With today's electronic products to increase the main frequency, increased wiring density and a large number of BGA package devices and high-speed logic devices, PCB designers have to increase the number of layers of the PCB board to reduce the signal to signal interaction. At the same time in a large number of portable terminal equipment, in order to reduce system power consumption must be used in a multi-level program, and these devices also have analog or RF circuitry, the need to use a variety of ground, and must use the power plane and ground plane division of the technology. Therefore, there is a large amount of radiation interference between the signals on the PCB board, resulting in equipment malfunction or unstable work, and all signals form a very strong electromagnetic radiation, making EMC testing has also become an obstacle to the product market.
At present, most hardware engineers are still only designing PCBs based on their experience, and in the debugging process, many of the signal lines or chip pins that need to be observed are buried in the middle layer of the PCB, which can't be detected using tools such as oscilloscopes, and if the product can't pass the functional test, they don't have effective means to find the cause of the problem. To verify the EMC characteristics of the product, only the product to the standard electromagnetic compatibility measurement room to measure, because this measurement can only measure the product radiation to the outside world, even if it does not pass can not provide useful information for solving the problem, so engineers can only be based on experience to modify the PCB, and repeat the test. This test method is very expensive and may delay the time to market.
Of course, there are many high-speed PCB analysis and simulation PCB design tools that can help engineers to solve some of the problems, but there are still many limitations in the device model, for example, to solve the signal integrity (SI) simulation of the IBIS model there are a lot of devices do not have a model or model is not accurate. To accurately simulate the EMC problem, it is necessary to use the SPICE model, but at present almost all the ASIC can not provide SPICE models, and if there is no SPICE model, the EMC simulation is unable to take into account the radiation of the device itself (device radiation is much larger than the radiation of the transmission line). In addition, simulation tools often have to make a compromise between accuracy and simulation time, with relatively high accuracy requiring a very long computation time, while tools with fast simulation speeds have very low accuracy. Therefore, simulation with these tools can not completely solve the problem of mutual interference in high-speed PCB design.VSPACE=12
HSPACE=12 ALT="Figure 1: Composition of the Emscan electromagnetic scanning system." >
We know that the return path of a high-frequency signal in a multilayer PCB should be in the reference ground plane (power or ground layer) adjacent to that signal line layer, which minimizes the return current and impedance, but in actuality there are splits and skeletonization in the ground or power layer, which can change the return path and lead to a larger return area, causing electromagnetic radiation and ground bounce noise. If engineers can be clear about the current path, large return paths can be avoided, thus effectively controlling electromagnetic radiation. However, the signal return path is determined by a variety of factors such as signal line wiring, PCB power and ground distribution structure, as well as power supply points, decoupling capacitors, and device placement locations and quantities, so it is very difficult to theoretically determine the return path of a complex system.
So in the PCB design stage to eliminate the radiation noise problem is very critical. We use an oscilloscope to see the waveform of the signal, which can help solve the signal integrity problem, then there is no equipment to see the radiation "graphics" and circuit board reflow?
Electromagnetic field high-speed scanning measurement technology
In a variety of electromagnetic radiation measurement methods, there is a near-field scanning measurement method to solve this problem, the method is based on the principle of PCB design, that is, the electromagnetic radiation is being measured on the device (DUT) on the formation of high-frequency current loop. Such as Canada's EMSCAN electromagnetic radiation scanning system Emscan is made according to this principle, it uses H-field array probes (with 32 × 40 = 1280 probes) to detect the current on the DUT, during the measurement period, the DUT is placed directly on top of the scanner. These probes detect changes in the electromagnetic field due to changes in high-frequency currents, and the system provides a visual image of the spatial distribution of RF currents on the PCB (Fig. 1).
The Emscan EMC scanning system has been widely used in telecommunications, automotive, office appliances, and consumer electronics industries. With the current density map provided by the system, engineers can identify areas with EMI problems and take appropriate measures before testing for EMC standards.
Near-field Scanning PrincipleEmscan measurements are made mainly in the active near-field region (r<<λ/2π), where most of the radiated signals emitted from the DUT are coupled to the magnetic field probe, with a small amount of energy diffused into free space. The magnetic field probe couples the flux lines of the near-H field as well as the current on the PCB, plus it acquires some trace components of the near-E field.
Large-current low-voltage current sources are mainly associated with magnetic fields, while high-voltage small-current voltage sources are mainly associated with electric fields, and pure electric or magnetic fields are rare on PCBs.RF and microwave circuits, the input impedance of circuits as well as microstrips or microstrip wires used for connection, their impedance is designed by PCBs to be 50 ohms, and this low impedance PCB design allows these components to generate high This low-impedance PCB design allows these components to generate large currents and low voltage variations. In addition, the trend in digital circuits is to use logic devices with lower voltage differentials, while the magnetic field wave impedance in the active near-field region is much smaller than the electric field wave impedance. Combining these factors, most of the PCB active near-field region energy is contained in the near magnetic field, making the magnetic field ring used by the Emscan scanning system suitable for near-field diagnostics of these PCBs.
All the rings are the same, however they are positioned differently in the feedback network so that the feedback network senses the response of the individual rings, and the response of each ring relative to the reference source is measured and considered as a filtered transfer function. To ensure linearity of the measurement, Emscan measures the inverse of this transfer function.
Because of the array antenna and electronic auto-switching antenna technology, measurements are greatly accelerated, thousands of times faster than manual single-probe measurement schemes, and hundreds of times faster than automated single-probe measurement schemes, allowing for fast and efficient determination of the effects before and after circuit modifications. The fast scanning technology and its advanced amplitude hold scanning and synchronous scanning techniques enable the system to effectively capture transient events, while it uses techniques that enhance the measurement accuracy of the spectrum analyzer to improve the accuracy and repeatability of the measurements.
Measurement method for evaluating PCB near-field radiated interference
The examination of the PCB radiated interference situation can be carried out in several steps. First determine the area to be scanned, then select a probe (grid 7.5mm) that adequately samples the scanned area, perform a spectrum scan in the frequency range of 100kHz to 3GHz, and store the maximum level at each frequency point. Note that relatively large frequency points can be further examined within the scanning area using spatial scanning, which can locate sources of interference as well as critical circuit paths.
The board under test must be placed as close as possible to the scanner board, as the received signal-to-noise ratio decreases as the distance increases, and there is a "separation" effect. In practice, this distance should be less than 1.5 cm. As we can see, measurements on the component side can sometimes be problematic due to the height of the component, so the height of the component must be taken into account in order to correct the measured voltage level. In the basic check, the separation distance correction factor needs to be considered.
We can get the measurement results very quickly, but these results can't judge whether the product meets the EMC characteristics or not, because it measures the value of the electromagnetic near-field generated by the high-frequency current on the PCB board. Standard EMC testing, on the other hand, is required to be conducted in an open field (OATS) or in a dark room at a distance of 3 meters (i.e., far field).
While Emscan's measurements are not a substitute for standard EMC testing, they do prove to have many uses. By analyzing the measurement results, many conclusions can be drawn for subsequent product development. In addition to obtaining voltage levels, the following information is important: interference generation points, interference distribution, interference conduction paths covering large areas, interference confined to narrow areas on the PCB, and coupling between internal structures or adjacent I/O modules, etc. It is also possible to see the effect of separating digital and analog circuits.
The above measurements can be used as a criterion for PCB design quality assessment. Further, if we already know the EMC characteristics of a similar PCB, it is perfectly possible to make a more reliable assessment of the EMC characteristics early in the product development phase, e.g., whether or not shielding should be used.
Special mention should be made of the fact that EMF high-speed scanning systems can also reveal transient EMI problems, which are often not detected in EMC measurements, but which can affect product performance and reliability.
Assessment of PCB interference immunity
In practice, all electronic devices are subject to electromagnetic field interference. If a device fails to meet the interference immunity requirements and is not shielded, the performance of the device will be affected by electromagnetic interference. It has been shown that the frequency of interference signals may be several hundred MHz, and these interferences are mainly coupled through the connected conductors, so the anti-interference PCB design of I/O modules is very important. In order to enhance the anti-interference performance of a product, sometimes means such as filtering 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 appropriately modifying the measurement methods mentioned above, it is possible to correctly assess a product's immunity during the product development and testing phase. The improved method is as follows: Place the PCB on a scanner board for spectral scanning to determine the PCB's interference frequency, and then couple the frequency sine wave interference signal to the I/O lines or conductors with clips or appropriate coupling equipment (e.g., T-LISN on balanced lines), using a step size of 10MHz, a frequency range that meets the 10MHz to 150MHz (to avoid overlapping with the PCB's interference Using a generator with a step of 10MHz, a frequency range of 10MHz to 150MHz (to avoid overlapping with the interference frequency of the PCB), and a power of -20 to 0dBm (depending on the type of coupling device and PCB), a spatial sweep is performed at a frequency consistent with the added interference signal. Interference signal from the coupling point to the distribution of the PCB can be very clear in the spatial scanning graphic, and then can be based on some of the following principles of spatial scanning results of the interpretation, including which areas of the distribution of the PCB coupled up to the distribution of interference signals, insertion of filter effectiveness (attenuation of interference signals), the proximity of the I/O conductor coupling, and the effectiveness of the PCB grounding layer or area, and so on.
Economic
Economic benefits
Electromagnetic field high-speed scanning technology in the PCB design and debugging applications, can help early detection of problems, timely and effective measures to eliminate or inhibit the system internal and external electromagnetic interference, to ensure that the product EMC test a pass, thus speeding up the product PCB design process, improve the quality of the product PCB design, save product development costs, reduce the product's After-sales service workload.
On average, a new product PCB design needs 2 to 4 times to EMC field test, each time to EMC field test need to wait 3 to 4 weeks. If the product does not pass the test, the entire process will be very long and will have to be repeated until it does. EMI scanning technology allows engineers to consider product EMC during the "PCB design process", thus shortening product development time and reducing the number of trips to the EMC field test. Typically, this can advance time to market by 3 to 12 weeks or more.
Additionally, the identification of alternative components is important to ensure continued production and cost control. In many cases, alternative components do not have the same electromagnetic radiation characteristics as the preferred components, and electromagnetic scanning technology provides a means to quickly and effectively evaluate the availability of alternative components to ensure that the electromagnetic integrity of the product does not affect the premise of reducing the product's production costs.