Interference Analysis And Countermeasures In High Frequency PCB Design
In the design of PCB board, with the rapid increase of frequency, there will be a lot of interference that is different from the design of low-frequency PCB board. In the actual research, we conclude that there are mainly four aspects of interference, including power noise, transmission line interference, coupling and electromagnetic interference (EMI). Through the analysis of various interference problems of high frequency PCB, combined with the practice of work, the effective solution is put forward.
In the high frequency circuit, the noise of the power supply has an obvious influence on the high frequency signal. Therefore, the first requirement is that the power supply is low noise. Obviously, the power supply has some impedance, and the impedance is distributed throughout the power supply, so the noise will also be added to 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 layer and connecting layer.
In high frequency circuit design, the power supply is designed as a layer, in most cases much better than a bus, so that the circuit can always follow the path with the least impedance. In addition, the power board must provide a signal circuit for all generated and received signals on the PCB. This minimizes the signal circuit and thus reduces noise, which is often overlooked by low-frequency circuit designers.
There are several ways to eliminate power noise in PCB design:
Note the through-hole on the plate: the through-hole makes it necessary to etch an opening on the power layer to leave space for the through-hole to pass through. If the power layer opening is too large, it will affect the signal loop, signal is forced to bypass, loop area increases, noise increases. At the same time, if some signal lines are concentrated near the opening and share the same circuit, the common impedance will cause crosstalk.
Connecting wires should have enough ground wires: each signal should have its own proprietary 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.
Analog and digital power supply of power to separate: high-frequency components in general are very sensitive to digital noise and to separate, so both at the entrance of the power supply to meet together, if the signal to cross analog and digital two parts, can put a cross in the signal circuit to minimize the loop area, used for cross between the analog signal circuit.
Avoid overlapping of separate power sources between different layers; otherwise, circuit noise is easy to pass through parasitic capacitance coupling.
Isolation of sensitive components: such as PLL.
Power line placement: To reduce the signal loop, the power line is placed beside the signal line to reduce noise.
In PCB, there are only two kinds of transmission lines: ribbon line and microwave line. The biggest problem of transmission lines is reflection, which will cause many problems. For example, load signal will be the superposition of original signal and echo signal, which increases the difficulty of signal analysis. Reflection will cause return loss (return loss), and its influence on the signal is as serious as that of additive noise interference:
The signal reflected back to the signal source will increase the system noise, making it more difficult for the receiver to distinguish noise from signal;
Any reflected signal basically degrades the signal quality and changes the shape of the input signal. Generally speaking, the solution is mainly impedance matching (for example, the interconnect impedance should match the system impedance very well). However, sometimes the calculation of impedance is troublesome, so you can refer to the calculation software of transmission line impedance.
The methods to eliminate transmission line interference in PCB design are as follows:
Avoid impedance discontinuity of transmission line.
The point of impedance discontinuity is the point of transmission line mutation, such as straight corner, through hole, etc., should be avoided as far as possible. The methods are as follows: avoid straight corners of the line, and try to take an Angle of 45° or an arc, or a large curve;
Use as few through-holes as possible because each through-hole is an impedance discontinuity and the outer signal avoids passing through the inner layer and vice versa.
Don’t use pile thread.
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, which will generate a lot of reflection and complicate the problem. It is recommended not to use it.
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 ground, etc.). On this channel, the IC’s fallback causes a common mode voltage in the current loop in series, affecting the receiver.
Field common-mode coupling will cause the radiation source to cause common-mode voltage on the loop formed by the interfered circuit and the common reference surface. If the magnetic field is dominant, the value of common mode voltage generated in the series ground loop is Vcm=-(△B/△ T)* area (△B = changes in magnetic induction intensity). If it is electromagnetic field, given its electric field value, its induced voltage: Vcm=(L* H *F*E)/48, the formula applies to L(m)= less than 150MHz, beyond this limit, the calculation of maximum induced voltage can be simplified as: Vcm=2*h*E.
Differential mode field coupling: refers to the direct radiation is induced and received by the conductor pair or the lead on the circuit board and its loop. If you go as close as you can to two wires. This coupling is greatly reduced, so the two wires can be twisted together to reduce interference.
Wire coupling (crosstalk) can make any wire equal to the undesired coupling between parallel circuits, which will seriously damage the performance of the system. It can be divided into capacitive crosstalk and perceptual crosstalk. The former is because the stray capacitance between the lines causes the noise from the noise source to be coupled to the noise receiving line through the current injection. The latter can be thought of as a signal coupling between the first stages of an unwanted parasitic transformer. The size of the sensible crosswalk depends on the proximity of the two loops and the size of the loop area, as well as the impedance of the affected load.
Power line coupling: refers to the AC or DC power line subjected to electromagnetic interference, the power line will transmit these interferences to other equipment.
There are several ways to eliminate crosstalk in PCB design:
The size of the two kinds of crosstalk increases with the increase of load impedance, so the signal lines sensitive to interference caused by crosstalk should be terminated appropriately.
Increase the distance between signal lines as much as possible to effectively reduce capacitive crosstalk. Conduct ground connection management, conduct spacing between wiring (e.g., isolate active signal lines from ground, especially between signal lines with state transitions and ground) and reduce lead inductance.
The insertion of a ground wire between adjacent signal lines can also effectively reduce capacitively crosstalk. This ground wire needs to be connected to the formation at every 1/4 wavelength.
For sensible crosstalk, minimize the loop area and eliminate the loop if possible.
Avoid signal sharing loop.
Focus on signal integrity: The designer should realize end-to-end welding in order to solve signal integrity. The designer using this method can concentrate on shielding the microstrip length of the copper foil in order to obtain good performance of signal integrity. For systems with dense connectors in the communication structure, the designer can use a PCB as an end – on.
As speed increases, EMI will become more and more serious and will show many aspects (such as electromagnetic interference at interconnects). High-speed devices are particularly sensitive to this, and thus will receive false signals at high speeds, while low-speed devices will ignore such false signals.
There are several methods to eliminate electromagnetic interference in PCB design:
Reduce the number of loops: Each loop is equivalent to an antenna, so we need to minimize the number of loops, the area of loops and the antenna effect of loops. Ensure that the signal has only one circuit path at any two points, avoid artificial loops, and use the power layer whenever possible.
Filtering: Filtering can be used on power lines and signal lines to reduce EMI. There are three methods: decoupling capacitance, EMI filter and magnetic components.
Shield and minimize the speed of high-frequency devices.
Increase the dielectric constant of PCB board to prevent the outward radiation of high-frequency parts such as transmission lines near the board; Increasing the thickness of PCB board and minimizing the thickness of microstrip line can prevent the overflow of electromagnetic line and also prevent radiation.
With this discussion, we can summarize the following principles that should be followed in the design of HIGH-FREQUENCY PCB:
Unification and stability of power supply and ground.
Carefully considered wiring and proper terminations can eliminate reflections.
Carefully considered wiring and proper terminations can reduce capacitive and perceptual crosstalk.
Noise suppression is required to meet EMC requirements.
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