Microwave Tech - Interference in high frequency PCB design

In the design of high frequency PCB, engineers need to consider power noise, transmission line interference, coupling, electromagnetic interference (EMI) interference in four aspects.

1. Power supply noise

In the high frequency circuit, the noise from the power supply has a significant impact on the high frequency signal. Therefore, the first requirement is that the power supply is low noise. Here, clean land and clean power are equally important. Why? The power characteristics are shown in Figure 1. Obviously, the power supply has a certain impedance, and the impedance is distributed throughout the power supply, so the noise will also be overlaid on the power supply. Then we should minimize the impedance of the power supply as much as possible, so it is best to have a proprietary power layer and a contact layer. In the high frequency circuit design, the power supply is designed in the form of layers, which in most cases is much better than in the form of buses, so that the circuit can always follow the path with the least impedance. In addition, the power board also provides a signal loop for all generated and received signals on the PCB, which can minimize the signal loop and reduce noise, which is often overlooked by low frequency circuit designers.

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

1.1. Notice board through holes: Through holes make it necessary to etch the openings on the power layer to make room for the through holes to pass through. If the opening of the power 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 loop, the common impedance will cause crosstalk.

1.2 Connection lines need enough ground wires: each signal needs its own unique signal loop, and the area of the loop of the signal and loop is as small as possible, that is, the signal and loop should be parallel.

1.3. The power supply of analog and digital power supply should be separated: High frequency devices are generally very sensitive to digital noise, so they should be separated and connected at the entrance of the power supply. If the signal needs to span both analog and digital parts, a loop can be placed at the signal crossing to reduce the area of the loop. Spanning between the numbers used in the signal loop.

1.4 Avoid overlapping of separate power supply layers: otherwise circuit noise can easily be coupled through parasitic capacitance.

1.5. Isolate sensitive elements such as PLL.

1.6. Place the power cord: To reduce the signal circuit, reduce the noise by placing the power cord on the side of the signal line.

high frequency PCB design

2. Transmission lines

There are only two kinds of transmission lines in PCB: strip line and microwave line. The biggest problem of transmission line is reflection. Reflection can cause many problems. For example, the load signal will be the overlay of the original signal and the echo signal, which increases the difficulty of signal analysis. Reflections cause return loss, which is as severe as additive noise interference.

2.1 Reflecting the signal back to the source increases the system noise, making it more difficult for the receiver to distinguish the noise from the signal;

2.2 Basically, any reflected signal will degrade the signal quality and change the shape of the input signal. In general, the main solution is impedance matching (for example, the interconnect impedance should match the system impedance very well), but sometimes the calculation of impedance is more difficult, you can refer to some transmission line impedance calculation software.

The methods to eliminate transmission line interference in PCB design are as follows:

2.2.1 Avoid impedance discontinuity of transmission lines. Points with discontinuous impedance, such as right corners and through holes, should be avoided as much as possible. The methods are as follows: Avoid straight corners of the line, and walk at 45 degrees or arcs as far as possible, even at large bends; Use as few holes as possible, since each hole is an impedance discontinuity, as shown in Fig. 5; The outer signal avoids passing through the inner layer, and vice versa.

2.2.2 Do not use stake lines. Because any post line is a source of noise. If the stake line is short, it can be connected at the end of the transmission line. If the length of the stake line is long, it will take the main transmission line as the source, which will produce a large reflection and complicate the problem. It is not recommended to use it.

3. PCB Coupling

3.1 Common impedance coupling: A common coupling channel, i.e. interference sources and interfered devices, often share certain conductors (e.g., circuit power supply, bus, common grounding, etc.).

On this channel, the descent of the Ic back in series causes a common mode voltage in the current loop, which affects the receiver.

3.2 Field common mode coupling will cause the radiation source to cause common mode voltage on the loops and common reference surfaces formed by the interfered circuit. If the magnetic field plays a dominant role, the common mode voltage generated in the series ground circuit is Vcm=-(Delta B/Delta t)* area (Delta B=the variation of magnetic induction strength in the formula). If the electromagnetic field is known, its induced voltage is Vcm=(L*h*F*E)/48. The formula is suitable for L(m)=less than 150MHz, beyond this limit, the calculation of maximum induced voltage can be simplified to Vcm=2*h*E.

3.3 Differential mode field coupling: Refers to the direct radiation received by the lead pair or by the lead and its circuit on the circuit board. If as close as possible to two wires. This coupling is greatly reduced, so you can twist the two wires together to reduce the interference.

3.4 Inter-line coupling (crosstalk) can cause undesirable coupling between any line equaling a parallel circuit, which will seriously impair the performance of the system. They can be classified into capacitive and sensory 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 through the injection of current. The latter can be imagined as signal coupling between the primary stages of an unwanted parasitic transformer. The magnitude of the sensory crosstalk depends on the proximity of the two loops, the area of the road, and the impedance of the load affected.

3.5 Power line coupling: refers to the AC or DC power lines are subject to electromagnetic interference, and then the power lines transmit these interference to other devices.

There are several ways to eliminate crosstalk in PCB design:

3.5.1, the size of both crosstalk increases with the increase of load impedance, so signal lines sensitive to the interference caused by crosstalk should be properly terminated.

3.5.2. Increasing the distance between signal lines as much as possible can effectively reduce the tolerance crosstalk. Ground layer management, separation between wiring (e.g. active signal lines and ground lines, especially between signal lines and ground with state hopping) and reduction of lead inductance.

3.5.3 Inserting a ground line between adjacent signal lines can also effectively reduce tolerance crosstalk, which requires that the ground line be connected to the stratum every quarter of the wavelength.

3.5.4 For perceptual crosstalk, the annulus area should be minimized and eliminated if allowed.

3.5.5 Avoid signal sharing loops.

3.5.6. Concern about signal integrity: Designers need to end-connect during the welding process to solve the signal integrity. Designers of 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 their communication architecture, designers can use a PCB as a terminal.

4. Electromagnetic interference

With the increase of speed, EMI will become more and more serious and manifest in many aspects (e.g. electromagnetic interference at the interconnection). High-speed devices are particularly sensitive to this, so they will receive high-speed false signals, while low-speed devices will ignore such false signals.

There are several ways to eliminate EMI in PCB design:

4.1, 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. Ensure that the signal has only one loop path at any two points, avoid artificial loops and use the power layer as much as possible.

4.2, Filtering: Filtering can be used to reduce EMI on both power and signal lines. There are three methods: decoupling capacitance, EMI filter, magnetic element.

4.3. Shielding. Due to the length of the article plus many articles discussing blocking, it will not be introduced in detail.

4.4. Minimize the speed of high frequency devices.

4.5. Increase the dielectric constant of the PCB board to prevent the high frequency parts such as transmission lines near the board from radiating outward. Increasing the thickness of PCB plates and minimizing the thickness of microstrip lines can prevent the overflow of electromagnetic lines and radiation as well.

5. Summary:

In high frequency PCB design, we should follow the principles:

5.1. Unification and stability of power supply and land.

5.2. Careful wiring and proper termination can eliminate reflection.

5.3. Careful wiring and proper termination can reduce tolerance and sensory crosstalk.

5.4. Noise suppression is required to meet EMC requirements.