Emc test for entering the market of electronic products is very important for a test, but can test can only be obtained through the result of the past, can't provide more useful information. In this paper, the advantage of high speed automatic scanning measuring electromagnetic radiation, detecting the change of electromagnetic field on PCB board, make the engineering and technical personnel in the electromagnetic compatibility can be found before the standard test questions and promptly corrected.
With today's electronic products, wiring density increased frequency and a large number of BGA encapsulation device and the use of high speed logic devices, the designers had to increase the PCB layer to reduce the mutual influence between the signal and the signal. Across a large number of portable terminal devices at the same time, in order to reduce the power consumption of the system must use multilevel schemes, and these devices and analog or RF circuit, need to use a variety of land, and must use the power plane and plane segmentation technology. So a large number of radiated interference between the signal on the PCB, cause the failure or work function of the equipment is not stable, and all external signal form strong electromagnetic radiation, the EMC test has become an obstacle to product.
At present most of the hardware engineer to design the PCB is only by experience, in the process of debugging, many need observation signal lines or chip pins were buried in the middle tier of the PCB, unable to use tools to detect, such as the oscilloscope, if the product cannot pass functional test, they also have no effective means to find the cause of the problem. To verify the EMC characteristics of products, only the product to measure the standard of electromagnetic compatibility measurement room, since this measurement can only product external radiation condition, if not by also cannot provide useful information to solve the problem, so the engineer can only with experience to modify the PCB, and repeated trials. This test method is very expensive, and may delay the product time to market.
Analysis and simulation, of course, there are a lot of high speed PCB design tool, can help engineers to solve some problems, but at present there are still many restrictions in the device model, for example, can solve the signal integrity (SI) simulation of the IBIS model as there are many devices no model or model is not accurate. To accurate simulation of EMC problems, it is necessary to use the SPICE model, but nearly all of the ASIC can't provide the SPICE model, but without the SPICE model, EMC simulation is unable to take into consideration the radiation device itself (components of radiation is much bigger than the transmission line radiation). In addition, the simulation tools tend to compromise on the accuracy and the simulation time, relatively high accuracy and computation time is long, and the simulation speed of the tool, its precision is very low. So with these tools, simulation can't completely solve the interference problem in high speed PCB design. Figure 1: Emscan electromagnetic scanning system.
We know that the high frequency signal in the multilayer PCB should return path in the signal wire layer near the reference ground plane (power supply layer or stratum), such as minimum reflux and impedance, but the actual formation or power supply layer will be split and hollow out, so as to change the flow path, causing reflux area is larger, play the noise caused by electromagnetic radiation and the earth. If the engineer can clear current path, can avoid large flow path, thus effectively control electromagnetic radiation. But the signal return path from signal wiring, PCB power supply and distribution structure and power supply, the decoupling capacitor and the device is placed and quantity is determined by many factors, such as complex system in return path is very difficult to determine in theory.
Electromagnetic field scanning measurement technology at a high speed
Among various kinds of electromagnetic radiation measurement, there is a kind of near field scanning measurement method to solve the problem, the method based on the principle of design, the electromagnetic radiation is the high frequency current on the equipment under test (DUT) circuit. Electromagnetic radiation such as Canada EMSCAN company EMSCAN scanning system is made according to this principle, it USES the H field array probe (32 x 40 = 1280 probe) to detect the current on the DUT, during a measurement, DUT directly at the top of the scanner. These probes can detect changes due to the high frequency current caused by the change of the electromagnetic field, the system can provide the RF current in the visual image of the spatial distribution of the PCB.
Emscan emc scanning system has been in communication, auto, office appliances and consumer electronics and other industrial fields widely used, through the system to provide the figure of current density, engineers in the electromagnetic compatibility can be found before the standard test EMI problem areas and take corresponding measures.
Near-field scanning principle Emscan measuring main activity in the near field area (r < < lambda / 2 PI), most radiation signals on the DUT is coupled to the magnetic field sensor, a small amount of energy spread to the free space. Magnetic field probe nearly H coupling field of flux lines and electric current on the PCB, and it also get some nearly E field of trace elements.
High current low voltage current source is mainly related to the magnetic field, and the high voltage low current voltage source is mainly related to the electric field, on the PCB, pure electric field or magnetic field is rare. RF and microwave circuit, the input impedance of the circuit and connect with microstrip or microstrip lines, the impedance is designed as 50 ohm, the low impedance design make these components produce large current and low voltage change, in addition to the trend of the digital circuit is to use a lower voltage difference logic devices, at the same time, active magnetic wave impedance in the near field area is far less than the electric field wave impedance. Integrated these factors, most of the PCB active energy is contained in the near field area close to magnetic field, thus Emscan scanning system adopts CiChangHuan is suitable for the PCB near field diagnosis.
All of the ring is the same, but their place in the feedback network is different, so each ring can be induced by the feedback network response, each ring relative reference source response have been measured and consider as the filter transfer function. In order to guarantee measuring linearity, Emscan measurement is the reciprocal of the transfer function.
With the automatic switch antenna array antenna and electronic technology, thus greatly accelerate the measuring speed, thousands of times faster than manual single probe measurement scheme, also faster than single probe measurement scheme automatically hundreds, can quickly and effectively judge the effect of the circuit before and after modification (figure 2). Maintain rapid scanning technology and advanced amplitude scanning technique and synchronous scanning technology to enable the system to capture the transient events effectively, at the same time it can promote a spectrum analyzer is used to measuring precision technology, improve the measurement accuracy and repeatability.
Evaluate PCB near field radiated interference measuring method is shown in figure 2: Emscan measurement according to the results.
Check to PCB radiation interference can be divided into a few steps. To determine the need to scan the area first, and then choose to sampling probe (7.5 mm) grid scanning area, in 100 KHZ ~ 3 GHZ spectrum scanning frequency range, and stores the maximum level of each frequency point. Note that the larger frequency point scanning available space in the scanning area for further checking, so that we can locate interference sources and key circuit path.
Under test plate must be as close to the scanner, because with the increase of distance, receiving signal-to-noise ratio decreases, and there will be "separation" effect. In practical measurement, this distance should be less than 1.5 cm. We can see that the components surface measurement sometimes may be because the height of the components and make the measurement problem, so the height of the components must be considered, for measuring the voltage level of correction. In the basic check, separation distance correction factor should be considered.
We can quickly get the measured results, but these results can't judge whether products conform to the EMC characteristics, because of its measuring value is produced by the high frequency current on the PCB electromagnetic near field. While the standard EMC test is required in open fields (OATS) or in a dark room, a distance of 3 m (i.e., the far field).
Although Emscan measurement cannot replace standard EMC test, but the practice has proved that it has many USES. Through the analysis of measurement results, many conclusions can facilitate the subsequent development of product. In addition to get the voltage level, the following information is also very important: interference distribution points, interferences, covering large area of interference conduction path, the interference is limited in a narrow area on the PCB and the internal structure or coupling between adjacent I/O module, etc., can also see digital circuit and analog circuit separate effects.
These measurements can be used as a PCB design quality evaluation standard, furthermore, if we already know of a similar PCB EMC characteristics, we can completely in the early product development more reliable assessment on EMC characteristics, such as whether shielding methods should be used.
In particular, high-speed scanning system can also reveal the transient electromagnetic field EMI problems, transient EMI problems in electromagnetic compatibility measurement tend not to be detected, but they will influence the performance and reliability of the products.
The anti-jamming performance of PCB evaluation
In actual use, all electronic equipment is the interference of electromagnetic field, if a device can not meet the requirement of the anti-interference, also not blocking, then the device's performance will be affected by electromagnetic interference. Facts show that there may be several hundred MHz, the frequency of the disturbance signal mainly through the interference coupling connection of conductors, therefore I/O modules of anti-interference design is very important. In order to enhance anti-jamming performance of products, sometimes have to increase the filtering methods, which means that would increase the cost of the product. From this point of view, to find a solution that can optimize all the circuits and components is very important.
Through proper modification measure methods mentioned above, in product development and testing phase will be able to correctly evaluate anti-jamming performance of products. The improved method is as follows: put the PCB on the scanner plate on the interference of the frequency spectrum scan to determine the PCB, and then put the frequency sine wave interference signal with tongs or the appropriate coupling device (such as balance of online with T - LISN) coupled to the I/O line or on a conductor, the interval of 10 MHZ, frequency range can satisfy the 10 MHZ to 150 MHZ (to avoid the interference of PCB frequency overlap), power - 20 DBM 0 (depend on the type of coupling and PCB) generator, execution and the jamming signal frequency scanning space.Coupled interference signals from the distribution of points in the PCB can very clearly see on dimensional scanning pattern, then we can according to the following principles to explain spatial scan results, including the regional distribution of PCB, which has coupling up the effectiveness of the interference signal, insert filter (disturbance signal attenuation), near the I/O coupling conductor and the effectiveness of the PCB ground plane or area, etc.
economic benefit
Electromagnetic field scanning technology in high-speed PCB design and debugging, the application of can help early detection problem, promptly take effective measures to eliminate or internal and external electromagnetic interference suppression system, ensure product EMC test a through, so as to accelerate the process of product design, improve the quality of the product design, save cost of product development, reduce the product after-sales service work.
On average, a new product design requires 2 to 4 times go to the EMC test, every time go to the EMC testing needs to be 3 ~ 4 weeks. If the product didn't pass the test, the whole process will be very long, but also had to be repeated, until they pass the test. Electromagnetic interfere with the scanning technology can let the engineers in the "design" in the process of considering products electromagnetic compatibility, thus shortening the product development time and reduce to EMC test the number of times. Typically can advance the product time to market of 3 to 12 weeks or more.
The determination of other alternative components to ensure continuous production and cost control is also very important, in many cases, replacement components do not agree with electromagnetic radiation characteristics of preferred components, electromagnetic scanning technology provides a rapid and effective means of assessing alternative components availability, can guarantee the integrity of the product in does not affect the electromagnetic premise reduce the production cost of the products.
