PCB Blog - Acquisition and Application of Electromagnetic Information of PCB Board

The traditional tools for debugging PCB board include: time-domain oscilloscopes, TDR (time-domain reflectometry) oscilloscopes, logic analyzers, and frequency-domain spectrum analyzers, etc., but none of these methods can provide a reflection of the overall information of the PCB board. data. PCB board, also known as printed circuit board, printed circuit board, referred to as printed board, English abbreviation PCB (printed circuit board) or PWB (printed wiring board), with insulating board as the base material, cut into a certain size, at least with A conductive pattern with holes (such as component holes, fastening holes, metallized holes, etc.) is used to replace the chassis of the previous electronic components and realize the interconnection between electronic components. Because such boards are made using electronic printing techniques, they are called "printed" circuit boards. It is inaccurate to call a "printed circuit board" a "printed circuit" because there are no "printed components" on the printed circuit board, but only wiring. Emscan electromagnetic compatibility scanning system adopts the advanced array antenna technology and electronic switching technology, which can measure the current of PCB at high speed. The key to Emscan is the use of an array antenna to measure the near-field radiation of a working PCB placed on the scanner. The antenna array consists of 40 x 32 (1280) small H-field probes embedded in an 8-layer circuit board with a protective layer to accommodate the PCB under test. The results of the spectrum scan can give us a general idea of the spectrum produced by the EUT: how many frequency components are there and roughly what is the magnitude of each frequency component.

Full frequency scan
The design of the PCB board is based on the circuit schematic diagram to realize the functions required by the circuit designer. The design of the printed circuit board mainly refers to the layout design, which needs to consider various factors such as the layout of external connections, the optimal layout of internal electronic components, the optimal layout of metal connections and through holes, electromagnetic protection, and heat dissipation. The layout design can save production costs and achieve good circuit performance and heat dissipation performance. Simple layout design can be realized by hand, and complex layout design needs to be realized by computer-aided design. When performing the spectral/spatial scanning function, place the working PCB on the scanner, the PCB is divided into 7.6mm*7.6mm grids by the scanner grid (each grid contains an H-field probe), execute After scanning the full frequency range of each probe (the frequency range can be from 10kHz to 3GHz), Emscan finally gives two images, which are the synthetic spectrogram and the synthetic spatial graph. Spectral/spatial scans obtain all spectral data for each probe in the entire scan area.

Quickly locate sources of electromagnetic interference
Spectrum analyzer is an instrument for studying the spectral structure of electrical signals. It is used to measure signal parameters such as signal distortion, modulation, spectral purity, frequency stability and intermodulation distortion. It can be used to measure certain parts of circuit systems such as amplifiers and filters. Parameter is a multi-purpose electronic measuring instrument. It can also be called frequency domain oscilloscope, tracking oscilloscope, analysis oscilloscope, harmonic analyzer, frequency characteristic analyzer or Fourier analyzer. Modern spectrum analyzers can display analysis results in analog or digital form, and can analyze electrical signals in all radio frequency bands from very low frequencies below 1 Hz to sub-millimeter bands. Using a spectrum analyzer and a single near-field probe, "interference sources" can also be located. The method of "extinguishing fire" is used here as an analogy. The far-field test (EMC standard test) can be compared to "detecting fire". If there are frequency points exceeding the limit value, it is considered to be "found fire". The traditional "spectrum analyzer + single probe" solution is generally used by EMI engineers to detect "where the flames come out of the chassis". "Flame" is covered inside the product. EMSCAN allows us to detect the source of the interference source - "tinder", and also to see the "fire", that is, the transmission path of the interference source.
The general method is as follows: quickly locate the source of electromagnetic interference.
(1) Check the spatial distribution of the fundamental wave, and find the physical location of the amplitude on the spatial distribution diagram of the fundamental wave. For broadband interference, specify a frequency in the middle of the broadband interference (for example, a 60MHz-80MHz broadband interference, we can specify 70MHz), check the spatial distribution of the frequency point, and find the physical location of the amplitude.
(2) Specify the position and see the spectrogram of the position. Check that the amplitudes of the individual harmonic points at that location coincide with the total spectrogram. If they coincide, it means that the specified location is a strong place for these disturbances. For wideband interference, check whether the location is the location of the entire wideband interference.
(3) In many cases, not all harmonics are generated at one location, sometimes even harmonics and odd harmonics are generated at different locations, and it is also possible that each harmonic component is generated at different locations. In this case, you can find the location of strong radiation by looking at the spatial distribution of the frequency points you care about.
(4) Taking measures in places with strong radiation is undoubtedly effective to solve EMI/EMC problems.
This EMI troubleshooting method, which can really trace the "source" and propagation path, allows engineers to eliminate EMI problems at low cost and fast speed. In an actual measurement of a communication device, radiated interference radiated from the telephone line cable. After the above-mentioned tracking scan with EMSCAN, a few more filter capacitors were installed on the processor board, which solved the EMI problem that the engineer could not solve.

Quickly locate circuit fault location
As the complexity of the PCB increases, so does the difficulty and workload of debugging. Using an oscilloscope or logic analyzer, only one or a limited number of signal lines can be observed at the same time. However, there may be thousands of signal lines on the PCB. Engineers can only find the problem by experience or luck. the problem. If we have the "complete electromagnetic information" of the normal board and the faulty board, by comparing the data of the two, we can find the abnormal frequency spectrum, and then use the "interference source location technology" to find out the location of the abnormal frequency spectrum. Find the location and cause of the failure. Then find the location where this "abnormal spectrum" occurs on the spatial distribution map of the faulty board, as shown in Figure 6. In this way, the fault location is located on a grid (7.6mm * 7.6mm), and the problem can be solved. Diagnosed soon.

Applications for Assessing PCB Design Quality
A good PCB needs to be carefully designed by engineers, and the issues to be considered include:
(1) Reasonable stacking design: especially the arrangement of the ground plane and the power plane, as well as the design of the layer where the sensitive signal lines and the signal lines that generate a lot of radiation are located. There are also divisions of ground planes, power planes, and routing of signal lines across the division area.
(2) Keep the signal line impedance as continuous as possible: as few vias as possible; as few right-angle traces as possible; and as small a current return area as possible, which can generate fewer harmonics and lower radiation intensity.
(3) Good power supply filtering: Reasonable type, capacitance, quantity, and placement of filter capacitors, as well as a reasonable stacking arrangement of ground planes and power planes, can ensure that electromagnetic interference is controlled in the smallest area possible.
(4) Ensure the integrity of the ground plane as much as possible: as few vias as possible; reasonable safety spacing of vias; reasonable device layout; reasonable arrangement of vias to ensure the integrity of the ground plane. On the contrary, dense vias and excessive safety spacing of vias, or unreasonable device layout, will seriously affect the integrity of the ground plane and power plane, resulting in a large amount of inductive crosstalk, common mode radiation, and make the circuit more sensitive to external disturbances.
(5) Find a compromise between signal integrity and electromagnetic compatibility: On the premise of ensuring the normal function of the equipment, increase the rising edge and falling edge time of the signal as much as possible, and reduce the amplitude and number of harmonics of the electromagnetic radiation generated by the signal. For example, it is necessary to select a suitable damping resistor, suitable filtering means, etc. In the past, using the complete electromagnetic field information generated by the PCB, the quality of the PCB design can be scientifically evaluated. Using the complete electromagnetic information of the PCB, the design quality of the PCB can be evaluated from the following four aspects: 1. The number of frequency points: the number of harmonics. 2. Transient interference: unstable electromagnetic interference. 3. Radiation intensity: the magnitude of electromagnetic interference at each frequency point. 4. Distribution area: The size of the distribution area of the electromagnetic interference at each frequency point on the PCB.

Summary of this article
The complete electromagnetic information of PCB allows us to have a very intuitive understanding of the overall PCB, which not only helps engineers solve EMI/EMC problems, but also helps engineers debug PCB and continuously improve the design quality of PCB board.