The traditional tools for debugging PCB include: time domain oscilloscope, TDR (time domain reflectometry) oscilloscope, logic analyzer, and frequency domain spectrum analyzer and other equipment, but these methods can not give a reflection of the overall information of the PCB board. data. PCB board is also called printed circuit board, printed circuit board, printed circuit board for short, PCB (printed circuit board) or PWB (printed wiring board) for short, using insulating board as the base material, cut into a certain size, and at least attached A conductive pattern with holes (such as component holes, fastening holes, metallized holes, etc.) is used to replace the chassis of the electronic components of the previous device and realize the interconnection between the electronic components. Because this board is made using electronic printing, it is called a “printed” circuit board. It is not accurate to call “printed circuit board” as “printed circuit” because there are no “printed components” but only wiring on the printed circuit board.

How to obtain and apply PCB electromagnetic information

The Emscan electromagnetic compatibility scanning system uses a patented array antenna technology and electronic switching technology, which can measure the current of the PCB at a high speed. The key to Emscan is the use of a patented array antenna to measure the near-field radiation of the working PCB placed on the scanner. This antenna array consists of 40 x 32 (1280) small H-field probes, which are embedded in an 8-layer circuit board, and a protective layer is added to the circuit board to place the PCB under test. The results of spectrum scanning can give us a rough understanding of the spectrum generated by the EUT: how many frequency components there are, and the approximate magnitude of each frequency component.

Full band 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 optimized layout of internal electronic components, the optimized layout of metal connections and through holes, electromagnetic protection, and heat dissipation. Excellent layout design can save production cost and achieve good circuit performance and heat dissipation performance. Simple layout design can be realized by hand, while complex layout design needs to be realized with the aid of computer-aided design.

When performing the spectrum/spatial scanning function, place the working PCB on the scanner. The PCB is divided into 7.6mm×7.6mm grids by the grid of the scanner (each grid contains an H-field probe), and execute After scanning the full frequency band of each probe (the frequency range can be from 10kHz-3GHz), Emscan finally gives two pictures, namely the synthesized spectrogram (Figure 1) and the synthesized space map (Figure 2).

How to obtain and apply PCB electromagnetic information

Spectrum/spatial scanning obtains all the spectrum data of each probe in the entire scanning area. After performing a spectrum/spatial scan, you can get the electromagnetic radiation information of all frequencies at all spatial locations. You can imagine the spectrum/spatial scan data in Figure 1 and Figure 2 as a bunch of spatial scan data or a bunch of spectrum Scan the data. you can:

1. View the spatial distribution map of the specified frequency point (one or more frequencies) just like viewing the spatial scanning result, as shown in Figure 3.

2. View the spectrogram of the specified physical location point (one or more grids) just like viewing the spectrum scan result.

The various spatial distribution diagrams in Fig. 3 are the spatial abdomen diagrams of the frequency points viewed through designated frequency points. It is obtained by specifying the frequency point with × in the uppermost spectrogram in the figure. You can specify a frequency point to view the spatial distribution of each frequency point, or you can specify multiple frequency points, for example, specify all the harmonic points of 83M to view the total spectrogram.

In the spectrogram in Figure 4, the gray part is the total spectrogram, and the blue part is the spectrogram at the specified position. By specifying the physical location on the PCB with ×, comparing the spectrogram (blue) and the total spectrogram (gray) generated at that position, the location of the interference source is found. It can be seen from Figure 4 that this method can quickly find the location of the interference source for both broadband interference and narrowband interference.

Quickly locate the source of electromagnetic interference

How to obtain and apply PCB electromagnetic information

A spectrum analyzer is an instrument for studying the spectrum structure of electrical signals. It is used to measure signal distortion, modulation, spectral purity, frequency stability, and intermodulation distortion. It can be used to measure certain 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 ways, and can analyze electrical signals in all radio frequency bands from very low frequency to sub-millimeter wave bands below 1 Hz.

Using a spectrum analyzer and a single near-field probe can also locate “interference sources”. Here we use the method of “extinguishing fire” as a metaphor. The far-field test (EMC standard test) can be compared to “detecting fire”. If a frequency point exceeds the limit value, it is considered as “a fire has been found.” The traditional “spectrum analyzer + single probe” solution is generally used by EMI engineers to detect “from which part of the chassis the flame is coming out”. After the flame is detected, the general EMI suppression method is to use shielding and filtering. “Flame” is covered inside the product. Emscan allows us to detect the source of the interference source-“fire”, but also to see the “fire”, that is, the way the interference source spreads.

It can be clearly seen that using “complete electromagnetic information”, it is very convenient to locate electromagnetic interference sources, not only can solve the problem of narrowband electromagnetic interference, but also effective for broadband electromagnetic interference.

The general method is as follows:

How to obtain and apply PCB electromagnetic information

(1) Check the spatial distribution of the fundamental wave, and find the physical position with the largest amplitude on the spatial distribution map 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 with the largest amplitude.

(2) Specify the location and look at the spectrogram of the location. Check whether the amplitude of each harmonic point at this position coincides with the total spectrogram. If they overlap, it means that the designated location is the strongest place that produces these interferences. For broadband interference, check whether the location is the maximum location of the entire broadband interference.

(3) In many cases, not all harmonics are generated at one 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 location with the strongest radiation by looking at the spatial distribution of the frequency points you care about.

(4) Taking measures in the places with the strongest radiation is undoubtedly the most effective solution to EMI/EMC problems.

This kind of EMI investigation method that can truly trace the “source” and propagation path allows engineers to eliminate EMI problems at the lowest cost and fastest speed. In an actual measurement case of a communication device, radiated interference radiated from the telephone line cable. After using EMSCAN to carry out the above-mentioned tracking and scanning, a few more filter capacitors were finally installed on the processor board, which solved the EMI problem that the engineer could not solve.

Quickly locate the circuit fault location

How to obtain and apply PCB electromagnetic information

With the increase of PCB complexity, the difficulty and workload of debugging are also increasing. With 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, we can compare the data of the two to 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.

Figure 5 shows the frequency spectrum of the normal board and the faulty board. Through comparison, it is easy to find that there is an abnormal broadband interference on the faulty board.

Then find the location where this “abnormal frequency spectrum” is generated 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 very serious. The diagnosis will be made soon.

How to obtain and apply PCB electromagnetic information

Application cases for evaluating PCB design quality

A good PCB needs to be carefully designed by an engineer. The issues that need to be considered include:

(1) Reasonable cascading design

Especially the arrangement of the ground plane and the power plane, and the design of the layer where the sensitive signal lines and signal lines that generate a lot of radiation are located. There are also the division of the ground plane and the power plane, and the routing of signal lines across the divided 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 as possible current return area, it can produce less harmonics and lower radiation intensity.

(3) Good power filter

Reasonable filter capacitor type, capacitance value, quantity, and placement position, as well as a reasonable layered arrangement of ground plane and power plane, can ensure that electromagnetic interference is controlled in the smallest possible area.

(4) Try to ensure the integrity of the ground plane

How to obtain and apply PCB electromagnetic information

As few vias as possible; reasonable via safety spacing; reasonable device layout; reasonable via arrangement to ensure the integrity of the ground plane to the greatest extent. On the contrary, dense vias and too large via safety spacing, 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 will cause the circuit More sensitive to external interference.

(5) Find a compromise between signal integrity and electromagnetic compatibility

On the premise of ensuring the normal function of the equipment, increase the rising and falling edge time of the signal as much as possible to reduce the amplitude and the number of harmonics of electromagnetic radiation generated by the signal. For example, you need to select a suitable damping resistor, a suitable filtering method, and so on.

In the past, the use of the complete electromagnetic field information generated by the PCB can scientifically evaluate the quality of the PCB design. 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 ​​electromagnetic interference at each frequency point on the PCB.

In the following example, the A board is an improvement of the B board. The schematic diagrams of the two boards and the layout of the main components are exactly the same. The results of the spectrum/spatial scanning of the two boards are shown in Figure 7:

From the spectrogram in Figure 7, it can be seen that the quality of the A board is obviously better than that of the B board, because:

1. The number of frequency points of the A board is obviously less than that of the B board;

2. The amplitude of most frequency points of the A board is smaller than that of the B board;

3. The transient interference (frequency points that are not marked) of the A board is less than that of the B board.

How to obtain and apply PCB electromagnetic information

It can be seen from the space diagram that the total electromagnetic interference distribution area of ​​the A plate is much smaller than that of the B plate. Let’s take a look at the electromagnetic interference distribution at a certain frequency point. Judging from the electromagnetic interference distribution at the 462MHz frequency point shown in Figure 8, the amplitude of the A plate is small and the area is small. The B board has a large range and a particularly wide distribution area.

Summary of this article

The complete electromagnetic information of the 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 the PCB and continuously improve the design quality of the PCB. Similarly, there are many applications of EMSCAN, such as helping engineers solve electromagnetic susceptibility issues and so on.