How to deal with grounding in high frequency design ?

How to deal with grounding in high frequency design ?

Noise reduction is an important design issue in most electronic systems. As with power consumption limits, ambient temperature variations, size limits, and speed and accuracy requirements, ubiquitous noise factors must be addressed to make the final design a success.

 

Here, we do not consider techniques for reducing "external noise" (to the system with the signal) because its presence is generally not directly controlled by the design engineer. In contrast, it is the direct responsibility of the design engineer to prevent "internal noise" (noise generated or coupled within the circuit or system) from disturbing the signal. Today we will talk about "grounding" and it is "grounding" for high frequency operation.

 

"Grounding" generally refers to the connection of a circuit, device, or system to a good conductor that acts as a reference or reference potential plane, creating a low impedance path between the circuit or system and the ground.

 

The ground line is an equipotential body that serves as a reference point for the circuit or system potential. It is the common conductor of each circuit in the system. The current of any circuit will form a loop through the ground. However, any conductor has a certain impedance. When there is current in the local line, according to Ohm's law, there will be voltage on the ground, then the ground is not an equipotential body. Therefore, in the actual design of the circuit or system, the assumption that the potentials at the points on the ground line must be equal is not true. The actual situation is that there is a potential difference at each point on the bottom line, and some phase differences may be large. The common impedance of the ground will cause a certain voltage to form between the grounding points, which will cause grounding interference.

 

As mentioned above, the ground wire acts as a conductor and has a certain impedance. As the name suggests, the impedance is composed of two parts: resistance and inductive reactance, namely:

resistance and inductive reactance

The impedance of a conductor is a function of frequency, and as the frequency increases, the impedance increases rapidly. For high-speed digital circuits, the clock frequency of the circuit is very high, and the pulse signal contains a high frequency component, so a large voltage is generated on the ground line, and the ground line impedance is very disturbing to the digital circuit.

 

In the PCB design of electronic products, suppressing or preventing ground interference is one of the most important issues to consider. The so-called interference must occur between different unit circuits, components or systems, while ground-line interference refers to signal interference generated by means of a common ground. Note that the signals mentioned here usually refer to AC signals or hopping signals. There are many forms of ground interference. Some people have classified it into two categories: ground loop interference and common impedance interference. In fact, electromagnetic coupling interference of the ground loop should be added, so it is of three types. The figure below is a good illustration of the causes of the three types of ground interference.

three types of ground interference.

First, the ground loop interference. Lateral, the current on each wire is different, so differential mode voltage is generated, which affects the circuit. Specifically, the ground current of "other circuit unit B" affects amplifiers A1 and A2 in the "ground loop" formed by J, N, L, and M. Since this interference is caused by the loop current composed of the cable and the ground, it becomes a ground loop interference.

 

Second, the ground loop electromagnetic coupling interference. On the PCB of the actual circuit, the "ground loop" formed by J, N, L, M will surround a certain area. According to the law of electromagnetic induction, if there is a changing magnetic field in the area surrounded by this loop, An induced current is generated in the loop to form an interference. The change of the spatial magnetic field is ubiquitous, so the larger the area enclosed, the more serious the interference.

 

Third, the common impedance interference. Careful examination of the circuit structure shown in the above figure, we will find that one of J, N, L, M is redundant, just remove one, still can meet the connection relationship of each ground point, and at the same time eliminate the ground Line loop. So, which one is more reasonable to remove? At this time, another type of interference problem should be considered - common impedance interference.

 

1. Remove J: This is the worst solution. After J is removed, the ground loop seems to disappear, but another more terrible loop is formed (I, N, L, M), where I is the signal line, so the interference is more serious than the original cable J.

 

2. Remove M: The loop disappears, but we find that the ground current of amplifier A2 needs to flow through J and N to reach the ground zero. Note that the N segment is the common ground wire of A1 and A2, so the ground current of A2 is formed on N. The voltage drop is added to A1, creating interference. This interference caused by sharing a ground line is called "common impedance interference."

 

3. Removal of L: Not only can not solve the common impedance interference problem between A2 and A1, but also cause the common impedance interference problem between "B unit circuit" and A1, A2.

 

4. Remove N: It seems that this is the last method. In fact, doing so will make M become the "common impedance" of A1 and A2, and also form interference. Still have problems! However, we noticed that the interference in this method is the interference of A1 to A2, A2 is the latter stage, and the working signal strength is much larger than A1. Therefore, the interference of A1 to A2 is difficult to cause adverse consequences.

 

The most reasonable routing scheme is to remove N and then connect the lower end of M directly to the "ground signal zero".

 

The above is the cause of grounding interference. The following common grounding methods, combined with the previous understanding of the causes of grounding interference, help us to correctly select the grounding with the least interference when actually designing the PCB board circuit. Way, design a reasonable circuit or system.

 

Signal grounding methods can be broadly divided into: single-point grounding, multi-point grounding, hybrid grounding, and floating grounding.

 

First, a single point of grounding. Single point grounding is to use a certain point in the real circuit system as the grounding reference point. All the ground lines of the circuit and equipment must be connected to this point, and use this point as the zero potential reference point of the circuit and equipment. Single-point grounding is further divided into series single-point grounding and parallel single-point grounding. As shown below:

Series single point grounding

Figure 2. Series single point grounding

 

For the series single-point grounding method, if the power of the circuit is large, a large circuit reflow will occur, and a voltage drop will occur at the finite impedance, causing a difference in the voltage reference between the circuit and the reference ground. Can't work as expected. If there are multiple circuits of different power levels, the series single-point grounding method cannot be used, because the high-power circuit generates a large return current, which will affect the low-power devices and circuits. If this grounding method must be used, the most sensitive circuit must be placed directly at the power input location and as far away as possible from low power devices and circuits. The series single-point grounding method and structure are relatively simple. If the grounding leads of each circuit are relatively short, the impedance will be relatively small. This grounding method can be used if the grounding level of each circuit does not differ much.

circuit

 

In the parallel single-point grounding mode, each circuit unit is connected to the agreed location by a separate ground wire. The advantage is that the location of each circuit is only related to the ground current and ground impedance of the circuit, and is not affected by other circuits. Low-impedance interference between circuit units can be effectively avoided at low frequencies, but there are also many disadvantages. The main performances are as follows: First, each circuit is grounded by an independent ground wire, which requires more grounding, which will increase the length of the grounding wire, thereby increasing the ground impedance, and the complicated structure is troublesome. Secondly, this grounding method will cause the lines to be mutually Coupling, and as the frequency increases, the ground impedance, ground inductance, and wire capacitance increase. This grounding method is not suitable for high frequency circuits.

Second, multi-point grounding. Multi-point grounding means that each circuit and equipment in a system that needs to be grounded is directly connected to the ground plane closest to it. The grounding length is the shortest and the grounding impedance is minimized.

circuit

When the operating frequency of the electronic system is higher than 1 MHz, so that the working wavelength is comparable to the length of the system ground lead, the ground line is like a short-circuited transmission line, the current and voltage of the ground line are distributed, and the ground line becomes The antenna is radiated and cannot function as a ground. In order to reduce the grounding impedance and avoid radiation, the length of the ground wire should be less than 1/20 wavelength. Therefore, the single-point grounding method is unreasonable, and multi-point grounding technology is usually adopted. The multi-point grounding circuit has a simple structure, and the high-frequency standing wave phenomenon that may occur on the grounding wire is significantly reduced, but multi-point grounding may cause many grounding loops to be formed inside the device, which is easy to cause ground loop interference in sensitive stores inside the device.

 

Generally, a single-point grounding method can be used when the frequency is below 1 MHz, and a multi-point grounding method can be used when the frequency is higher than 10 MHz, and a hybrid grounding method is usually used when the frequency is 1 to 10 MHz.

 

Third, mixed grounding. Hybrid grounding is a combination of single-point grounding and multi-point grounding. This type of grounding is often used when there are high and low frequency mixing frequencies in the PCB.

mixed grounding

Figures 5 and 6 provide two hybrid grounding methods. For the capacitive coupling type circuit, a single-point grounding structure is exhibited at a low frequency and a multi-point grounding state at a high frequency. This is because the capacitor shunts the high frequency current to ground. The key to the success of this method is to clearly understand the frequency of use and the expected flow direction of the ground current. The use of capacitors and inductors in the ground topology allows us to control the RF current in an optimized design. The routing of the PCB can be controlled by determining the path through which the RF current will pass. A lack of knowledge of the RF current loop can cause problems with radiation or sensitivity.

 

Fourth, suspension grounding.

Suspended means that the grounding system of the device is electrically insulated from the grounding system of the housing member to prevent electromagnetic interference in the housing member from being conducted into the device. However, since the device is not connected to the public ground, the suspension grounding tends to cause static electricity accumulation between the two. When the charge accumulates to a certain extent, the potential difference between the device and the common ground may cause severe electrostatic discharge, causing interference. Discharge current. Suspension grounding is not suitable for use in communication systems.

 

Understand the above content, then in the actual printing of BCB board, the ground line should be rationally laid out based on the above theoretical basis. When laying out the ground line, you should pay attention to the following points:

 

1. The digital ground and the simulated ground should be separated;

Second, the digital circuit ground wire does not constitute a closed loop;

Third, in the multilayer PCB, try to place the ground layer and the power layer in adjacent layers;

Fourth, the ground wire, power line and signal line width design should be reasonable.

 

These points of attention should be slowly studied in practice.

 

A reasonable grounding design is the most efficient electromagnetic compatibility design technique when designing high speed circuits. According to statistics, 90% of electromagnetic compatibility problems are caused by improper wiring and grounding. Good wiring and grounding can improve the immunity and reduce the interference emission. At the same time, it is possible to solve many electromagnetic interference problems at a lower cost. Therefore, reasonable design grounding is required when designing the PCB of high-speed circuits. It is vital.

 

It is generally recommended that the power and signal currents be returned through the "ground plane" and that the layer also provides a reference node for the converter, voltage reference, and other sub-circuits. However, even the extensive use of the ground plane does not guarantee a high quality ground reference for the AC circuit.

 

Figure 1 shows a simple circuit fabricated using a two-layer printed circuit board with an AC and DC current source on the top layer, one end connected to the via 1 and the other end connected to the via 2 via a U-shaped copper trace. Both vias pass through the board and are connected to the ground plane. Ideally, the impedance is zero and the voltage on the current source is 0 V.

Schematic and layout of the current source

Figure 1. Schematic and layout of the current source. U-shaped traces are placed on the PC board and returned through the ground plane.

 

This simple schematic is far from reflecting the real situation, but understanding how current flows from via 1 to via 2 in the ground plane will help us see the actual problem and find ways to eliminate ground noise from high frequency layouts. method.

 

The inductance is proportional to the area of the current loop, and the relationship between the two can be illustrated by the right-hand rule and magnetic field shown in Figure 2. Within the loop, the magnetic fields generated by the current flowing along all parts of the loop reinforce each other. Outside the loop, the magnetic fields generated by the different parts weaken each other. Therefore, the magnetic field is in principle limited to within the loop. The larger the loop, the larger the inductance, which means that for a given current level, it stores more magnetic energy and higher impedance, which will produce more voltage at a given frequency.

Magnetic lines and inductive loops

Figure 2. Magnetic lines and inductive loops

 

In the simple example shown in the figure, the smallest area loop is clearly the loop formed by the U-shaped top trace and the portion of the ground plane directly below it. Figure 3 (left) shows the path most of the AC current is selected in the ground plane, which is the smallest enclosed area, just below the U-shaped top conductor. In practical applications, the ground plane resistance will cause the low IF current to flow somewhere between the direct return path and directly below the top wire (right). However, even with frequencies as low as 1-2 MHz, the return path is near the top of the top trace.

AC current

How to avoid layout problems? Once you understand the return path of the current in the ground plane, you can find and correct common layout problems. For example, in Figure 4, path A is considered to be a critical path and should be kept to a minimum, away from digital lines, and must not have vias. Path B is not that important, but it needs to go through path A. Usually cut the ground plane below path A, then pass through two vias and route path B below path A.

Typical PCB layout issues when paths are crossed

Figure 4. Typical PCB layout issues when paths are crossed

 

But the result is unfortunate, the inductance is introduced in the ground loop of both signals, because the ground layer of the interrupt makes the area of both loops larger. Path A conducts a high frequency signal, so an induced voltage drop will appear on the opening of the ground plane. For a typical ECL or TTL signal, this voltage drop can be greater than a few hundred millivolts, enough to severely impact the performance of a 12-bit, 10 MHz converter or an 8-bit, 20-MHz converter. A simple remedy is to add a wire to the ground plane cutout to keep the loop area small.

 

Power interference is another issue of concern. The characteristic impedance of the power line must be as low as possible. To make this ratio small, the ground plane must always be below the power line to reduce inductance and increase capacitance. Selectively placing the bypass capacitor in a critical position further increases the capacitance. If only capacitors are considered, such as placing a 0.1 μF capacitor on the power supply pin to reduce its impedance, a 30 nH inductor will have approximately 3 MHz of damped oscillation after each transient.