Example 1: Radar Equipment

In the reality of dense and complex digital circuits, interference generated by circuits is inevitable, and digital circuits are also susceptible to external interference. So it is necessary to consider electromagnetic compatibility in product design, in order to take corresponding measures to reduce the energy of interference sources in the early stages of design. Below is an introduction to the example problems that are prone to occur in the design of a certain radar device (see Figure 1).

The curve of the radar equipment chassis during the initial testing of GJB-151A RE102 is shown in Figure 2.

It can be seen that most frequency bands of the device exceed the standard at 30MHz, 60MHz -90MHz. In order to quickly identify the cause of exceeding the standard, we can use a spectrum analyzer to locate the location of their leakage.

Due to the fixed structure and circuit of the equipment, it is not possible to change its structure and circuit. Improvements can only be made on the basis of the original structure and circuit.

As is well known, the structure of the chassis usually has an impact on radiation emissions. The components, integrated blocks, wiring of printed circuit boards, and areas with signal currents inside the chassis may radiate electromagnetic energy into the surrounding space, and the higher the frequency, the more likely it is to generate electromagnetic radiation. If an unshielded chassis is used, these electromagnetic energies will radiate to the outside of the chassis. If a metal chassis is used or a layer of metal is sprayed inside a non-metallic chassis as a shielding layer, electromagnetic energy may be limited within the chassis

Figure 1 GJB151A RE102 test curve of a radar equipment before reinforcement

Figure 2 Physical photo of a radar device

1. Take chassis reinforcement measures

To determine whether the radiation interference of the equipment is mainly caused by chassis leakage, the equipment cables and control connection lines can be removed, and only the power line (which has been filtered) can be retained to allow the equipment to work normally, and then the radiation interference field strength can be measured. The results show that some frequency points still exceed the standard, indicating a leakage in the chassis or obvious leakage in the power cord. To further determine whether it is a power cord or a chassis leak or both. At this point, a near-field magnetic field probe (connected to a spectrum analyzer) can be used to move along the hole gap to find the leakage point and observe the leakage situation at different frequencies. As a result, a large leakage field strength was found in the gap near the power supply of the chassis. A metal conductive tape should be temporarily attached to this area, which should have a good conductive overlap with the metal surface of the chassis. It was found that the radiation field strength significantly decreased, indicating a leakage in the chassis.

The reinforcement measure is to ensure that the gap size meets the requirements, and conductive pads can be added. Waveguide design, shortening the spacing between connecting screws, and so on can also be used. On the premise of solving the shielding of the chassis, connect the cables and control lines (the external cables and control lines have been shielded), and continue testing RE102. It was found that there is a significant improvement, as shown in Figure 3, but there are still superscripts.

Figure 3 RE102 test curve after taking reinforcement measures for chassis leakage

2. Take filtering reinforcement measures

Due to the electromagnetic interference inside the shielding body, it can be coupled to the wires (or cables) connected to the I/O interface and power lines, generating interference currents that are conducted outside the shielding body, causing conducted and radiated interference to the outside world; Similarly, external electromagnetic interference can also enter the shielding body through wires (or cables) connected to the I/O interface and power lines, or generate interference currents through electromagnetic induction, causing radiation interference inside the shielding body.

Low pass filters are an important measure to suppress and prevent interference, which can significantly reduce the level of conducted interference.

For power lines, the interference spectrum component is much higher than the power frequency, so low-pass filters have good suppression ability for the interference spectrum component, and provide the power frequency with no attenuation to pass through.

For I/O interface wires, their interference spectrum components are also higher than the operating frequency band, so low-pass filters have good suppression ability for interference spectrum components, and provide the operating frequency band with no attenuation to pass through.

Due to the working frequency band of I/O interface wires being much higher than the power frequency of power lines, the insertion loss characteristics of low-pass filters they use are different, and the soft magnetic materials they use are also different. The former uses nickel iron magnetic materials while the latter uses manganese zinc magnetic materials.

2.1 Reinforcement of cables and control lines

a) Replace the existing connectors with connectors of the same model with filters.

b) Reorganize the input and output lines of the filter to prevent coupling between the input and output lines, ensuring that the filtering effect of the filter remains unchanged.

C) The shielding layer of the shielded cable adopts multi-point grounding.

d) Connect the suspended pin of the connector to the ground potential to prevent antenna effects.

2.2 Reinforcement of Power Cords

For metal shielded chassis, an independent metal casing power filter should be selected and installed at the power line inlet, ensuring good electrical contact between the filter casing and the equipment chassis (ground). The grounding point of the filter is usually fixed on the common grounding metal component at the chassis or cable outlet. As shown in Figure 4.

Figure 4 Correct grounding method for power filter

After the above filtering and reinforcement processing, the results of the RE102 test are very ideal, as shown in Figure 5.

Figure 5 RE102 test curve after implementing filtering reinforcement measures

In summary, the previous methods are beneficial for improving electromagnetic compatibility, but the most widely used methods are to change the structure of the ground wire and the classification and reinforcement of wires and cables. These methods not only save costs, but are also the most effective methods for rectification and reinforcement. Although shielding may increase costs, its shielding effectiveness is sometimes unmatched by other methods. Therefore, in actual rectification and reinforcement, the main methods should be to change the structure of the ground wire, classify the reinforcement of wires and cables, and improve shielding, supplemented by other methods, to master and apply EMC reinforcement technology.

Example 2: Crystal oscillators for digital circuits

In digital circuit design, everyone cannot do without crystal oscillators. Crystal oscillators can provide a reference signal for digital circuits. The higher the oscillation frequency, the richer the harmonics generated, in other words, the greater the interference. In many digital circuits, the housing of the crystal oscillator is not grounded, and the PCB board does not provide sufficient grounding area.

More and more electronic engineers are paying attention to the fact that grounding the crystal oscillator casing can effectively reduce interference. Therefore, a ground wire with an area equivalent to that of the crystal oscillator is left in the PCB wiring to remove the solder mask layer, and the crystal oscillator casing is soldered onto it.

The level of crystal oscillation must meet a certain amplitude in order for digital circuits to operate in a certain sequence. However, the harmonics generated by crystal oscillators can cause disturbance, and in severe cases, can also interfere with sensitive circuits.

Figure 1 shows the amplitude of frequency doubling interference generated by a 25MHz crystal oscillator. It can be seen from the figure that high peaks are generated at frequencies of 2, 3, 4, 5, 6... These are typical spectrograms of 25MHz frequency doubling. This spectrogram shows that the crystal oscillator circuit or ground wire is poorly handled, so it is necessary to reduce the harmonic radiation of the crystal oscillator.

1. Take grounding reinforcement measures

The grounding treatment of the crystal oscillator shell should ensure that the grounding area around the crystal oscillator is as large as possible.

After grounding the crystal oscillator shell, if the frequency point still exceeds but the amplitude decreases, then the harmonic radiation of the crystal oscillator should also be reduced.

2. Take measures to suppress amplitude reinforcement

Figure 1 Typical spectrum diagram of crystal oscillator circuit before reinforcement

a) Adding an attenuator can be used while ensuring sensitivity and signal-to-noise ratio. Crystal oscillators in VCD and DVD players have a serious impact on electromagnetic compatibility. Reducing the frequency of crystal oscillators is one of the feasible methods, but not the only solution.

We can also take other measures to reduce the amplitude of crystal oscillator frequency doubling:

b) Observe whether the crystal oscillator waveform is close to a sine wave through a high-frequency oscilloscope, otherwise adjust the grounding capacitance of the crystal oscillator to improve the waveform;

C) On the pins connected to the crystal oscillator and decoding chip, add magnetic beads with low reactance and high impedance in series. When the reactance does not increase much, it is required that the impedance of the magnetic beads should be as high as possible after exceeding 50 MHz;

d) Check if the area of the power supply circuit of the decoding chip, the clock circuit of the decoding chip crystal oscillator, the high-speed signal circuit, and the wiring circuit next to the crystal oscillator are too large. If so, try to reduce their circuit area;

e) If the above measures have already been partially implemented and other measures are difficult to implement, magnetic beads and magnetic rings can only be strung on the output line, but it is only a temporary measure.

As shown in Figure 2:

Figure 2

Figure 3 Spectral diagram of crystal oscillator circuit after reinforcement