Agilent near-field electromagnetic interference source detection and location solution

Overview

If a new product fails in electromagnetic interference (EMI) pre-compliance testing or standard compliance testing, fault diagnosis and improvement are urgent. The most common and feasible method is to use a near-field probe with a spectrum analyzer to find the interference source and verify the improvement effect.

Figure 1 Agilent X-Series signal analyzer and N9311X-100 near-field probe

Near Field Test Overview

In the certification body, the radiation leakage test is performed using various calibrated antennas, which are all far-field measurements. The standard far-field radiation leakage test can accurately and quantitatively tell us whether the device under test meets the corresponding EMI standards. However, the far-field test cannot tell engineers whether the serious radiation problem comes from the gap in the shell, from the connected cables, or from the communication interfaces such as USB and LAN. In this case, we can locate the real source of radiation through the near-field test method.

The problem with near-field EMI measurement is that the measurement results using near-field probes and the results of far-field measurements using antennas cannot be directly mathematically converted. However, there is a basic principle: the greater the radiation in the near field, the greater the radiation in the far field. Therefore, the measurement using a near-field probe is actually a relative measurement, not an accurate absolute measurement. When using a near-field probe for EMI pre-compliance testing, we often compare the test results of a new DUT with the results of a near-field probe test (near-field test) of a known qualified DUT to predict the results of the EMI radiation leakage test (far-field test), rather than directly comparing it with the limit line that meets the EMI compatibility standard. At the same time, the absolute value of the test is not very meaningful, because this test result is closely related to many variables, including the position and direction of the probe, the shape of the DUT, etc.

Types and main features of near-field probes

The electromagnetic field is composed of the electric field and the magnetic field. In the near field, the electric field and the magnetic field coexist, and their strengths do not form a fixed relationship. Whether the electric field is dominant or the magnetic field is dominant is mainly determined by the type of the emission source. In short, in the area of ​​high voltage and low current, the electric field is greater than the magnetic field. In the area of ​​high current and low voltage, the magnetic field is greater than the electric field. At the same time, in the main EMI test frequency band, the magnetic field changes faster than the electric field with distance.

Because magnetic fields are generated by electric current, the most common emission sources include chips, device pins, wiring on PCBs, power lines, and signal cables. The most common magnetic field probes are mostly ring-shaped. When the magnetic field propagation line is perpendicular to the probe ring surface, the measured value is the largest. Therefore, during the measurement process, engineers generally need to rotate the direction of the probe to measure the maximum magnetic field value while avoiding missing important emission sources.

The electric field is generated by voltage, and the main emission sources include some cables with unterminated devices, PCB wiring connected to high-resistance devices, etc. The simplest electric field probe is similar to a small antenna. Some people even peel off a small section of the shielding layer at the front end of the coaxial cable to expose the core wire to form a simple electric field probe for use. In the absence of shielding equipment, the problem with the electric field probe is that it is easy to pick up electromagnetic wave signals in the environment, such as the uplink and downlink signals of cellular communications, thereby affecting the measurement dynamic range of the entire test system.

When selecting a near-field probe, several important factors need to be considered, including resolution, sensitivity, and frequency response.

The sensitivity of the near-field probe is not an absolute indicator. The key is whether the probe and the spectrum analyzer or receiver used in conjunction can easily measure the radiation leakage signal and have enough margin to observe the changes after improvement. If the sensitivity of the spectrum analyzer is very high, we can choose a probe with relatively low sensitivity. Otherwise, we must choose a probe with high sensitivity, and even consider an external preamplifier to improve the sensitivity of the overall system.

Resolution refers to the ability of the probe to distinguish the location of the interference source. Generally speaking, resolution and sensitivity are contradictory. Taking the most commonly used annular magnetic field probe as an example, the larger the size of the annular probe, the higher the sensitivity is, and the larger the test area is, the lower the resolution will be. The more recommended method is to use a set of probes of multiple sizes, use a larger probe when testing a large area, find the suspected area, and then gradually reduce the probe size to finally locate the interference source.

Frequency response is an important factor that is often overlooked. The so-called frequency response is the difference in amplitude obtained when the probe measures signals of the same amplitude but different frequencies. As we mentioned earlier, using a probe for EMI analysis is a relative, qualitative test. However, if the probe's frequency response is poor or not flat enough, the full-band test results will be unintuitive, causing us to ignore some important radiation leakage signals.

The shape and diversity of the probe are also important factors. In addition to conventional electric field and magnetic field probes, we often need some special probes when performing EMI analysis. Engineers often encounter such a situation. After finding the location of an interference source and shielding it, they find that the radiation leakage of the whole machine has not been improved enough. Then the most common reason is that the interference signal is transmitted to other areas through the signal cable or power cable, and finally radiates into space. Conventional near-field probes are difficult to detect interference inside the cable, so it is often necessary to use the No. 4 probe in the N9311X-100 near-field probe group introduced below, which is specifically used for cable interference testing.

Agilent N9311X-100 Near Field Probe Set

In order to cooperate with the Agilent X-Series signal analyzer with powerful EMI analysis function and quickly locate the interference source, Agilent has recently launched a set of near-field probes covering 30 MHz to 3 GHz, option N9311X-100. This probe set includes 4 magnetic field probes, providing users with the best choice of comprehensive sensitivity and resolution. It can not only be used to detect and locate electromagnetic interference sources, but also meet the needs of professional cable interference testing, and can help users analyze most EMI problems conveniently and quickly.

Table 1 Technical Specifications of N9311X-100 Near Field Probe Set

If users need higher test sensitivity, Agilent also provides an external preamplifier that can be used with the N9311X-100 near-field probe, option N9311X-110. The amplifier has a noise figure of only 4.5 dB in the 100 kHz to 3 GHz frequency band and provides 30 dB gain.

Figure 2 N9311X-100 preamplifier

in conclusion

Agilent's N9311X-100 near-field probe set and its matching N9311X-110 preamplifier are lightweight, quick to install and easy to use. At the same time, the probe has a wide frequency range, combines good resolution and sensitivity, and can quickly analyze and find various EMI interference sources. Combined with the good performance and professional EMI measurement and analysis functions of Agilent's X-Series signal analyzer, it provides users with the most cost-effective professional EMI analysis tool.

Recommended Configuration

N9311X-100: 30 MHz to 3 GHz Near Field Probe Set

N9311X-110: External preamplifier (optional)

Agilent X-Series Signal Analyzers and N6141A/W6141A EMI Receiver Software

Typical configuration of signal analyzer

N9000A-503 or 507: 3 GHz or 7.5 GHz CXA Signal Analyzer

N9000A-P03 or P07: 3 GHz or 7.5 GHz CXA Preamplifier

W6141A-2FP: EMI Receiver Application Software