PCB Tech - Application of automatic scanning technology in high-speed PCB

The electromagnetic compatibility test is a very important test for the electronic products that are about to enter the market, but the previous tests can only get the results of whether they can pass, and can not provide more useful information. This article introduces the use of high-speed automatic scanning technology to measure electromagnetic radiation and detect changes in the electromagnetic field on the PCB, so that PCB engineering and technical personnel can find relevant problems and correct them in time before conducting electromagnetic compatibility standard tests.

At present, most hardware engineers only use experience to design the PCB. During the debugging process, many signal lines or chip pins that need to be observed are buried in the middle layer of the PCB and cannot be detected with tools such as oscilloscopes. If the product fails the functional test, They also have no effective means to find the cause of the problem. If you want to verify the EMC characteristics of the product, you can only take the product to a standard electromagnetic compatibility measurement room for measurement. Because this measurement can only measure the product's external radiation, even if it does not pass, it cannot provide useful information for solving the problem. Therefore, the engineer can only Modify the PCB based on experience and repeat the test. This test method is very expensive and may delay the time to market of the product.

Of course, there are many high-speed PCB analysis and simulation design tools that can help engineers solve some problems, but there are still many limitations on device models. For example, the IBIS model that can solve signal integrity (SI) simulation has many devices without models. Or the model is not accurate. To accurately simulate EMC problems, you must use the SPICE model, but currently almost all ASICs cannot provide SPICE models, and without the SPICE model, EMC simulation cannot take the radiation of the device into account (the radiation of the device is higher than that of the transmission line Radiation is much greater).

We know that the return path of high-frequency signals in a multi-layer PCB should be on the reference ground plane (power layer or ground layer) adjacent to the signal line layer, so that the return flow and impedance are the smallest, but there will be divisions in the actual ground layer or power layer And hollow out, thereby changing the return path, causing the return area to become larger, causing electromagnetic radiation and ground bounce noise. If engineers can understand the current path, 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 and quantity. Therefore, it is very difficult to theoretically determine the return path of a complex system. .

High-speed scanning measurement technology of electromagnetic field

Among various electromagnetic radiation measurement methods, there is a near-field scanning measurement method that can solve this problem. The method is designed based on the principle that electromagnetic radiation is formed by a high-frequency current loop on the device under test (DUT). Such as the Canadian EMSCAN company

The electromagnetic radiation scanning system Emscan 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 scanner. These probes can detect changes in electromagnetic fields caused by changes in high-frequency currents, and the system can provide 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 standards testing. Take corresponding measures.

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 the 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 obtains some trace components of the near E field.

The high-current low-voltage current source is mainly related to the magnetic field, while the high-voltage low-current voltage source is mainly related to the electric field. On the PCB, pure electric field or pure magnetic field 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 have an impedance of 50 ohms. This low impedance design makes these components produce large currents and low voltage changes. In addition, digital circuits The trend is to use logic devices with a lower voltage difference, and 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 energy in the active near-field area of the PCB is contained in the near magnetic field, so the magnetic field loop used in the Emscan scanning system is suitable for the near-field diagnosis of these PCBs.

All the loops are the same, but their positions in the feedback network are different, so the feedback network can sense the response of each loop, and the response of each loop to the reference source is measured and considered as a filter transfer function. In order to ensure the linearity of the measurement, Emscan measures the reciprocal of this transfer function.

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

Measurement methods for evaluating PCB near-field radiation interference

The inspection of PCB radiation interference can be carried out in several steps. First determine the area to be scanned, and then select a probe (grid 7.5mm) that can fully sample the scanned area, perform a spectrum scan in the frequency range of 00kHz～3GHz, and store the maximum level of each frequency point. Note that relatively large frequency points can be further checked in the scanning area using spatial scanning, so that interference sources and critical circuit paths can be located.

The board under test must be as close as possible to the scanner board, because as the distance increases, the received signal-to-noise ratio will decrease, and there will be a "separation" effect. In actual measurement, this distance should be less than 1.5cm. We can see that the measurement of the component surface may sometimes cause measurement problems due to the height of the component, so the height of the component must be considered to correct the measured voltage level. In the basic inspection, the separation distance correction factor needs to be considered.

We can get the measurement results quickly, but these results cannot judge whether the product meets the EMC characteristics, because the measured value 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 (that is, far field).

Although the measurement of Emscan cannot replace the standard EMC test, practice has proved that it does have many uses. 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: interference generation point, interference distribution, interference conduction path covering a large area, interference limited to a narrow area on the PCB, internal structure or coupling between adjacent I/O modules, etc., You can also see the effect of separating the digital circuit from the analog circuit.

The above measurement can be used as a standard for PCB design quality evaluation. Furthermore, if we already know the EMC characteristics of a similar PCB, we can make a relatively reliable evaluation of the EMC characteristics in the early stage of product development, such as whether shielding should be used. Means etc.