Interference Analysis and Countermeasures in High Frequency PCB Design

Interference Analysis and Countermeasures in HIGH FREQUENCY PCB Design

With the development of PCB board design, and the rapid increase of frequency, in addition to the design of low-frequency PCB board, the inconsistency between many interferences, the increase of frequency, the miniaturization and cost reduction of PCB board become more obvious.

These disturbances are becoming increasingly complex. The current study concluded that there are four main types of interference: voltage noise, transmission line interference, coupling, and electromagnetic interference. In this article, we analyze various interference problems of high-frequency circuit boards, and propose effective solutions based on practice.

In high frequency circuits with power supply noise, power supply noise is particularly important for high frequency signals. Therefore, firstly, the power supply is required to have low noise. Here, clean ground is as important as clean electricity, why? Obviously, the power supply has some impedance, and the impedance is distributed across the power supply, so the noise is also superimposed on the power supply.

Then we should keep the impedance of the power supply as low as possible, so it's best to have dedicated power and ground planes. In high frequency circuit design, the power supply is designed in layers, which in most cases is much better than the bus form, so the loop can always follow the path with minimal impedance.

Additionally, the power board must provide a signal loop for all generated and received signals on the PCB, minimizing signal loops and reducing noise, often overlooked by low frequency circuit designers.

Power characteristics

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

Note the via on the board: the via must be etched with an opening on the power plane to leave room for the via to pass through. If the power plane 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 the loop, the common impedance will cause crosstalk.

The cable needs adequate grounding: each signal needs its own dedicated signal loop, and the loop area for the signal and loop should be as small as possible, i.e. the signal is parallel to the loop.

Analog and digital power supplies should be kept separate: High frequency equipment is often very sensitive to digital noise, so the two should be separated and connected together at the power inlet. If the signal goes through both analog and digital crossovers, it's probably at the signal crossover. Place a ring to reduce ring area.

Place the power cable on the side of the signal cable

Transmission Lines There are only two types of transmission lines in a PCB: striplines and microwave lines. The biggest problem with transmission lines is reflections. Reflections can cause a lot of problems. For example, the load signal will be a superposition of the original signal and the echo signal, and the signal analysis will increase. Difficulty; reflections cause return loss (return loss), which affects the signal as badly as additive noise interference:

Signals reflected back to the source add noise to the system, making it more difficult for the receiver to noise, and the signal is separated; any reflected signal radically degrades the signal, changing the shape of the incoming signal. In principle, the solution is mainly impedance matching (for example, the interconnect impedance should match the impedance of the system), but sometimes the calculation of the impedance is cumbersome.

Ways to eliminate transmission line interference: Do not use stubs. Because any stub is a source of noise. If the stub line is short, it can be terminated at the end of the transmission line. If the stub line is long, the main transmission line is the source, which will cause a large reflection, which complicates the problem and is not recommended.

coupling:

Generic Impedance Coupling: This is a generic coupling channel. That is, the source of interference and the faulty device often share specific conductors (loop voltage, bus, common ground, etc.). Field common-mode coupling creates a common-mode voltage in the loop formed by the noisy circuit and the common reference plane.

If the magnetic field is dominant, the common-mode voltage value generated in the series loop is Vcm = -(ΔB/Δt)*area (where ΔB=change in magnetic induction). If it is an electromagnetic field, it is known. Its electric field value, its induced voltage: Vcm=(L*h*F*E)/48, this formula is applicable for L(m)=150MHz or lower, beyond this limit, the maximum induced voltage can be calculated simplified as : Vcm = 2*h*E.

Differential Mode Field Coupling: Means that direct radiation is received by pairs or leads on the board and its circuitry. If as close as possible to both wires. This coupling is greatly reduced, so the two wires can be twisted together to reduce interference.

Line-to-line coupling allows any line equals undesired coupling between parallel circuits, which can severely impair system performance. Its types can be divided into capacitive crosstalk and inductive crosstalk.

The former is because the parasitic capacitance between the lines causes the noise on the noise source to couple to the noise receiving line through the injection current. The latter can be thought of as unwanted signal coupling between the primary and secondary of the parasitic transformer.

The magnitude of 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 AC or DC power lines are exposed to electromagnetic interference, the power lines transmit these interferences to other equipment. There are several ways to eliminate crosstalk in PCB designs: The magnitude of both types of crosstalk increases with load impedance, so signal lines sensitive to interference caused by crosstalk should transmit correctly.

Increasing the distance between signal lines as much as possible can effectively reduce capacitive crosstalk. Perform ground plane management and spacing between traces (e.g., isolate active signal lines and ground lines, especially between signal lines that transition state and further increase ground distance), and set lead inductance to reduce.

Inserting ground lines between adjacent signal lines can also effectively reduce capacitive crosstalk, which needs to enter the ground plane every 1/4 wavelength.

For inductive crosstalk, loop area should be minimized and eliminated as much as possible. Avoid signal sharing loops. Focus on signal integrity: Designers need to implement termination during the soldering process to address signal integrity issues.

Designers using this approach can focus on the microstrip length of the shielded copper foil to achieve good signal integrity. For systems that use dense connectors in the communication fabric, designers can use a single PCB termination.

Electromagnetic Interference As speeds increase, EMI will become more severe and manifest in many ways (eg EMI at interconnects). High-speed equipment is particularly sensitive to this, so it will receive high-speed error signals. Low-speed devices ignore such error signals.

There are several ways to eliminate EMI in PCB designs:

Reduce loops: Each loop is equivalent to an antenna, so we need to minimize the number of loops, the area of the loops, and the antenna effect of the loops. Make sure the signal has only one loop at any two points, avoid artificial loops, and use power planes whenever possible.

Filtering: Filtering can be used on power and signal lines to reduce EMI. There are three methods: decoupling capacitors, EMI filters and magnetics. The EMI filter is shown in the figure.

Due to the length of the question and the many articles on shielding issues, we will not be specific about reducing the speed of high frequency devices as much as possible. Increasing the dielectric constant of the PCB board prevents the high frequency portion of the transmission line close to the board from radiating out.

Increasing the thickness of the PCB board and minimizing the thickness of the microstrip line can prevent the overflow of the magnet wire and also prevent radiation.

In the discussion, we can conclude that in high-frequency PCB design, we should follow the following principles: unity and stability of power and ground. Careful routing and proper termination can eliminate reflections. Careful consideration of routing and proper termination can reduce capacitive and inductive crosstalk. To meet EMC requirements, noise suppression is required.