edges, resulting in significant crosstalk. Therefore, in the design, clock lines should be surrounded by ground lines and multiple ground vias should be used to reduce distributed capacitance and thus reduce crosstalk.
For high-frequency clock signals, use low-voltage differential clock signals and grounding them whenever possible, and pay attention to the integrity of the grounding vias.
Do not leave unused input terminals floating; instead, ground them or connect them to a power supply (the power supply is also ground in high-frequency signal loops). This is because a floating wire can act as a transmitting antenna, and grounding can suppress transmission. Practice has shown that this method can sometimes eliminate crosstalk immediately.
[Sixth Tip] Add high-frequency decoupling capacitors to the power supply pins of integrated circuits.
Add a high-frequency decoupling capacitor near the power supply pin of each integrated circuit block. Adding a high-frequency decoupling capacitor to the power supply pin can effectively suppress high-frequency harmonic interference on the power supply pin.
[Seventh Tip] Isolate the ground wires of high-frequency digital signals from those of analog signals.
When connecting analog ground wires, digital ground wires, etc., to a common ground wire, a high-frequency choke bead should be used for connection, or they should be directly isolated and interconnected at a suitable single point. The ground potentials of the ground wires for high-frequency digital signals are generally inconsistent, and there is often a certain voltage difference between the two. Moreover, the ground wires for high-frequency digital signals often carry very rich harmonic components of high-frequency signals. When the digital signal ground wire and the analog signal ground wire are directly connected, the harmonics of the high-frequency signal will interfere with the analog signal through ground coupling.
Therefore, under normal circumstances, the ground wires of high-frequency digital signals and analog signals should be isolated. This can be achieved by single-point interconnection at appropriate locations or by interconnection using high-frequency choke beads.
[Eighth Tip] Avoid Wiring Loops
Avoid forming loops in the routing of various high-frequency signals as much as possible. If it is unavoidable, the loop area should be minimized.
[Ninth Tip] Ensure good signal impedance matching.
During signal transmission, when there is an impedance mismatch, the signal will be reflected in the transmission channel. The reflection will cause the synthesized signal to overshoot, resulting in the signal fluctuating around the logic threshold.
The fundamental solution to eliminate reflections is to ensure good impedance matching of the transmitted signal. Since the greater the difference between the load impedance and the characteristic impedance of the transmission line, the greater the reflection, the characteristic impedance of the signal transmission line should be made as equal to the load impedance as possible. At the same time, it is crucial to avoid abrupt changes or bends in the transmission lines on the PCB, and to maintain impedance continuity at all points along the transmission line; otherwise, reflections will occur between different segments of the transmission line. This necessitates adhering to the following routing rules when performing high-speed PCB routing:
USB wiring rules: USB signals must be routed differentially, with a line width of 10mil, a line spacing of 6mil, and a ground and signal line spacing of 6mil.
HDMI cabling rules: HDMI signals must be routed differentially with a cable width of 10mil, a cable spacing of 6mil, and a spacing of more than 20mil between any two HDMI differential signal pairs.
LVDS routing guidelines require differential LVDS signals to be routed with a trace width of 7 mil and a trace spacing of 6 mil. This is to control the impedance of the HDMI differential signal, which is 100 ± 15 ohms. DDR1 routing guidelines require that signals avoid vias as much as possible, signal traces be of equal width and spacing, and traces must adhere to the 2W rule to reduce crosstalk. For high-speed devices such as DDR2 and above, high-frequency data traces must also be of equal length to ensure impedance matching.
[Tenth Tip] Maintain the integrity of signal transmission
Maintain the integrity of signal transmission and prevent "ground bounce" caused by ground wire splitting.
