**Abstract**: This article provides a systematic analysis of the various forms and causes of power noise interference in high-frequency PCB boards. Through mathematical derivations and practical engineering insights, it proposes effective countermeasures and concludes by outlining general principles for power noise suppression. In high-frequency PCBs, power supply noise is one of the most significant sources of interference.
Building on a thorough analysis of the characteristics and causes of power noise in high-frequency PCBs, and incorporating engineering applications, the article suggests several effective and simple solutions.
**Power Supply Noise Analysis**
Power supply noise refers to unwanted signals generated either by the power supply itself or through external interference. This interference manifests in the following ways:
1) **Distributed noise due to the inherent impedance of the power supply**. In high-frequency circuits, power supply noise can significantly affect high-frequency signals, making a low-noise power supply essential.
A clean ground is just as important as a clean power source. Ideally, a power supply would have no impedance, thus generating no noise. However, in reality, the power supply always has some impedance, which is distributed throughout the system, causing noise to be superimposed on the supply. Therefore, minimizing the impedance of the power supply is crucial. The use of a dedicated power supply layer and ground layer is recommended.
1) **Power Supply in High-Frequency Circuit Design**
In high-frequency circuit design, the power supply implemented as a layer design typically performs better than a bus design. This approach ensures that the circuit always follows the path of least impedance. Additionally, the power plane serves to provide a signal loop for all signals generated and received on the PCB, effectively minimizing signal loops and reducing noise.
2) **Common Mode Field Interference**
Common mode field interference refers to the noise between the power supply and ground. This occurs due to common mode voltage interference generated by the loop formed between the interference circuit and the common reference plane. The magnitude of this interference depends on the relative strength of the electric and magnetic fields. In such channels, reducing Ic leads to the appearance of common-mode voltage in the series current loop, which in turn can impact the receiving section.
If the magnetic field is the dominant factor, the common-mode voltage generated in the series loop is:
[
Delta B = text{change in magnetic induction, Wb/m}^2; , s = text{area, m}^2.
]
In the presence of an electromagnetic field, when the electric field strength is known, the induced voltage can be determined by the following formula:
[
V_{text{induced}} = text{induced voltage in the electric field}.
]
Equation (2) is typically applicable when ( L = 150 / F ), where ( F ) is the frequency in MHz.
PCB designers often find that exceeding this limit results in a reduction of induced voltage.
3) **Differential Mode Field Interference**
Differential mode field interference pertains to the interaction between the power supply and the input/output power lines. In practical PCB design, this form of interference is usually minimal in comparison to other types of power supply noise, and will therefore not be discussed in detail here.
4) **Inter-line Interference**
Inter-line interference occurs between power lines when there is mutual capacitance ( C ) and mutual inductance ( M_{1-2} ) between two parallel circuits. If there is a voltage ( V_C ) and current ( I_C ) in the source circuit, interference is introduced into the system:
A) Capacitive impedance coupling voltage:
[
V = text{Voltage due to capacitive coupling}.
]
Here, ( R_V ) is the parallel resistance value of the near-end and far-end resistances in the interference circuit.
B) Inductive coupling via series resistor:
If common mode noise is present in the interference source, line-to-line interference generally manifests as both common mode and differential mode.
5) **Power Line Coupling**
Power line coupling refers to the phenomenon where AC or DC power cables are subjected to electromagnetic interference, which is then transmitted to other devices. This is a form of power supply noise indirectly affecting high-frequency circuits.
It’s important to note that power supply noise isn’t always generated internally; it can also result from external interference, which then combines with the self-generated noise (radiated or conducted) to disrupt other circuits or equipment.
**Countermeasures to Eliminate Power Supply Noise Interference**
To effectively suppress power supply noise interference, solutions should address the different manifestations and causes of the noise, as well as the conditions under which it occurs. Below are some common strategies:
1) **Minimize Through-Hole Impact**
Care should be taken with through-holes on the PCB. A through-hole requires an opening in the power layer to accommodate its passage. If the opening is too large, it will inevitably affect the signal loop, forcing the signal to bypass it. This increases the loop area and the potential for noise. When signal traces are placed near such openings, shared impedance can lead to crosstalk.
2) **Ensure Adequate Grounding for Cables**
Each signal line should have its own dedicated signal loop, with the signal loop area kept as small as possible. This ensures that the signal runs parallel to its loop to minimize interference.
3) **Install Power Supply Noise Filters**
Power supply noise filters can effectively suppress internal power supply noise and enhance the system’s immunity to interference. These filters are bidirectional RF filters that not only block noise coming from the power line (preventing external interference) but also filter out self-generated noise (preventing interference with other devices). Additionally, common-mode interference in series circuits can be effectively suppressed.
4) **Use Power Isolation Transformers**
A power isolation transformer can isolate the common-mode ground loop of both the power circuit and signal cable. This helps in effectively separating high-frequency common-mode currents and minimizing interference.
5) **Implement Power Regulators**
Using power regulators to obtain a cleaner power supply can significantly reduce power supply noise, thus improving overall system performance.
6) **Optimize PCB Layout**
Power supply input and output lines should not be routed along the edge of the PCB. Otherwise, radiation can easily occur, leading to interference with other circuits or devices.
7) **Separate Analog and Digital Power Supplies**
High-frequency devices are particularly sensitive to digital noise. Therefore, analog and digital power supplies should be separated, with a connection point provided at the power entry. When signals span across both analog and digital sections, it’s beneficial to include a loop in the signal path to reduce the loop area.
8) **Avoid Overlapping Power Supplies Across Different Layers**
Power supplies should not overlap between layers. Staggering them as much as possible helps reduce the likelihood of power supply noise coupling through parasitic capacitance.
9) **Isolate Sensitive PCB Components**
It’s essential to isolate sensitive components on the PCB to prevent external interference from affecting their operation.
Building on a thorough analysis of the characteristics and causes of power noise in high-frequency PCBs, and incorporating engineering applications, the article suggests several effective and simple solutions.
**Power Supply Noise Analysis**
Power supply noise refers to unwanted signals generated either by the power supply itself or through external interference. This interference manifests in the following ways:
1) **Distributed noise due to the inherent impedance of the power supply**. In high-frequency circuits, power supply noise can significantly affect high-frequency signals, making a low-noise power supply essential.
A clean ground is just as important as a clean power source. Ideally, a power supply would have no impedance, thus generating no noise. However, in reality, the power supply always has some impedance, which is distributed throughout the system, causing noise to be superimposed on the supply. Therefore, minimizing the impedance of the power supply is crucial. The use of a dedicated power supply layer and ground layer is recommended.
1) **Power Supply in High-Frequency Circuit Design**
In high-frequency circuit design, the power supply implemented as a layer design typically performs better than a bus design. This approach ensures that the circuit always follows the path of least impedance. Additionally, the power plane serves to provide a signal loop for all signals generated and received on the PCB, effectively minimizing signal loops and reducing noise.
2) **Common Mode Field Interference**
Common mode field interference refers to the noise between the power supply and ground. This occurs due to common mode voltage interference generated by the loop formed between the interference circuit and the common reference plane. The magnitude of this interference depends on the relative strength of the electric and magnetic fields. In such channels, reducing Ic leads to the appearance of common-mode voltage in the series current loop, which in turn can impact the receiving section.
If the magnetic field is the dominant factor, the common-mode voltage generated in the series loop is:
[
Delta B = text{change in magnetic induction, Wb/m}^2; , s = text{area, m}^2.
]
In the presence of an electromagnetic field, when the electric field strength is known, the induced voltage can be determined by the following formula:
[
V_{text{induced}} = text{induced voltage in the electric field}.
]
Equation (2) is typically applicable when ( L = 150 / F ), where ( F ) is the frequency in MHz.
PCB designers often find that exceeding this limit results in a reduction of induced voltage.
3) **Differential Mode Field Interference**
Differential mode field interference pertains to the interaction between the power supply and the input/output power lines. In practical PCB design, this form of interference is usually minimal in comparison to other types of power supply noise, and will therefore not be discussed in detail here.
4) **Inter-line Interference**
Inter-line interference occurs between power lines when there is mutual capacitance ( C ) and mutual inductance ( M_{1-2} ) between two parallel circuits. If there is a voltage ( V_C ) and current ( I_C ) in the source circuit, interference is introduced into the system:
A) Capacitive impedance coupling voltage:
[
V = text{Voltage due to capacitive coupling}.
]
Here, ( R_V ) is the parallel resistance value of the near-end and far-end resistances in the interference circuit.
B) Inductive coupling via series resistor:
If common mode noise is present in the interference source, line-to-line interference generally manifests as both common mode and differential mode.
5) **Power Line Coupling**
Power line coupling refers to the phenomenon where AC or DC power cables are subjected to electromagnetic interference, which is then transmitted to other devices. This is a form of power supply noise indirectly affecting high-frequency circuits.
It’s important to note that power supply noise isn’t always generated internally; it can also result from external interference, which then combines with the self-generated noise (radiated or conducted) to disrupt other circuits or equipment.
**Countermeasures to Eliminate Power Supply Noise Interference**
To effectively suppress power supply noise interference, solutions should address the different manifestations and causes of the noise, as well as the conditions under which it occurs. Below are some common strategies:
1) **Minimize Through-Hole Impact**
Care should be taken with through-holes on the PCB. A through-hole requires an opening in the power layer to accommodate its passage. If the opening is too large, it will inevitably affect the signal loop, forcing the signal to bypass it. This increases the loop area and the potential for noise. When signal traces are placed near such openings, shared impedance can lead to crosstalk.
2) **Ensure Adequate Grounding for Cables**
Each signal line should have its own dedicated signal loop, with the signal loop area kept as small as possible. This ensures that the signal runs parallel to its loop to minimize interference.
3) **Install Power Supply Noise Filters**
Power supply noise filters can effectively suppress internal power supply noise and enhance the system’s immunity to interference. These filters are bidirectional RF filters that not only block noise coming from the power line (preventing external interference) but also filter out self-generated noise (preventing interference with other devices). Additionally, common-mode interference in series circuits can be effectively suppressed.
4) **Use Power Isolation Transformers**
A power isolation transformer can isolate the common-mode ground loop of both the power circuit and signal cable. This helps in effectively separating high-frequency common-mode currents and minimizing interference.
5) **Implement Power Regulators**
Using power regulators to obtain a cleaner power supply can significantly reduce power supply noise, thus improving overall system performance.
6) **Optimize PCB Layout**
Power supply input and output lines should not be routed along the edge of the PCB. Otherwise, radiation can easily occur, leading to interference with other circuits or devices.
7) **Separate Analog and Digital Power Supplies**
High-frequency devices are particularly sensitive to digital noise. Therefore, analog and digital power supplies should be separated, with a connection point provided at the power entry. When signals span across both analog and digital sections, it’s beneficial to include a loop in the signal path to reduce the loop area.
8) **Avoid Overlapping Power Supplies Across Different Layers**
Power supplies should not overlap between layers. Staggering them as much as possible helps reduce the likelihood of power supply noise coupling through parasitic capacitance.
9) **Isolate Sensitive PCB Components**
It’s essential to isolate sensitive components on the PCB to prevent external interference from affecting their operation.
