PCB Blog - High frequency PCB board interference problems and solutions

1. Power supply noise
In the high-frequencyPCB board, the noise of the power supply has a particularly obvious impact on the high-frequency signal. Therefore, firstly, the power supply is required to be low-noise. Here, clean ground is as important as clean power, why? The power supply characteristics are shown in Figure 1. Obviously, the power supply has a certain impedance, and the impedance is distributed over the entire power supply, so the noise will also be superimposed on the power supply. Then we should reduce the impedance of the power supply as much as possible, so there must be a dedicated power supply layer and ground plane. In high frequency circuit design, the power supply is designed in the form of layers, which in most cases is much better than the form of buses, so that the loop can always follow the path of the impedance. In addition, the power supply board must provide a signal loop for all the generated and received signals on the PCB, which can reduce the signal loop and reduce noise, which is often overlooked by low-frequency circuit designers.

There are several ways to eliminate power supply noise in PCB board design.
1.1 Pay attention to the through holes on the board: the through holes make the openings need to be etched in the power plane to leave room for the through holes to pass through. If the opening of the power layer is too large, it will inevitably affect the signal loop, the signal will be forced to be bypassed, the loop area will increase, and the noise will increase. At the same time, if some signal lines are concentrated near the opening and share this loop, the common impedance will cause crosstalk.
1.2 The connection line needs enough ground wires: each signal needs to have its own dedicated signal loop, and the loop area of the signal and the loop should be as small as possible, that is, the signal and the loop should be parallel.
1.3 The power supplies of analog and digital power supplies should be separated: high-frequency devices are generally very sensitive to digital noise, so the two should be separated and connected together at the entrance of the power supply. Place a loop at the position to reduce the loop area.
1.4 Avoid separate power supplies overlapping between layers: otherwise circuit noise can easily be coupled through parasitic capacitances.
1.5 Isolate sensitive components: such as PLL.
1.6 Placing the power cable: To reduce the signal loop, reduce the noise by placing the power cable on the side of the signal cable.

2. Transmission line
There are only two kinds of transmission lines that can appear in the PCB: strip line and microwave line. The problem of transmission line is reflection, which will cause many problems. For example, the load signal will be the superposition of the original signal and the echo signal, which increases the difficulty of signal analysis. ; Reflections cause return loss (return loss), which affects the signal as badly as additive noise interference:
2.1 The reflection of the signal back to the signal source will increase the system noise, making it more difficult for the receiver to distinguish the noise from the signal;
2.2 Any reflected signal will basically degrade the signal quality and change the shape of the input signal. In principle, the solution is mainly impedance matching (for example, the impedance of the interconnection should be very matched with the impedance of the system), but sometimes the calculation of the impedance is cumbersome, you can refer to some transmission line impedance calculation software.
2.3 The method of eliminating transmission line interference in PCB board design is as follows:
1) Avoid impedance discontinuities in the transmission line. The point of discontinuous impedance is the point of sudden change of the transmission line, such as straight corners, vias, etc., which should be avoided as much as possible. The methods are: avoid the straight corners of the traces, and try to take 45° angles or arcs as much as possible, and large corners are also acceptable; use as few vias as possible, because each via is a discontinuous point of impedance, and the outer layer signal avoids passing through inner layer and vice versa.
2) Do not use stake lines. Because any stub is a source of noise. If the stub wire is short, it can be terminated at the end of the transmission line; if the stub wire is long, the main transmission line will be used as the source, resulting in a large reflection, which complicates the problem and is not recommended.

3. Coupling
3.1 Common impedance coupling: It is a common coupling channel, that is, the interference source and the interfered device often share some conductors (such as loop power supply, bus, common ground, etc.).
3.2 Field common-mode coupling will cause the radiating source to cause a common-mode voltage on the loop formed by the disturbed circuit and on the common reference plane. If the magnetic field is dominant, the value of the common-mode voltage generated in the series ground loop is Vcm=-(△B/△t)*area (△B=change in the magnetic induction intensity in the formula) If it is an electromagnetic field, it is known When its electric field value, its induced voltage: Vcm=(L*h*F*E)/48, the formula is applicable to L(m)=150MHz or less, beyond this limit, the calculation of induced voltage can be simplified as: Vcm=2 *h*E.
3.3 Differential Mode Field Coupling: Refers to the direct radiation received by the wire pair or the leads and their loops on the circuit board. If as close as possible to both wires. This coupling is greatly reduced, so twist the two wires together to reduce interference.
3.4 Coupling between lines (crosstalk) can make any line equal to the undesired coupling between parallel circuits, which will seriously damage the performance of the system. Its types can be divided into capacitive crosstalk and inductive crosstalk. The former is due to the parasitic capacitance between the lines that couples the noise on the noise source to the noise receiving line through the injection of current; the latter can be thought of as the coupling of the signal between the primary and secondary of an unwanted parasitic transformer. The magnitude of inductive crosstalk depends on the proximity of the two loops and the size of the loop area, as well as the impedance of the load affected.
3.5 Power Line Coupling: It means that after the AC or DC power line is subjected to electromagnetic interference, the power line transmits the interference to other devices.
3.6 There are several ways to eliminate crosstalk in PCB board design:
1) The magnitude of both types of crosstalk increases with the increase of load impedance, so the signal lines that are sensitive to interference caused by crosstalk should be properly terminated.
2) Increasing the distance between signal lines as much as possible can effectively reduce capacitive crosstalk. Conduct ground plane management, space between traces (such as isolation of active signal lines and ground lines, especially between signal lines and ground where state transitions occur) and reduce lead inductance.
3) Inserting a ground wire between adjacent signal wires can also effectively reduce capacitive crosstalk. This ground wire needs to be connected to the ground layer every 1/4 wavelength.
4) For inductive crosstalk, the loop area should be minimized, and if allowed, the loop should be eliminated.
5) Avoid signal sharing loops.
6) Focus on signal integrity: Designers address signal integrity by implementing termination during the soldering process. Designers using this approach can focus on the microstrip length of the shielding copper foil for good signal integrity performance. For systems that use dense connectors in the communications fabric, designers can use a single PCB for termination.

4. Electromagnetic Interference
As speed increases, EMI will become more severe and manifest in many ways (such as electromagnetic interference at interconnects), and high-speed devices are particularly sensitive to this, which will receive high-speed false signals, while low-speed The device ignores such glitches.
There are several ways to eliminate electromagnetic interference in PCB board design:
4.1 Reduce the loop: Each loop is equivalent to an antenna, so we need to minimize the number of loops, the area of the loop and the antenna effect of the loop. Ensure that the signal has only one loop path at any two points, avoid artificial loops, and use the power plane as much as possible.
4.2 Filtering: Filtering can be used on both power lines and signal lines to reduce EMI. There are three methods: decoupling capacitors, EMI filters, and magnetic components.
4.3 Shielding. Due to space problems and many articles discussing shielding, I will not introduce them in detail.
4.4 Minimize the speed of high-frequency devices.
4.5 Increasing the dielectric constant of the PCB board can prevent the high-frequency parts such as transmission lines close to the board from radiating outward; increasing the thickness of the PCB board and minimizing the thickness of the microstrip line can prevent the overflow of the electromagnetic wire and also prevent radiation.

5. To summarize in the design of high-frequency PCB board, we should follow the following principles:
5.1 The unity and stability of power supply and ground.
5.2 Careful wiring and proper termination can eliminate reflections.
5.3 Careful wiring and proper termination can reduce capacitive and inductive crosstalk.
5.4 The noise in the high-frequencyPCB board needs to be suppressed to meet the EMC requirements.