Things to know when designing circuits that operate at high frequencies (or in circuits with signal lines – data transmission lines running at high frequencies)
The circuit operates at high frequency, if the PCB is poorly designed, it will cause noise, causing the data transmitted on the board to be attenuated or erroneous, and sometimes the circuit is unstable, even running completely wrong
Example: Considering a single conductor carrying AC current, weak or no electric coupling will allow the transverse electromagnetic wave (TEM wave) generated by the current, to dissipate the external environment. , causing severe EMI electromagnetic field interference.
The Control of Noise in High Frequency PCB
Suppose there is a second conductor, near the old one, carrying the same current as wire 1 but in the opposite direction. In this case, the magnetic fields of the two wires are suppressed, and the electric fields are tightly coupled. The TEM waves of the two wires are suppressed, not scattering to the external environment. The small dissipation field can only be linked at a distance. Therefore, the EMI noise is much smaller.
Therefore, for High Frequency PCB, or at least on circuits with high frequency signal transmission lines, it becomes extremely important to design the layout and wiring it.
For low-frequency running circuits, when designing PCBs, it is still recommended to follow the guidelines in the following document (minimize EMI noise, ensure stable operation of the circuit).
What is EMI noise in High Frequency PCB?
Electromagnetic interference (EMI, also known as radio frequency interference or RFI) is a disorder that affects High Frequency PCB due to electrical induction or electromagnetic radiation emitted from an external source. These disturbances can interfere, interrupt, or degrade the signal. The noise source can be any object, man-made or natural.
For digital boards operating at frequencies of 150Mbps or less (or the rise time and fall time of signals greater than 1ns), and circuits up to 10 inches in length, it is possible to use board FR-4 (glass – epoxy) to make PCB.
Transmission line with controllable impedance in High Frequency PCB
– A line with impedance Z0 that can be controlled based on changes in the geometry of the circuit (trace).
– Often used to match impedances at intermediate connections in transmission lines, such as between cables and jacks, to minimize signal loss.
– For Digital Isolators, the circuits are impedance controlled so that they are close to the output impedance (Z0~r0) – according to the source impedance matching theorem.
The importance of mass coating (GND coating)
In a 2-layer layout, the BOTTOM face should have less wiring, and most of the area on the BOTTOM face should be covered with mass. Because the mass will act as a return path, eliminating the interfering signals.
(Actually, to make High Frequency PCB, reduce noise, we need PCBs with at least 4 layers, but currently, making circuits with small numbers in us is only convenient for 2-layer PCBs. )
Meaning of capacitor types in EMI noise reduction:
– Bulk capacitors: large capacitance capacitors, called bulk capacitors, (eg 10 uF), near the supply source, for example at the voltage regulator chip, or at the place of power supply to the PCB: to stabilize the source voltage.
Decoupling capacitors: as a local source of charge, provide ICs that need a significant current to meet internal switching operations. Lack of decoupling capacitors can cause a lack of current to supply the IC for it to work properly, as a result, the signal is not preserved (attenuated), and data errors occur.
Wiring angle in High Frequency PCB
It is recommended to wire with an angle of 45 or 135 degrees without going at a right angle
Because the 90 degree angle increases the actual width of the line → increase in impedance → impedance bias, resulting in more reflection.
The signal path should not be traversed on multiple layers.
When going signal lines, it is necessary to avoid changing layers (avoiding signal traces on many different layers) because it will lead to increase inductance of signal lines => increase EMI noise
If it is unavoidable to wire the signal through multiple layers, each via of the signal line must be linked to a via of the return-trace. In this case, use the smallest via size possible so that the increase in mutual inductance is minimal.
The concept of High Frequency PCB noise
Noise in electronic circuit design is a type of random generated impurity signal that adversely affects the information signal. There are two types of circuit board noise to be noticed: Radiation noise and reception noise. Radiated noise is interference caused by the device or circuit itself, and received noise is the noise received by that device or circuit when operating near noise sources.
Two types of noise are of interest in PCBs
The concept of interference is often associated with the concept of EMC (Electromagnetic compatibility) – electromagnetic compatibility which is understood to include two types: electromagnetic reception – EMI (Electromagnetic interference) and electromagnetic immunity – EMS (Electromagnetic Susceptibility). Noise reduction is a combination of solving two problems: reducing EMI and increasing EMS.
Anti-interference High Frequency PCB for what!
In the design of electronic circuits, especially high-speed electronic circuits, anti-interference is a must and must be done from the beginning of the project. We cannot wait until the circuit design is complete and then start testing for interference because if we want to improve anything, we will have to change the whole design, which consumes design time, circuit making and circuit installation costs.
Noise cannot be pre-simulated because it is random by design. The calculation of noise cancellation in High Frequency PCB is relatively difficult for inexperienced people, so it requires the circuit maker to have certain knowledge in circuit design.
It is also worth noting that combining too many noise cancellation methods on a single PCB can cost you quite a bit more than the initial cost of creating that device. Therefore, it is necessary to effectively suppress noise just enough for the equipment to operate stably and save costs.
What is the reason for the circuit board noise?
Hidden inductors and hidden capacitors are virtual inductances that exist in printed circuit boards (PCBs) which are one of the causes of interference. These inductive components exist between two circuits or between two layers of material for multilayer circuits.
Existence of hidden capacitors and hidden inductors in High Frequency PCB
Due to the PCB design process, the designer has carried out two or more parallel signal wires too close together or due to the characteristics of the printed circuit board PCB that can produce different values of hidden capacitor and hidden inductors; these components contribute to the reciprocal effect of signals from one printed circuit onto another:
How to prevent circuit board interference?
Therefore, at the first step of making a printed circuit board, the designer must pay attention to the distance between the signal wires and the thickness and material of the printed circuit board. For companies that make PCBs, it is recommended that their CNC equipment be able to travel the minimum distance of lines, with what thickness.
Circuit makers need to study this carefully before placing circuits at circuit companies to avoid having to redraw the design because the circuit maker cannot meet the requirements, each time redrawing will be very time consuming. For example, Woopcb is a large circuit company in China. The recommended printed circuit specification is at least 8mil, the distance between the two circuits is at least 8mil.