The traditional debugging tools of PCB include: time domain oscilloscope, TDR(time domain reflectometry) oscilloscope, logic analyzer, and frequency domain spectrum analyzer and other equipment, but these means can not give a reflection of the overall information of PCB board data. This paper introduces the method of obtaining complete electromagnetic information of PCB with EMSCAN system, and describes how to use this information to help design and debugging.

EMSCAN provides spectrum and space scanning functions. The results of the spectrum scan can give us a general idea of the spectrum produced by EUT: how many frequency components there are, and what is the approximate amplitude of each frequency component. The result of spatial scanning is a topographic map with color representing amplitude for a frequency point. We can see the dynamic electromagnetic field distribution of a certain frequency point generated by PCB in real time.

The “interference source” can also be located by using a spectrum analyzer and a single near-field probe. Here use the method of “fire” to carry out a metaphor, can compare the far field test (EMC standard test) to “detect a fire”, if there is a frequency point beyond the limit, it is considered as “found a fire”. The traditional “Spectrum analyzer + single probe” scheme is generally used by EMI engineers to detect which part of the chassis a flame is escaping from. When a flame is detected, EMI suppression is generally carried out by shielding and filtering to cover the flame inside the product. EMSCAN allows us to detect the source of an interference, the “kindling,” as well as the “fire,” which is the propagation path of the interference. When EMSCAN is used to check the EMI problem of the whole system, the tracing process from flame to flame is generally adopted. For example, first scan the chassis or cable to check where the interference comes from, then trace the inside of the product, which PCB is causing the interference, and then trace the device or wiring.

The general method is as follows:

(1) Quickly locate electromagnetic interference sources. Look at the spatial distribution of the fundamental wave and find the physical location with the largest amplitude on the spatial distribution of the fundamental wave. For broadband interference, specify a frequency in the middle of the broadband interference (such as a 60MhZ-80mhz broadband interference, we can specify 70MHz), check the spatial distribution of this frequency point, find the physical location with the largest amplitude.

(2) Specify the position and see the spectrum map of the position. Check that the amplitude of each harmonic point at that location coincides with the total spectrum. If overlapped, it means that the specified location is the strongest place to produce these disturbances. For broadband interference, check whether this position is the maximum position of the entire broadband interference.

(3) In many cases, not all harmonics are generated at the same location, sometimes even harmonics and odd harmonics are generated at different locations, or each harmonic component may be generated at different locations. In this case, you can find the strongest radiation by looking at the spatial distribution of the frequency points you care about.

(4) It is undoubtedly the most effective to solve EMI/EMC problems by taking measures in the place with the strongest radiation.

This EMI detection method, which can truly trace the “source” and propagation route, enables engineers to troubleshoot EMI problems at the lowest cost and fastest. In the case of a communications device, where radiation radiated from a telephone cable, it became apparent that adding shielding or filtering to the cable was not feasible, leaving engineers helpless. After EMSCAN was used to carry out the above tracking and scanning, a few more yuan was spent on the processor board and several more filter capacitors were installed, which solved the EMI problem that engineers could not solve before. Quick locating circuit fault location Figure 5: Spectrum diagram of normal board and fault board.

As the complexity of PCB increases, the difficulty and workload of debugging also increases. With an oscilloscope or logic analyzer, only one or a limited number of signal lines can be observed at a time, whereas nowadays there may be thousands of signal lines on a PCB, and engineers have to rely on experience or luck to find the problem. If we have the “complete electromagnetic information” of the normal board and the faulty board, we can find the abnormal frequency spectrum by comparing the two data, and then use the “interference source locating technology” to find out the location of the abnormal frequency spectrum, and then we can quickly find the location and cause of the fault. Then, the location of the “abnormal spectrum” was found on the spatial distribution map of the fault plate, as shown in FIG.6. In this way, the fault location was located to a grid (7.6mm×7.6mm), and the problem could be quickly diagnosed. Figure 6: Find the location of “abnormal spectrum” on the spatial distribution map of the fault plate.

This article summary

PCB complete electromagnetic information, can let us have a very intuitive understanding of the whole PCB, not only help engineers to solve EMI/EMC problems, but also help engineers to debug PCB, and constantly improve the design quality of PCB. EMSCAN also has many applications, such as helping engineers solve electromagnetic sensitivity problems.