In the design of PCB board, with the rapid increase of frequency, there will be a lot of interference which is different from that of low-frequency PCB board. Moreover, with the increasing of frequency and the contradiction between the miniaturization and low cost of PCB board, these interference will become more and more complicated.

In the actual research, we can conclude that there are mainly four aspects of interference, including power supply noise, transmission line interference, coupling and electromagnetic interference (EMI). Through analyzing various interference problems of high-frequency PCB and combining with practice in work, effective solutions are put forward.

First, power supply noise

In the high frequency circuit, the noise of power supply has an obvious influence on the high frequency signal. Therefore, the first requirement of the power supply is low noise. Clean floors are just as important as clean electricity. Why? The power characteristics are shown in Figure 1. Obviously, the power supply has a certain impedance, and the impedance is distributed over the whole power supply, therefore, the noise will be added to the power supply.

Then we should minimize the impedance of the power supply, so it is best to have a dedicated power supply layer and grounding layer. In hf circuit design, it is much better to design the power supply as a layer than as a bus in most cases, so that the loop can always follow the path of minimal impedance.

In addition, the power board must provide a signal loop for all generated and received signals on the PCB. This minimizes the signal loop and thus reduces noise, which is often overlooked by low-frequency circuit designers.

High-frequency PCB design occurs interference solutions

Figure 1: Power characteristics

There are several ways to eliminate power noise in PCB design:

1. Note the through hole on the board: the through hole requires etched openings on the power supply layer to leave space for the through hole to pass through. If the opening of the power supply layer is too large, it is bound to affect the signal loop, the signal is forced to bypass, the loop area increases, and the noise increases. At the same time, if several signal lines are clustered near the opening and share the same loop, the common impedance will cause crosstalk. See Figure 2.

High-frequency PCB design occurs interference solutions

Figure 2: Common path of bypass signal loop

2. The connection line needs enough ground: each signal needs to have its own proprietary signal loop, and the loop area of the signal and loop is as small as possible, that is to say, the signal and loop should be parallel.

3. Analog and digital power supply to separate: 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 across the analog and digital parts of the words, can be placed in the signal across a loop to reduce the loop area. The digital-analog span used for the signal loop is shown in Figure 3.

High-frequency PCB design occurs interference solutions

Figure 3: Digital – analog span for signal loop

4. Avoid overlapping of separate power supplies between layers: otherwise circuit noise can easily pass through parasitic capacitive coupling.

5. Isolate sensitive components: such as PLL.

6. Place the power cable: To reduce the signal loop, place the power cable on the edge of the signal line to reduce the noise, as shown in Figure 4.

High-frequency PCB design occurs interference solutions

Figure 4: Place the power cord beside the signal line

Two, transmission line

There are only two possible transmission lines in a PCB:

The biggest problem of ribbon line and microwave line is reflection. 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 causes return loss (return loss), which affects the signal as badly as additive noise interference:

1. The signal reflected back to the signal source will increase the noise of the system, making it more difficult for the receiver to distinguish noise from signal;

2. Any reflected signal will basically degrade the signal quality and change the shape of the input signal. Generally speaking, the solution is mainly impedance matching (for example, the impedance of the interconnection should very match the impedance of the system), but sometimes the calculation of impedance is more troublesome, you can refer to some transmission line impedance calculation software. The methods of eliminating transmission line interference in PCB design are as follows:

(a) Avoid impedance discontinuity of transmission lines. The point of discontinuous impedance is the point of transmission line mutation, such as straight corner, through hole, etc., should be avoided as far as possible. Methods: To avoid straight corners of the line, as far as possible to go 45° Angle or arc, large Angle can also be; Use as few through holes as possible, because each through hole is an impedance discontinuity, as shown in FIG. 5; Signals from the outer layer avoid passing through the inner layer and vice versa.

High-frequency PCB design occurs interference solutions

Figure 5: Method for eliminating transmission line interference

(b) Do not use stake lines. 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 great reflection, which will complicate the problem. It is recommended not to use it.

Third, the coupling

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, and common grounding), as shown in Figure 6.

High-frequency PCB design occurs interference solutions

Figure 6: Common impedance coupling

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

2. The field common-mode coupling will cause the radiation source to cause common-mode voltages in the loop formed by the interfered circuit and on the common reference surface.

If the magnetic field is dominant, the value of the common-mode voltage generated in the series ground circuit is Vcm=-(△B/△t)* area (where △B= change in magnetic induction intensity). If it is an electromagnetic field, when its electric field value is known, its induced voltage: Vcm=(L* H *F*E)/48, the formula is suitable for L(m)=150MHz, beyond this limit, the calculation of the maximum induced voltage can be simplified as: Vcm=2* H *E.

3. Differential mode field coupling: refers to the direct radiation by wire pair or circuit board on the lead and its loop induction received. If you get as close to the two wires as possible. This coupling is greatly reduced, so the two wires can be twisted together to reduce interference.

4. Inter-line coupling (crosstalk) can cause unwanted coupling between any line or parallel circuit, which will greatly damage the performance of the system. Its type can be divided into capacitive crosstalk and perceptual 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 current injection. The latter can be thought of as the coupling of signals between the primary stages of an unwanted parasitic transformer. The size of inductive crosstalk depends on the proximity of the two loops, the size of the loop area, and the impedance of the load affected.

5. Power cable coupling: The ac or DC power cables are interfered by electromagnetic interference

Transfer to other devices.

There are several ways to eliminate crosstalk in PCB design:

1. Both types of crosstalk increase with the increase of load impedance, so the signal lines sensitive to interference caused by crosstalk should be properly terminated.

2. Maximize the distance between signal lines to effectively reduce capacitive crosstalk. Ground management, spacing between wiring (such as active signal lines and ground lines for isolation, especially in the state of jump between the signal line and ground to interval) and reduce lead inductance.

3. Capacitive crosstalk can also be effectively reduced by inserting a ground wire between adjacent signal lines, which must be connected to the formation every quarter of a wavelength.

4. For sensible crosstalk, the loop area should be minimized, and if allowed, the loop should be eliminated.

5. Avoid signal sharing loops.

6. Pay attention to signal integrity: the designer should implement ends in the welding process to solve signal integrity. Designers using this approach can focus on the microstrip length of the shielding 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 the terminal.

Four, electromagnetic interference

As the speed increases, EMI becomes more and more serious and presents in many aspects (such as electromagnetic interference at interconnects). High-speed devices are particularly sensitive to this and will receive high-speed spurious signals, while low-speed devices will ignore such spurious signals.

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

1. Reduce 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. Make sure the signal has only one loop path at any two points, avoid artificial loops and use the power layer whenever possible.

2. Filtering: Filtering can be used to reduce EMI on both the power line and the signal line. There are three methods: decoupling capacitor, EMI filter and magnetic element. EMI filter is shown in Figure 7.

High-frequency PCB design occurs interference solutions

Figure 7: Filter types

3. The shielding. As a result of the length of the issue plus a lot of discussion shielding articles, no longer specific introduction.

4. Reduce the speed of high-frequency devices.

5. Increase the dielectric constant of PCB board, which can prevent the high frequency parts such as transmission line near the board from radiating outward; Increase the thickness of PCB board, minimize the thickness of microstrip line, can prevent electromagnetic line spillover, can also prevent radiation.

At this point, we can conclude that in hf PCB design, we should follow the following principles:

1. Unification and stability of power supply and ground.

2. Carefully considered wiring and proper terminations can eliminate reflections.

3. Carefully considered wiring and proper terminations can reduce capacitive and inductive crosstalk.

4. Noise suppression is required to meet EMC requirements.