Electromagnetic interference measurement and diagnosis - Database & Sql Blog Articles

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When your product cannot be shipped due to the electromagnetic interference emission intensity exceeding the electromagnetic compatibility standard, or when the system cannot work normally due to electromagnetic interference between circuit modules, we must solve the electromagnetic interference problem. To solve the electromagnetic interference problem, we must first be able to "see" the electromagnetic interference and understand the magnitude and source of the electromagnetic interference. This article will introduce methods for measuring electromagnetic interference and determining the source of interference.

Measuring instrument

When it comes to measuring electrical signals, the first thing an electrical engineer might think of is an oscilloscope. An oscilloscope is an instrument that displays the law of voltage amplitude over time. It is equivalent to the eyes of an electrical engineer, allowing you to see the changes in current and voltage in the line, and thus master the working state of the circuit. But oscilloscopes are not an ideal tool for electromagnetic interference measurement and diagnosis. This is because:

A. The electromagnetic interference limits in all EMC standards are defined in the frequency domain, while the oscilloscope displays the time domain waveform. Therefore, the results obtained from the test cannot be directly compared with the standard. In order to compare the test results with the standard, the time domain waveform must be transformed into the frequency domain spectrum.

B. The electromagnetic interference is often small relative to the working signal of the circuit, and the frequency of electromagnetic interference is often higher than the signal, and when some low-frequency high-frequency signals are superimposed on a large amplitude low-frequency signal, use an oscilloscope. It is impossible to measure.

C. The sensitivity of the oscilloscope is at the mV level, and the amplitude of the electromagnetic interference received by the antenna is usually V level, so the oscilloscope cannot meet the sensitivity requirements.

A more suitable instrument for measuring electromagnetic interference is a spectrum analyzer. A spectrum analyzer is an instrument that displays the law of voltage amplitude as a function of frequency. The waveform displayed is called the spectrum. The spectrum analyzer overcomes the shortcomings of the oscilloscope in measuring electromagnetic interference, and it can accurately measure the interference intensity at each frequency.

For the analysis of electromagnetic interference problems, spectrum analyzers are more useful instruments than oscilloscopes. The spectrum analyzer can directly display the various spectral components of the signal.

1.1 Principles of the spectrum analyzer

A spectrum analyzer is a receiver that scans and receives over a range of frequencies.

The spectrum analyzer uses a frequency sweeping superheterodyne approach. The mixer mixes the signal received on the antenna with the signal generated by the local oscillator. When the frequency of the mixing is equal to the intermediate frequency, the signal can be amplified by the intermediate frequency amplifier for peak detection. The detected signal is amplified by the video amplifier and then displayed. Since the oscillation frequency of the local oscillator circuit changes with time, the frequency spectrum of the spectrum analyzer is different at different times. When the frequency of the local oscillator is scanned over time, the amplitude of the measured signal at different frequencies is displayed on the screen. The amplitude of the signal at different frequencies is recorded, and the spectrum of the measured signal is obtained.

According to this spectrum, it is possible to know whether the device under test has an interference emission exceeding the standard, or the frequency of the signal that causes the interference.

1.2 How to use the spectrum analyzer

To get the correct measurement results, the spectrum analyzer must be operated correctly. This section briefly describes how to use the spectrum analyzer. The key to proper use of the spectrum analyzer is to properly set the parameters of the spectrum analyzer. The meaning and setting method of the main parameters in the spectrum analyzer are explained below.

Frequency scan range:

The upper and lower limits of the spectrum of the spectrum analyzer are specified. By adjusting the scan frequency range, you can observe the frequency of interest in detail. The wider the scan frequency range, the longer the time required to scan one pass, and the lower the measurement accuracy of each point on the spectrum, so try to use a smaller frequency range if possible. When setting this parameter, it can be determined by setting the scan start frequency and the stop frequency, for example: start frequency = 1MHz, stop frequency = 11MHz. It can also be determined by setting the sweep center frequency and frequency range, for example: center frequency = 6MHz, span = 10MHz. The result of these two settings is the same.

IF resolution bandwidth:

The IF bandwidth of the spectrum analyzer is specified. This metric determines the selectivity and scan time of the instrument. Adjusting the resolution bandwidth can serve two purposes. One is to increase the selectivity of the instrument to distinguish between two signals that are close in frequency. Another purpose is to increase the sensitivity of the instrument. Because any circuit has thermal noise, these noises will drown the weak signal, making it impossible for the instrument to observe weak signals. The amplitude of the noise is proportional to the bandwidth of the instrument, and the wider the bandwidth, the greater the noise. Therefore, reducing the resolution bandwidth of the instrument can reduce the noise of the instrument itself, thereby enhancing the detection capability of weak signals.

The resolution bandwidth is typically expressed in 3 dB bandwidth. When the resolution bandwidth is changed, the amplitude of the signal displayed on the screen may change. If the bandwidth of the measurement signal is greater than the bandwidth of the passband, as the bandwidth increases, the display amplitude will increase due to the increase in the total energy of the signal passing through the intermediate frequency amplifier. If the bandwidth of the measurement signal is less than the bandwidth of the passband, such as for a single spectral line, the amplitude of the displayed signal does not change regardless of the resolution bandwidth. A signal whose signal bandwidth exceeds the IF bandwidth is called a wideband signal, and a signal whose signal bandwidth is smaller than the IF bandwidth is called a narrowband signal. The interference source can be effectively located depending on whether the signal is a wideband signal or a narrowband signal.

Scan time:

The time it takes for the instrument to receive a signal from the lowest end of the sweep frequency range to the highest end is called the scan time. The scan time matches the scan frequency range. If the scan time is too short, the measured signal amplitude is smaller than the actual signal amplitude.

Video bandwidth:

The video bandwidth has the same effect as the IF bandwidth, which can reduce the in-band noise of the instrument itself, thereby improving the instrument's ability to detect weak signals.

2. Analyze the source of the interference with a spectrum analyzer

2.1 Determine the interference source based on the frequency of the interference signal

When solving the electromagnetic interference problem, the most important problem is to judge the source of the interference. Only when the interference source is accurately located can the measures for solving the interference be proposed. Determining the source of interference based on the frequency of the signal is the easiest method because the frequency characteristics are most stable among all the characteristics of the signal, and the circuit designer tends to be very clear about the signal frequencies at various parts of the circuit. Therefore, as long as the frequency of the interference signal is known, it can be estimated which part of the interference is generated.

For electromagnetic interference signals, because the amplitude is often much smaller than the normal working signal, it is difficult to measure the frequency of the interference signal with an oscilloscope. Especially when a small interference signal is superimposed on a large working signal, the oscilloscope cannot synchronize with the interference signal, so it is impossible to obtain an accurate interference signal frequency.

It is very simple to do this measurement with a spectrum analyzer. Since the spectrum analyzer has a narrow intermediate frequency bandwidth, it is possible to filter out signals different from the interference signal frequency and accurately measure the interference signal frequency to determine the circuit that generates the interference signal.

2.2 Determine the interference source based on the bandwidth of the interference signal

Determining the bandwidth of the interference signal is also an effective way to determine the source of the interference. For example, there may be a single high-intensity signal in the transmission of a broadband source, and if it can be judged that the high-intensity signal is a narrow-band signal, it cannot be generated from a broadband source. The source of the interference may be an oscillator in the power supply, or a circuit that is unstable, or a resonant circuit. When there is only one line in the passband of the instrument, it can be concluded that this signal is a narrowband signal.

According to the Fourier transform, the signal corresponding to a single root line is a periodic signal. Therefore, when a single spectral line is encountered, attention is focused on the periodic signal circuit in the circuit.

3. Use a near field test method to determine the source of radiation

In addition to the above method of judging the interference source according to the signal characteristics, the source of the interference can be directly found by finding the radiation source in the near field. Tools for finding radiation sources in the near field are near field probes and current calipers. Check the source on the cable to use a current caliper. To check for leaks in the chassis gap, use a near field probe.

3.1 Current calipers and near field probes

A current probe is a sensor that is built using the principle of a transformer to detect current on a wire. When the current probe is stuck on the conductor under test, the conductor is equivalent to the primary of the transformer, and the coil in the probe is equivalent to the secondary of the transformer. The signal current on the wire induces a current on the coil of the current probe, producing a voltage at the input of the instrument. The spectrum of the interfering signal can then be seen on the spectrum analyzer's screen. The voltage value read on the instrument and the current value in the wire are converted by the transmission impedance. The transmission impedance is defined as: Instrument 50? The ratio of the voltage induced across the input impedance to the current in the conductor. For a specific probe, its transfer impedance ZT can be found in the probe instructions provided by the manufacturer. Therefore, the current in the wire is equal to:

I = V / ZT

If all physical quantities in the formula are expressed in dB, they are directly subtracted.

For chassis leaks, use a near field probe for probing. The near field probe can be seen as a small loop antenna. Because it is small, the sensitivity is very low and only the near-field radiation source can be detected. This facilitates precise positioning of the radiation source. Due to the low sensitivity of the near field probe, it is used with the preamplifier when in use.

3.2 Detecting common mode current with current calipers

One of the main causes of radiation from equipment is the common mode current on the cable. Therefore, when the equipment or system has excessive emission, the first thing that should be suspected is the various cables that are towed on the equipment. These cables include the power cable and the interconnecting cable between the devices.

The current probe is stuck on the cable. At this time, since the probe is stuck to the signal line and the return line at the same time, the differential mode current does not induce a voltage, and the voltage read on the instrument only represents the common mode current.

When measuring the common mode current, it is best to do it in a shielded room. If it is not in the shielded room, the electromagnetic field in the surrounding environment will induce current on the cable, causing misjudgment. Therefore, the power of the device should be disconnected first, and the background current on the cable should be measured in the state where the device is not powered, and recorded to compare with the measured results after the device is powered up, to eliminate the influence of the background.

If the spectrum of the spectrum analyzer is limited to a small range around the frequency of interest when measuring with an antenna, interference in the environment can be eliminated.

3.3 Using a near field probe to detect leakage from the chassis

If there is no strong common mode current on the external tow cable on the device, check the device chassis for electromagnetic leakage. The tool to check for leaks in the chassis is a near field probe. Place the near-field probe close to the seams and openings on the chassis to see if there is a signal of interest on the spectrum analyzer. Generally, because the sensitivity of the probe is low, that is, the amplifier is used, the weak signal induces a low voltage in the probe, so the sensitivity of the spectrum analyzer should be adjusted as high as possible during the measurement. According to the previous discussion, reducing the resolution bandwidth of the spectrum analyzer can increase the sensitivity of the instrument. However, it should be noted that when the resolution bandwidth is narrow, the scanning time becomes very long. In order to shorten the scanning time and improve the detection efficiency, the spectrum analyzer's scanning frequency range should be as small as possible. Therefore, when detecting a chassis leak with a near-field probe, the exact frequency of the leak signal is first measured by the antenna, and then the instrument is covered with the interference frequency as small as possible. Another benefit of doing this is that the background interference is not misjudged as a leak signal.

For the chassis, the gap near the filter installation location is the most prone to electromagnetic leakage. Because the filter bypasses the interference signal on the signal line to the chassis, a strong interference current is formed on the chassis. When these currents flow through the gap, electromagnetic leakage occurs at the gap.