高周波設計におけるアースの扱い方
高周波設計におけるアースの扱い方
High frequency designs need to address the issue of grounding. There are several issues that need to be addressed when it comes to grounding. These include the impedance of grounding conductors and grounding bonds, DC path dominating low-frequency signals, and single-point grounding.
Impedance of grounding conductors
The grounding electrode of a typical grounded electrical system is in parallel with the ground rods located on the line side of the service, transformers, and poles. The rod under test is connected to the grounding electrode. The equivalent resistance of the line side ground rods is negligible.
A single-point grounding method is acceptable for frequencies below one MHz, but it is less desirable for high frequencies. A single-point grounding lead will raise the ground impedance due to wire inductance and track capacitance, while stray capacitance will create unintended ground return paths. For high-frequency circuits, multipoint grounding is necessary. However, this method creates ground loops that are susceptible to magnetic field induction. Therefore, it is important to avoid using hybrid ground loops, especially if the circuit will contain sensitive components.
Ground noise can be a major problem in high frequency circuits, especially when the circuits draw large varying currents from the supply. This current flows in the common-ground return and causes error voltage, or DV. This varies with the frequency of the circuit.
Impedance of bonding conductors
Ideally, the resistance of bonding conductors should be less than one milli-ohm. However, at higher frequencies, the behavior of a bonding conductor is more complex. It can exhibit parasitic effects and residual capacitance in parallel. In this case, the bonding conductor becomes a parallel resonant circuit. It can also exhibit high resistance due to the skin effect, which is the flow of current through the outer surface of the conductor.
A typical example of a conducted interference coupling is a motor or switching circuit fed into a microprocessor with an earth return. In this situation, the earthing conductor’s impedance is higher than its operating frequency, and it is likely to cause the circuit to resonant. Because of this, bonding conductors are typically bonded at multiple points, with different bonding lengths.
DC path dominating for low-frequency signals
It is widely assumed that DC path dominating for low-frequency signals is easier to implement than high-frequency circuits. However, this method has several limitations, especially in integrated implementations. These limitations include flicker noise, DC current offsets, and large time constants. Moreover, these designs usually use large resistors and capacitors, which can produce large thermal noise.
In general, the return current of high-frequency signals will follow the path of least loop area and least inductance. This means that the majority of the signal current returns on the plane via a narrow path directly below the signal trace.
Single-point grounding
Single-point grounding is an essential element in protecting communications sites from lightning. In addition to effective bonding, this technique offers structural lightning protection. It has been extensively tested in lightning-prone areas and has proven to be an effective method. However, single-point grounding isn’t the only consideration.
If the power level difference between the circuits is large, it may not be practical to use series single-point grounding. The resulting large return current can interfere with low-power circuits. If the power level difference is low, a parallel single-point grounding scheme can be used. However, this method has many disadvantages. In addition to being inefficient, single-point grounding requires a larger amount of grounding, and it also increases the ground impedance.
Single-point grounding systems are generally used in lower frequency designs. However, if the circuits are operated at high frequencies, a multipoint grounding system can be a good choice. The ground plane of a high-frequency circuit should be shared by two or more circuits. This will reduce the chances of magnetic loops.
Power interference
Power interferences can degrade the performance of a circuit and can even cause serious signal integrity problems. Hence, it is imperative to deal with power interferences in high frequency design. Fortunately, there are methods for dealing with these problems. The following tips will help you reduce the amount of power interference in your high frequency designs.
First, understand how electromagnetic interferences occur. There are two main types of interference: continuous and impulse. Continuous interference arises from man-made and natural sources. Both types of interference are characterized by a coupling mechanism and a response. Impulse noise, on the other hand, occurs intermittently and within a short time.
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