Distributed noise caused by the inherent impedance of the power supply. In high-frequency circuits, power supply noise has a greater impact on high-frequency signals. Therefore, a low-noise power supply is required first. A clean ground is as important as a clean power supply; common mode field interference. It refers to the noise between the power supply and the ground. It is the interference caused by a power supply caused by the loop formed by the interfered circuit and the common mode voltage caused by the common reference plane. Its value depends on the relative relationship between the electric field and the magnetic field. Strong or weak.
In high frequency PCB boards, the most important type of interference is power supply noise. By systematically analyzing the characteristics and causes of power supply noise on high-frequency PCB boards, and combining with engineering applications, some very effective and simple solutions are proposed.
Analysis of Power Supply Noise
Power supply noise refers to the noise generated by the power supply itself or induced by disturbances. Its interference is manifested in the following aspects:
Distributed noise caused by the inherent impedance of the power supply itself. In high-frequency circuits, power supply noise has a greater impact on high-frequency signals. Therefore, a low-noise power supply is required first. Clean ground and clean power are equally important.
Ideally, a power supply has no impedance, so there is no noise. However, the actual power supply has a certain impedance, and the impedance is distributed on the entire power supply, so the noise will also be superimposed on the power supply. Therefore, the impedance of the power supply should be reduced as much as possible, and it is best to have a dedicated power supply layer and ground plane. In high-frequency circuit design, it is generally better to design the power supply in the form of a layer than in the form of a bus, so that the loop can always follow the path with the least impedance. In addition, the power board must provide a signal loop for all signals generated and received on the PCB, which can minimize the signal loop, thereby reducing noise.
Common mode field interference. It refers to the noise between the power supply and the ground. It is the interference caused by a power supply caused by the loop formed by the interfered circuit and the common mode voltage caused by the common reference plane. Its value depends on the relative relationship between the electric field and the magnetic field. Strong or weak.
On this channel, the drop in IC causes a common-mode voltage in the series-connected current loop, affecting the receiver section. If the magnetic field dominates, the value of the common-mode voltage developed in the series ground loop is:
Vcm = — (△B/△t) × S (1)
ΔB in the formula (1) is the variation of the magnetic induction intensity, Wb/m2; S is the area, m2.
If it is an electromagnetic field, when its electric field value is known, the induced voltage is:
Vcm = (L×h×F×E/48) (2)
Formula (2) is generally applicable to L=150/F or less, and F is the electromagnetic wave frequency MHz.
If this limit is exceeded, the calculation of the maximum induced voltage can be simplified to:
Vcm = 2×h×E (3)
Differential mode field interference. Refers to the interference between the power supply and the input and output power lines. In the actual PCB design, the author found that its proportion in the power supply noise is very small, so it can not be discussed here.
Interference between lines. Refers to interference between power lines. When there are mutual capacitance C and mutual inductance M1-2 between two different parallel circuits, if there are voltage VC and current IC in the interference source circuit, the interference circuit will appear:
a. The voltage coupled through the capacitive impedance is
Vcm = RvC1-2△Vc/△t (4)
In the formula (4), Rv is the parallel value of the near-end resistance and the far-end resistance of the disturbed circuit.
b. Series resistance via inductive coupling
V = M1-2*△Ic/△t (5)
If there is common mode noise in the interference source, the line-to-line interference generally manifests in two forms, common mode and differential mode.
Power line coupling. It refers to the phenomenon that after the AC or DC power line is subjected to electromagnetic interference, the power line transmits the interference to other devices. This is the interference of power supply noise to high frequency circuits indirectly. It should be noted that the noise of the power supply is not necessarily generated by itself, it may also be the noise induced by external interference, and then superimpose this noise with the noise generated by itself (radiation or conduction) to interfere with other circuits or devices.