In the design of the circuit board, with the rapid increase of frequency, there will be many interferences different from the design of the low frequency PCB. As the frequency increases and the contradiction between the miniaturization and cost reduction of PCB boards becomes more and more prominent, these interferences are more and more complicated. In the actual research, we conclude that there are four main types of interference, including power supply noise, transmission line interference, coupling, and electromagnetic interference (EMI). By analyzing various interference problems of high-frequency PCBs, combined with practice in practice, an effective solution is proposed.
I.Power supply noise
In high-frequency circuits, the noise of the power supply is particularly significant for high-frequency signals. Therefore, the power supply is first required to be low noise. Here, clean ground and clean power are just as important, 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 is also superimposed on the power supply. Then we should reduce the impedance of the power supply as much as possible, so it is best to have a proprietary power plane and ground plane. In high-frequency circuit design, the power supply is designed in layers, which is much better in most cases than in the form of a bus, so that the loop can always follow the path with the least impedance.In addition, the power board has to provide a signal loop for all generated and received signals on the PCB, which 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:
- Pay attention to the through hole on the board
The through holes make it necessary to etch the openings on the power supply layer to allow space for the through holes to pass. If the power supply layer is too large, it will affect the signal loop, the signal will be forced to bypass, the loop area will increase, and the noise will increase. At the same time, if some signal lines are concentrated near the opening, sharing this loop, the common impedance will cause crosstalk.
- The cable needs enough ground wire
Each signal needs its own proprietary signal loop, and the loop area of the signal and loop is as small as possible, that is, the signal is parallel to the loop.
- Analog and digital power supply 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 crosses both analog and digital, a loop can be placed at the signal crossing to reduce the loop area.
- Avoid separate power supplies that overlap between different layers: otherwise circuit noise is easily coupled through parasitic capacitance.
- Place the power cord
To reduce the signal loop, reduce the noise by placing the power line on the side of the signal line.
- Transmission line
Only two transmission lines are possible in the PCB: stripline and microwave.
The biggest problem with transmission lines is reflection. Reflections 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. The reflection will cause return loss (return loss), which is generated by the signal. The impact is equally severe with the effects of additive noise interference:
- Signal reflection back to the signal source will increase system noise, making it more difficult for the receiver to distinguish noise from signal.
- 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 method to eliminate transmission line interference in PCB design is as follows:
(a) Avoid impedance discontinuities in the transmission line.
The point at which the impedance is discontinuous is the point at which the transmission line is abrupt, such as a straight corner, a via, etc., and should be avoided as much as possible. The method is as follows: avoid straight corners of the traces, as far as possible to take a 45° angle or an arc, and a large corner can also be used; use vias as little as possible because each via is a discontinuous point of impedance; the outer layer signal is avoided 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 the source, which will cause a large reflection, which complicates the problem and is not recommended.
III. Coupling
Common impedance coupling
It is a common coupling channel, that is, the interference source and the interfered device often share certain conductors (such as loop power, bus, common ground, etc.), as shown in Figure 6. On this channel, the falling back of Ic causes a common mode voltage in the series current loop, affecting the receiver.
- Field common mode coupling will cause the radiation source to cause a common mode voltage on the loop formed by the disturbed circuit and the common reference plane.
If the magnetic field is dominant, the value of the common mode voltage generated in the series-connected loop is Vcm=-(ΔB/Δt)* area (where ΔB=the amount of change in the magnetic induction). If it is an electromagnetic field, it is known. Its electric field value, its induced voltage: Vcm = (L * h * F * E) / 48, the formula is suitable for L (m) = 150MHz or less, beyond this limit, the calculation of the maximum induced voltage can be simplified as: Vcm = 2*h*E.
- Differential mode field coupling
Refers to the direct radiation being sensed by the wire pair or the leads on the board and its loop. If possible as close as possible to the two wires. This coupling is greatly reduced, so the two wires can be twisted together to reduce interference.
- Inter-line coupling
Line-to-line coupling (crosstalk) can cause any line to be equal to unwanted coupling between parallel circuits, which can severely compromise system performance. The types can be classified into capacitive crosstalk and inductive crosstalk.
The former is because the parasitic capacitance between the lines causes the noise on the noise source to be coupled to the noise receiving line by the injection of current; the latter can be thought of as the coupling of the signal between an undesired primary and secondary of the parasitic transformer. The magnitude of the inductive crosstalk 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
When the AC or DC power line is subjected to electromagnetic interference, the power line transmits these interferences to other devices.
Method for eliminating crosstalk in PCB design
- The magnitude of both crosstalk increases as the load impedance increases, so the signal lines sensitive to interference caused by crosstalk should be properly terminated.
- Increase the distance between signal lines as much as possible, which can effectively reduce capacitive crosstalk. Ground layer management is performed, and the spacing between the wirings is made (for example, the active signal lines and the ground lines are isolated, especially between the signal lines where the state jumps and the ground), and the lead inductance is reduced.
- Inserting a ground line between adjacent signal lines can also effectively reduce capacitive crosstalk, which requires access to the ground plane every 1/4 wavelength.
- For inductive crosstalk, minimize the loop area and, if possible, eliminate this loop.
- Avoid signal sharing loops.
- Focus on signal integrity: The designer must implement termination during the soldering process to resolve signal integrity. Designers using this approach can focus on shielding the microstrip length of the copper foil for good signal integrity. For systems that use dense connectors in the communication structure, the designer can terminate with a single PCB.
- Electromagnetic interference
As speed increases, EMI will become more severe and manifest itself in many ways (such as electromagnetic interference at the interconnect), which is especially sensitive to high-speed devices, which will therefore receive high-speed spurious signals at low speeds. 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. Make sure that the signal has only one loop path at any two points, avoiding artificial loops and using the power plane as much as possible.
- Filtering
Filtering can be used on the power line and on the signal line to reduce EMI. There are three methods: decoupling capacitors, EMI filters, and magnetic components.
- Shield
- Minimize the speed of high frequency devices
- Increase the dielectric constant of the PCB board / Increase the thickness of the PCB board
Increasing the dielectric constant of the PCB board can prevent the high-frequency part of the transmission line 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 line and also prevent radiation.
Summary: In high frequency PCB design, we should follow the following principles:
- The power and the ground are unified and stable.
- Careful wiring and proper termination can eliminate reflections.
- Careful wiring and proper termination can reduce capacitive and inductive crosstalk.
- Need to suppress noise to meet EMC requirements.