Power Supply Noise Characteristics and Causes on High Frequency PCBs System Analysis
Distribution noise due to the inherent impedance of the power supply itself. In high-frequency circuits, power supply noise has a large impact on high-frequency signals. Therefore, a low-noise power supply is required first. Clean and clean power is just as important; common-mode field interference. Refers to the noise between the power supply and ground. It is caused by the interference of a power supply caused by the loop formed by the disturbing circuit and the common-mode voltage caused by the common reference plane. The value depends on the relative electric and magnetic fields. The strength will be determined. One of the more important types of interference in high-frequency PCBs is power supply noise. Through the systematic analysis of the power noise characteristics and the causes of the appearance of the high-frequency PCB board, combined with engineering applications, put forward some very effective and simple solutions. Power supply noise analysis Power supply noise refers to the noise generated by the power supply itself or induced. The interference is manifested in the following aspects: 1) Distributed noise caused by the inherent impedance of the power supply itself. In high-frequency circuits, power supply noise has a large impact on high-frequency signals. Therefore, a low-noise power supply is required first. Clean and clean power is just as important. The power supply in the ideal case has no impedance, so it does not have noise. However, in practice, the power supply has a certain impedance, and the impedance is distributed over the entire power supply. Therefore, the noise is also superimposed on the power supply. Therefore, the impedance of the power supply should be reduced as much as possible, preferably with a dedicated power supply layer and ground plane. In high-frequency circuit design, the design of the power supply in the form of a layer is generally better than the design in the form of a bus, so that the circuit can always follow the path with the lowest impedance. In addition, the power board has to provide a signal loop for all signals generated and received on the PCB. This minimizes the signal loop and reduces noise. 2) Power line coupling. It refers to the phenomenon that the power line transmits these interferences to other devices after the AC or DC power lines are subjected to electromagnetic interference. This is the interference of the power supply noise indirectly on the high frequency circuit. It should be noted that: the power supply noise is not necessarily generated by itself, it may also be external interference induced noise, and then add this noise and noise itself (radiation or conduction) to interfere with other circuits or devices. 3) Common mode field interference. Refers to the noise between the power supply and ground. It is caused by the interference of a power supply caused by the loop formed by the disturbing circuit and the common-mode voltage caused by the common reference plane. The value depends on the relative electric and magnetic fields. The strength will be determined. On this channel, a drop in Ic causes a common-mode voltage in the series current loop, affecting the receive portion. If the magnetic field dominates, the value of the common-mode voltage generated in the series ground loop is: Vcm = — (ΔB/Δt) × S (1) ΔB in Equation (1) is the amount of change in magnetic induction intensity, Wb/m2; S is area, m2. If it is an electromagnetic field and its electric field value is known, its induced voltage is: Vcm = (L×h×F×E/48) (2) Formula (2) is generally applicable to L=150/F or less, and F is an electromagnetic wave frequency of MHz. If this limit is exceeded, the calculation of the maximum induced voltage can be simplified as: Vcm = 2×h×E (3) 3) Difference mode field interference. Refers to the interference between the power supply and input and output power lines. In the actual PCB design, the author found that the proportion of power supply noise is very small, so it can not be discussed here. 4) Inter-line interference. Refers to interference between power lines. When there is mutual capacitance C and mutual inductance M1-2 between two different parallel circuits, if the interference source circuit has voltage VC and current IC, the disturbed circuit will appear: a. The voltage coupled through the capacitive impedance is Vcm = Rv*C1-2*ΔVc/Δt (4) In equation (4), Rv is the parallel value of the near-end resistance and the far-end resistance of the interfered circuit. b. Inductively coupled series resistance V = M1-2*ΔIc/Δt (5) If there is common-mode noise in the interference source, the line-to-line interference generally appears in both common mode and differential mode. Eliminating power noise interference In view of the different forms and causes of power noise interference analyzed above, the conditions for its occurrence can be targetedly targeted, and power noise interference can be effectively suppressed. The solution is: 1) Pay attention to the plate through hole. Through holes allow the power layer to be etched to leave space for the through hole. If the power plane opening is too large, it will inevitably affect the signal loop, the signal is forced to bypass, the loop area increases, the noise increases, and if some signal lines are concentrated near the opening and share this loop, the common impedance will cause crosstalk. 2) Place a power noise filter. It can effectively suppress the noise inside the power supply, improve the system's anti-jamming and security. And it is a bi-directional RF filter that can filter out the noise interference introduced from the power line (to prevent interference from other devices), and can also filter out the noise generated by itself (to avoid interference with other devices), and the common mode interference to the serial mode. All play inhibitory effects. 3) Power isolation transformer. By separating the common-mode ground loop of the power loop or signal cable, it can effectively isolate common-mode loop currents generated at high frequencies. 4) Power regulator. Regaining a cleaner power source can greatly reduce power supply noise. 5) Wiring. The input and output lines of the power supply should be avoided from being placed on the edge of the dielectric board, otherwise it is easy to generate radiation and interfere with other circuits or devices. 6) Separate analog and digital power supplies. High-frequency devices are generally very sensitive to digital noise, so the two should be separated and connected together at the power supply's entrance. If the signal is to span both analog and digital parts, a loop can be placed across the signal to reduce the loop area. 7) Avoid separate power supplies overlapping between different layers. Stagger it as much as possible, otherwise power supply noise can easily be coupled past parasitic capacitance. 8) Isolate sensitive components. Some components such as phase-locked loops (PLLs) are very sensitive to power supply noise and should be kept as far away from the power supply as possible. 9) The connection line needs enough ground. Each signal needs to have its own proprietary signal loop, and the loop area of ​​the signal and loop is as small as possible, that is, the signal and the loop must be parallel. 10) Place the power cord. In order to reduce the signal loop, noise reduction can be achieved by placing the power line on the edge of the signal line. 11) In order to prevent the noise caused by power noise from interfering with the circuit board and the external noise from the power supply, noise can be added to the noise path (except radiation) and connected to a bypass capacitor. This bypasses noise to the ground to avoid Interfere with other devices and devices. in conclusion Power supply noise is generated directly or indirectly from the power supply, and interferes with the circuit. When suppressing its effects on the circuit, it should follow a general principle that, on the one hand, the power supply noise should be prevented as much as possible. The impact of the circuit, on the other hand, should also minimize the influence of the outside world or the circuit on the power supply in order to avoid deterioration of the power supply noise. 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