Interference Analysis and Countermeasure in High-Frequency PCB Design
In the design of PCB board, with the rapid increase of frequency, there will be a lot of interference different from low frequency PCB board design. In practical research, we can conclude that there are mainly four aspects of interference, including power noise, transmission line interference, coupling, electromagnetic interference (EMI). By analyzing various interference problems of high frequency PCB and combining with work practice, an effective solution is put forward.
I. Power Noise
In the high frequency circuit, the noise of power supply has an obvious effect on the high frequency signal. Therefore, the first requirement is that the power supply is low noise. Here, clean and clean power is equally important. Why? Obviously, the power supply has a certain impedance, and the impedance is distributed throughout the power supply, so the noise will be superimposed on the power supply. Then we should reduce the impedance of the power supply as much as possible, so it's better to have a special power supply layer and connection layer. In the design of high frequency circuit, the power supply is designed in the form of layers, which is much better than that in the form of buses in most cases, so that the circuit can always follow the path with the smallest impedance. In addition, the power board has to provide a signal loop for all generated and received signals on PCB, which can minimize the signal loop and reduce noise, which is often ignored by low frequency circuit designers.
There are several methods to eliminate power noise in PCB design as follows:
1. Pay attention to the through holes on the board: the through holes make it necessary to etch the openings on the power layer to allow space for the through holes to pass through. If the opening of the power supply layer is too large, the signal circuit will be affected, the signal will be forced to bypass, the area of the circuit will increase, and the noise will increase. At the same time, if some signal lines are concentrated near the opening and share this circuit, the common impedance will cause crosstalk.
2. Connecting wires need enough ground wires: each signal needs its own special signal circuit, and the area of signal and loop is as small as possible, that is to say, signal and loop should be parallel.
3. The analog and digital power supplies should be separated: high-frequency devices are generally very sensitive to digital noise, so the two should be separated. Connected together at the entrance of the power supply. If the signal crosses both analog and digital, it can be in the signal. A loop is placed across the bridge to reduce the loop area for the span of the digital loop of the signal loop.
4. Avoid separate power supplies that overlap between different layers: otherwise circuit noise is easily coupled through parasitic capacitance.
5. Isolate sensitive components: such as PLL.
6. Place the power cable: To reduce the signal loop, reduce the noise by placing the power cable on the side of the signal line.
Ⅱ, the transmission line
There are only two kinds of transmission lines in the PCB: strip line and microwave line. The biggest problem of the transmission line is reflection. The reflection will cause many problems. For example, the load signal will be the superposition of the original signal and the echo signal, which will increase the difficulty of signal analysis. Reflection can cause return loss (return loss), and its effect on the signal is as severe as the effect of additive noise interference:
1. Signal reflection back to the signal source will increase system noise, making it more difficult for the receiver to distinguish noise from signal;
2. Any reflected signal will basically reduce the signal quality, which will change the shape of the input signal. In principle, the solution is mainly impedance matching (for example, the interconnection impedance should match the impedance of the system). However, sometimes the calculation of the impedance is troublesome. You can refer to some calculation software for transmission line impedance.
The methods of eliminating transmission line interference in PCB design are as follows:
1. Avoid impedance discontinuity of transmission line. The point of discontinuity of impedance is the point of sudden change of transmission line, such as right corner and through hole, which should be avoided as far as possible. The methods are as follows: avoid the right corner of the line, as far as possible go 45 degrees or arc, big corner can also be used; as little as possible through the hole, because each through hole is impedance discontinuity point, the outer signal avoids through the inner layer, and vice versa.
2. Don't use pile line. Because any pile line is a source of noise. If the pile line is short, it can be connected at the end of the transmission line; if the pile line is long, it will take the main transmission line as the source and produce a great reflection, which complicates the problem, so it is recommended not to use it.
III. COUPLING
1. Common impedance coupling: It is a common coupling channel, that is, the interference source and the interfered equipment often share some conductors (such as loop power supply, bus, common grounding, etc.). In this channel, the falling back of IC causes common-mode voltage in the series current loop, which affects the receiver.
2. Field common-mode coupling will cause the common-mode voltage of the radiator on the loop and common reference plane formed by the disturbed circuit. If the magnetic field is dominant, the value of common-mode voltage generated in the series earth circuit is Vcm=-(B/t)* area (the B=the variation of magnetic induction intensity in the formula). If the electromagnetic field is known, the induced voltage is Vcm=(L*h*F*E)/48. The formula is applicable to L(m)=less than 150 MHz, beyond this limit, the maximum induced electric field can be obtained. The calculation of pressure can be simplified to Vcm=2*h*E.
3. Differential mode field coupling: refers to the direct radiation received by the guide wire pair or by the lead and its circuit on the circuit board. If it is as close as possible to the two wires. This coupling will be greatly reduced, so two wires can be twisted together to reduce interference.
4. Inter-line coupling (crosstalk) can make any line equal to the unwanted coupling between parallel circuits, which will seriously damage the performance of the system. It can be divided into capacitive crosstalk and sensory crosstalk. The former is because the parasitic capacitance between the lines makes the noise on the noise source coupled to the noise receiving line through the injection of current; the latter can be imagined as the coupling of signals between the initial stages of an unwanted parasitic transformer. The magnitude of the inductive crosstalk depends on the proximity of the two loops, the area of the loop and the impedance of the load affected.
5. Power line coupling: refers to the AC or DC power lines are subject to electromagnetic interference, the power line will transmit these interference to other equipment.
There are several methods to eliminate crosstalk in PCB design as follows:
1. The magnitude of both crosstalk increases with the increase of load impedance, so the signal lines sensitive to crosstalk should be terminated appropriately.
3. Inserting a ground line between adjacent signal lines can also effectively reduce capacitive crosstalk. This ground line needs to be connected to the ground plane every 1/4 wavelength.
4. For inductive crosstalk, the loop area should be minimized and eliminated if allowed. Baineng Network is a subsidiary of Qinji Group. It is a leading electronic industry service platform in China. It provides components, sensor procurement, PCB customization, BOM distribution, material selection and other electronic industry supply chain solutions. The overall needs of small and medium-sized customers in the industry.
5. Avoid signal sharing loop.
6. Concern about signal integrity: Designers need to implement end-to-end connection in the welding process to solve the problem of signal integrity. Designers using this method can concentrate on the microstrip length of copper foil for shielding in order to obtain good signal integrity performance. For systems with dense connectors in communication architecture, the designer can use a PCB as terminal.
IV. EMI
With the increase of speed, EMI will become more and more serious and manifest itself in many aspects (e.g. electromagnetic interference at interconnection). High-speed devices are particularly sensitive to this, which will receive high-speed false signals, while low-speed devices will ignore such false signals.
There are several methods to eliminate EMI in PCB design as follows:
2. Filtering: The EMI can be reduced by filtering both on the power supply line and on the signal line. There are three methods: decoupling capacitor, EMI filter and magnetic components.
3. Shielding and minimizing the speed of high frequency devices.
4. Increasing the dielectric constant of PCB can prevent radiation from high frequency parts such as transmission lines near the PCB board. Increasing the thickness of PCB board and minimizing the thickness of microstrip line can prevent the overflow of electromagnetic lines and radiation as well.
In this discussion we can conclude that in high frequency PCB design, we should follow the following principles:
1. The unity and stability of power supply and ground.
2. Careful wiring and proper termination can eliminate reflections.
3. Careful consideration of wiring and proper termination can reduce capacitive and inductive crosstalk.
4. It is necessary to suppress noise to meet EMC requirements.
