PCB Blog - Electromagnetic radiation detection technology in high-speed PCB board

At present, most hardware engineers only design PCB board by experience. During the debugging process, many signal lines or chip pins that need to be observed are buried in the middle layer of the PCB board, which cannot be detected by tools such as oscilloscopes. If the product cannot pass the function Testing, they also have no effective means to find the cause of the problem. In order to verify the EMC characteristics of the product, the only way is to take the product to a standard electromagnetic compatibility measurement room for measurement. Since this measurement can only measure the external radiation of the product, even if it fails, it cannot provide useful information for solving the problem. Therefore, engineers can only Modify the PCB board empirically and repeat the test. This testing method is very expensive and can delay time to market. Of course, there are many high-speed PCB board analysis and simulation design tools that can help engineers solve some problems, but there are still many limitations on device models. For example, many devices do not have the IBIS model that can solve the signal integrity (SI) simulation. Model or model is inaccurate. To simulate the EMC problem, the SPICE model must be used, but at present almost all ASICs cannot provide the SPICE model, and without the SPICE model, the EMC simulation cannot take the radiation of the device itself into account (the radiation of the device is higher than the radiation of the transmission line). much larger). In addition, simulation tools often have to compromise between accuracy and simulation time. Relatively high precision requires a long calculation time, while tools with fast simulation speed have low precision. Therefore, simulation with these tools cannot completely solve the problem of mutual interference in high-speed PCB board design.

We know that the return path of high-frequency signals in a multi-layer PCB board should be on the reference ground plane (power layer or ground layer) adjacent to the signal line layer, such return flow and impedance, but the actual ground layer or power supply layer There will be divisions and hollowing out, thereby changing the return path, resulting in a larger return area, causing electromagnetic radiation and ground bounce noise. If engineers can understand the current paths, they can avoid large return paths and effectively control electromagnetic radiation. However, the signal return path is determined by many factors such as signal line wiring, PCB power supply and ground distribution structure, power supply point, decoupling capacitor, and device placement position and quantity. Therefore, it is very important to theoretically determine the return path of a complex system. difficulty. Therefore, it is very important to eliminate the radiated noise problem in the design stage. We can see the waveform of the signal with an oscilloscope to help with signal integrity issues, so is there any device that can see the "pattern" of radiation and reflow on the board?

Electromagnetic field high-speed scanning measurement technology
Among the various electromagnetic radiation measurement methods, there is a near-field scanning measurement method to solve this problem, which is designed based on the principle that electromagnetic radiation is formed by high-frequency current loops on the device under test (DUT). For example, the electromagnetic radiation scanning system Emscan of Canadian EMSCAN company is made according to this principle. It uses H-field array probes (32×40=1280 probes) to detect the current on the DUT. During the measurement, the DUT is directly placed on the scanning top of the device. These probes detect changes in electromagnetic fields due to changes in high-frequency currents, and the system provides a visual image of the spatial distribution of RF currents on the PCB. The Emscan electromagnetic compatibility scanning system has been widely used in industrial fields such as communications, automobiles, office appliances, and consumer electronics. Through the current density map provided by the system, engineers can find areas with EMI problems before conducting electromagnetic compatibility standard tests. Take appropriate action. Near-field scanning principle Emscan's measurement is mainly carried out in the active near-field region (r<<λ/2π). Most of the radiation signal emitted from the DUT is coupled to the magnetic field probe, and a small amount of energy is diffused into free space. The magnetic field probe couples the magnetic flux lines of the near H field and the current on the PCB, and it also acquires some trace components of the near E field.

High-current and low-voltage current sources are mainly related to magnetic fields, while high-voltage and low-current voltage sources are mainly related to electric fields. On PCB boards, pure electric fields or pure magnetic fields are rare. In RF and microwave circuits, the input impedance of the circuit and the microstrip or microstrip line used for connection are designed to be 50 ohms. This low impedance design enables these components to generate large currents and low voltage changes. In addition, digital circuits The trend is also to use logic devices with lower voltage differences, while the magnetic field wave impedance in the active near-field region is much smaller than the electric field wave impedance. Taking these factors into consideration, most of the active near-field energy of the PCB board is contained in the near-field magnetic field, so the magnetic field loop used by the Emscan scanning system is suitable for the near-field diagnosis of these PCB boards. All loops are the same, however their positions in the feedback network are different, so the feedback network can sense the response of each loop, the response of each loop relative to the reference source is measured and considered as a filtered transfer function. To ensure the linearity of the measurement, Emscan measures the inverse of this transfer function.

Due to the use of array antenna and electronic automatic switching antenna technology, the measurement speed is greatly accelerated, which is thousands of times faster than the manual single-probe measurement scheme and hundreds of times faster than the automatic single-probe measurement scheme, which can quickly and effectively judge the effect of the circuit before and after modification. . Fast sweep technology and its advanced amplitude-maintaining sweep technology and synchronous sweep technology enable the system to effectively capture transient events, and at the same time, it adopts technology that can improve the measurement accuracy of spectrum analyzers, improving measurement reliability and repeatability.

Measurement method to estimate near-field radiation interference of PCB board
The inspection of the radiated interference of the PCB board can be carried out in several steps. First determine the area to be scanned, then select a probe (7.5mm grid) that can fully sample the scanning area, perform spectrum scanning in the frequency range of 100kHz to 3GHz, and store the level of each frequency point. Note that larger frequency points can be further inspected within the scan area using spatial scanning, which can locate sources of interference and critical circuit paths. The board under test must be as close as possible to the scanner board because with increasing distance the received signal-to-noise ratio decreases and there is also a "separation" effect. In actual measurement, this distance should be less than 1.5cm. As we can see, the measurement of the component side can sometimes cause problems with the measurement due to the height of the component, so the height of the component must be considered in order to correct the measured voltage level. In the basic check, the separation distance correction factor is taken into account. We can get the measurement results very quickly, but these results cannot determine whether the product meets the EMC characteristics, because the value it measures is the electromagnetic near field generated by the high frequency current on the PCB board. The standard EMC test is required to be carried out in an open field (OATS) or in a dark room with a distance of 3 meters (ie, far field).

Although Emscan measurements cannot replace standard EMC testing, they have proven useful in many ways. Through the analysis of the measurement results, many conclusions can be drawn to facilitate the subsequent development of the product. In addition to obtaining the voltage level, the following information is also very important: the interference generation point, the interference distribution, the interference conduction path covering a large area, the interference confined to the narrow area of the PCB board, and the coupling between the internal structure or adjacent I/O modules etc., you can also see the effect of the separation of digital circuits and analog circuits. The above measurement can be used as a standard for the quality evaluation of PCB board design. Further, if we already know the EMC characteristics of a similar PCB board, we can make a more reliable evaluation of EMC characteristics in the early stage of product development, such as Whether shielding should be used, etc. In particular, the electromagnetic field high-speed scanning system can also reveal transient EMI problems, which are often not detected in electromagnetic compatibility measurements, but they can affect product performance and reliability.

* Estimation of the anti-interference performance of the PCB board
In actual use, all electronic devices will be interfered by electromagnetic fields. If a device cannot meet the anti-interference requirements and is not shielded, the performance of the device will be affected by electromagnetic interference. Facts show that the frequency of the interference signal may be several hundred MHz, and these interferences are mainly coupled through the connected conductors, so the anti-interference design of the I/O module is very important. In order to enhance the anti-interference performance of the product, means such as filtering sometimes have to be added, which means that the cost of the product will be increased. From this perspective, it is important to find a solution that optimizes all circuits and components.

By properly modifying the measurement methods mentioned above, the anti-jamming performance of the product can be correctly estimated during the product development and testing stages. The improved method is as follows: put the PCB board on the scanner board to perform spectrum scanning to determine the interference frequency of the PCB board, and then use a clip or appropriate coupling device (such as a T-LISN used on a balanced line for the sine wave interference signal of this frequency). ) is coupled to the I/O line or conductor, with a step size of 10MHz, the frequency range can meet the requirements of 10MHz to 150MHz (to avoid overlapping with the interference frequency of the PCB board), and the power is -20 to 0dBm (depending on the type of coupling device and PCB board) The generator performs a spatial scan at a frequency consistent with the applied interference signal. The distribution of the interference signal from the coupling point to the PCB can be clearly seen on the spatial scanning graph, and then the spatial scanning results can be interpreted according to the following principles, including which areas on the PCB are distributed with coupled interference Signals, effectiveness of inserted filters (attenuating interfering signals), coupling of adjacent I/O conductors, and effectiveness of PCB board ground planes or areas.