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Interference Analysis and Countermeasures in High Frequency PCB Design

Shenzhen Inno Circuit Co.,Ltd | Updated: May 26, 2018

Interference Analysis and Countermeasures in High Frequency PCB Design


In the PCB board design, with the rapid increase of the frequency, there will be a lot of interference different from the low-frequency PCB board design, and the contradiction between the increase in frequency and the miniaturization and low cost of the PCB board becomes increasingly prominent. These interferences are getting more and more complicated. In the actual research, we summarized that there are mainly four kinds of interference: power supply noise, transmission line interference, coupling, electromagnetic interference (EMI). This article analyzes various interference problems of high-frequency PCBs and combines practical work practices to propose effective solutions.


1, power supply noise


In high-frequency circuits, the noise associated with the power supply has a particularly noticeable effect on high-frequency signals. Therefore, the power supply is first required to be low noise. Here, clean and clean power is just as important. Why? Obviously, the power supply has a certain impedance, and the impedance is distributed over the entire power supply. Therefore, 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 it is better to have a proprietary power plane and ground plane. In high-frequency circuit design, the power supply is designed as a layer, and in most cases it is much better than designing in the form of a bus, so that the loop can always follow the path with the lowest impedance. In addition, the power supply board has to provide a signal loop for all signals generated and received on the PCB. This minimizes the signal loop and reduces noise, which is often overlooked by low-frequency circuit designers.


There are several ways to eliminate power supply noise in PCB design.


(1) Pay attention to the through hole on the board: The through hole allows the power layer to be etched to leave space for the through hole to pass through. If the power plane opening is too large, it will inevitably affect the signal loop, the signal is forced to bypass, the loop area increases, and the noise increases. At the same time, if some signal lines are concentrated near the opening and share this loop, the common impedance will cause crosstalk.


(2) The connection line needs enough ground lines: Each signal needs to have its own dedicated signal loop, and the loop area of the signal and the loop is as small as possible, that is, the signal and the loop must be parallel.


(3) The power supply 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. If the signal needs to cross the analog and digital parts, it can be Place a loop across the signal to reduce the loop area. For the signal loop across the number of models.


(4) Avoid separate power supplies overlapping between different layers: otherwise circuit noise can easily be coupled by parasitic capacitances.


(5) Isolate sensitive components: such as PLL.


(6) Place the power cord: To reduce the signal loop, reduce the noise by placing the power cord on the edge of the signal line.


2, transmission line


There are only two possible transmission lines in the PCB: stripline and microwave line. The biggest problem with the transmission line is reflection. Reflection can cause many problems. For example, the load signal will be the superposition of the original signal and the echo signal, increasing the difficulty of signal analysis. The reflection causes return loss (return loss), and its effect on the signal is as severe as the effect of additive noise interference:


(1) The signal is reflected back to the signal source will increase the system noise, making it more difficult for the receiver to separate the noise from the signal;


(2) Any reflected signal will basically degrade the signal quality, which will cause the shape of the input signal to change. In principle, the solution is mainly impedance matching (for example, the interconnect impedance should be very matched with the impedance of the system). However, sometimes the calculation of the impedance is troublesome. You can refer to the calculation software of some transmission line impedances.


The method of eliminating transmission line interference in PCB design is as follows:


(a) Avoid transmission line impedance discontinuities. The point of impedance discontinuity is the point where the transmission line mutates, such as straight corners, via holes, etc., should be avoided as much as possible. The methods include: avoiding the straight corners of the traces, using a 45 degree angle or arc as much as possible, and a large bend angle; using as few vias as possible, because each via is an impedance discontinuity point and the outer layer signal is prevented from passing through. The inner layer and vice versa.


(b) Do not use pile lines. Because any pile line is a source of noise. If the pile line is short, it can be terminated at the end of the transmission line; if the pile line 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


(1) Common impedance coupling: It is a common coupling channel that the interference source and the interfered equipment often share some conductors (such as loop power, bus, common ground, etc.),


On this channel, a drop in Ic causes a common-mode voltage in the series current loop, affecting the receiver.


(2) Field common-mode coupling will cause the radiation source to cause a common-mode voltage on the loop formed by the victim circuit and on the common reference plane. If the magnetic field prevails, the value of the common-mode voltage generated in the series ground loop is Vcm=-(ΔB/Δt)*area (where ΔB=change in magnetic induction intensity) if it is an electromagnetic field, it is known Its electric field value, the induced voltage: Vcm = (L * h * F * E) / 48, the formula applies to L (m) = 150MHz or less, beyond this limit, the calculation of the maximum induced voltage can be simplified as: Vcm = 2*h*E.


(3) Differential Mode Field Coupling: Direct radiation is induced by the wire pairs or the leads and circuit on the circuit board. If it is as close as possible to the two wires. This coupling is greatly reduced, so you can twist two wires together to reduce interference.


(4) Line-to-line coupling (crosstalk) can make any line equal to undesired coupling between parallel circuits, severely compromising system performance. Its type can be divided into capacitive crosstalk and inductive 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 the signal in an undesired parasitic transformer primary and secondary. The amount of inductive crosstalk depends on the proximity of the two loops and the size of the loop area, and the impedance of the affected load.


(5) Power line coupling: refers to the AC or DC power line is subjected to electromagnetic interference, the power line will in turn interfere with these


Transfer to other devices.


There are several ways to eliminate crosstalk in PCB design:


Both types of crosstalk increase in size as the load impedance increases, so proper termination of the interference-sensitive signal lines caused by crosstalk should be handled.


Increasing the distance between signal lines as much as possible can effectively reduce capacitive crosstalk. Ground layer management is performed with spacing between the wiring (for example, isolation between active signal lines and ground lines, especially between signal lines and ground in which state transitions occur) and reduction of lead inductance.


Inserting a ground line between adjacent signal lines can also effectively reduce capacitive crosstalk. This ground line needs to be connected to the ground every 1/4 wavelength.


For inductive crosstalk, the loop area should be reduced as much as possible, and if so, eliminate this loop.


Avoid signal sharing loops.


Focus on signal integrity: Designers need to implement termination during the soldering process to resolve signal integrity. Designers using this approach can focus on the microstrip length of the shielded copper foil for good signal integrity performance. For systems using dense connectors in communication structures, designers can use a PCB for termination.


4, electromagnetic interference


With the increase of speed, EMI will become more and more serious, and behave in many aspects (such as the electromagnetic interference at the interconnection), high-speed devices are particularly sensitive to this, it will receive high-speed false signals, and low speed The device will ignore such false signals.


There are several ways to eliminate electromagnetic interference in PCB design:


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.


Filtering: Filtering can be performed on the power line and on the signal line to reduce EMI. There are three methods: decoupling capacitors, EMI filters, and magnetic components. EMI filter.


shield. Due to the length of space and the number of articles discussing the screening, it is no longer specifically introduced


Minimize the speed of high frequency devices.


Increasing the dielectric constant of the PCB board can prevent the high-frequency part of the transmission line close to the board from radiating outwards; increasing the thickness of the PCB board to minimize the thickness of the microstrip line can prevent the electromagnetic wire from overflowing and can also prevent radiation.


Discussed this we can summarize in the high-frequency PCB design, we should follow the following principles:


The unity of power and ground is stable.


Careful consideration of wiring and proper termination eliminates reflections.


Careful consideration of wiring and proper termination can reduce capacitive and inductive crosstalk.


It is necessary to suppress noise to meet EMC requirements.