DF100 KW PSM shortwave transmitter is an important model in my country’s shortwave broadcasting industry, and its operating carrier frequency is 3.2~26.1 MHz. Because it works in the high frequency range and there are various transformers, power supplies and circuit boards in the chassis, many high-frequency electromagnetic radiations will be generated during the operation of the transmitter. In order to effectively suppress these interferences, the transmitter adopts the application of through-hole capacitor to reduce the impact of electromagnetic radiation. The measures taken by the transmitter in anti-electromagnetic interference are mainly analyzed, and the application of these measures in other high-frequency equipment also has certain reference significance.

Electromagnetic interference has always been an important problem faced by the electrical industry. It is ubiquitous in the process of equipment operation, and it will affect the operation of the equipment in severe cases. How to effectively suppress electromagnetic interference and take corresponding solutions at work are the basic skills required for personnel involved in the electromagnetic interference industry. DF100kWPSM shortwave transmitter occupies a very important position in the development of my country’s shortwave broadcasting industry. This type of transmitter is widely distributed throughout the country, and its operating frequency is in the high frequency range. Therefore, it will generate a lot of strong electromagnetic interference. If these electromagnetic radiations cannot be effectively handled, it is likely that the transmitter’s table values will be abnormal, and various indicators will not meet the technical requirements, and normal tuning cannot be performed. Therefore, relevant staff have proposed many measures for the transmitter to resist high frequencies, and these measures can also be applied to other working fields where electromagnetic interference exists. They can effectively solve most high-frequency electromagnetic interference problems.

1 Application of through-hole capacitor electromagnetic interference

Electromagnetic interference is electronic noise that interferes with signals in cables and effectively reduces the integrity of signals. The propagation mode of electromagnetic interference is generally divided into two types, namely conduction coupling mode and radiation coupling mode.

According to the way electromagnetic interference is generated, the basic measures to resist electromagnetic interference can be divided into four types: ① Suppressing radiation interference through the screen layer: ② Suppressing conducted interference through the magnetic ring: ③ Suppressing conducted interference through the application of through-hole capacitor: ④ Suppressing high-frequency interference through low-pass filters.

The following is a brief analysis of these four anti-electromagnetic interference methods combined with the work of the DF100KWPSM transmitter.

1.1 Suppressing radiated interference through shielding layer

Metal box shielding is the most important and effective means to suppress electromagnetic interference. It mainly shields and suppresses radiated electromagnetic interference. Electromagnetic shielding can be divided into the following three types according to its shielding principle: ① Electric field shielding. It can be divided into electrostatic shielding and alternating electric field shielding. ②) Magnetic field shielding. It includes low-frequency magnetic field shielding and high-frequency field shielding. ③ Electromagnetic field shielding. It is the sum of the previous types. There are various transformers, rectifier modules and circuit boards in the first unit chassis of the DF100KWPSM transmitter. When they are working, the electromagnetic spectrum in the first unit chassis becomes very complicated, and the electromagnetic power density per unit volume will be very large. If these electromagnetic radiations cannot be effectively handled, the transmitter cannot work properly. In the first unit, the power meter, reflected power meter and automatic gain control device use metal shell shielding to shield and isolate external radiation. The second and third unit chassis of the transmitter are independent metal boxes, which largely prevents the possibility of mutual interference between the units.

Shielding suppression is a bidirectional suppression technology that can not only limit the leakage of electromagnetic energy from internal radiators, but also prevent external radiation interference from entering. To achieve this effect, after selecting specific shielding suppression measures, the metal shell must be grounded, so that bidirectional suppression of electromagnetic interference can be achieved.

1.2 Suppressing conducted interference through magnetic rings

For wireless signals, the higher the frequency, the easier it is to generate radiation. If the signal line does not have a metal shielding layer, then these signal lines are equivalent to antennas, transmitting and receiving various messy high-frequency signals in the surrounding electromagnetic environment; if these signals are superimposed on useful transmission signals, they may change the original useful signals transmitted, and may cause incorrect operation actions during automatic control.

Magnetic rings, also known as ferrite magnetic rings, act as low-pass filters. They can better solve the problem of high-frequency interference suppression of power lines, signal lines and connectors, and are commonly used anti-interference components in electronic circuits. It has a good suppression effect on high-frequency noise, and the most common use in life is to use it on the connection line of the display.

Ferrite is a kind of material made of high magnetic conductivity and other metals. Its working characteristics are mainly manifested in the low frequency band. The ferrite anti-interference core has a very low inductive impedance value, which does not affect the transmission of useful signals on the data line or signal line; in the high frequency band, starting from about 10MHz, the impedance increases, and its inductive reactance component is still small, but the resistive component increases rapidly. When high-frequency energy passes through the magnetic material, the resistive component will convert this energy into heat energy and dissipate it, causing the high-frequency electromagnetic interference signal to attenuate, while the impedance to the low-frequency useful signal can be ignored because it does not affect the transmission of normal signals in the circuit. Especially in the field of automatic control, the use of this simple and effective method will greatly reduce the work of designing wave circuits in hardware.

In the actual working process, the number of turns of the signal line on the magnetic ring should be adjusted according to the frequency characteristics of electromagnetic interference. The more turns wound on the magnetic ring, the better the interference suppression effect on lower frequencies, and the weaker the noise suppression effect on higher frequencies. Usually, when the frequency band of the interference signal is wide, two magnetic rings can be put on the cable, each with a different number of turns, so that high-frequency interference can be suppressed while also suppressing low-frequency interference,further improving the application of through-hole capacitor.

1.3 Suppressing Conducted Interference by Through-hole Capacitors

The through-hole capacitor includes a three-terminal capacitor, two pins and a ground terminal. The connection between the two pins is the same as that of an ordinary capacitor, but it has a ground terminal and the ground terminal area is relatively large. The ground terminal installation methods include welding and crimping contact. It is precisely because of the existence of the ground terminal that the ground inductance of the through-hole capacitor is very small, the lead inductance is also very small, the input and output are isolated by the metal plate of the ground terminal, and the high-frequency root is eliminated, so that the through-hole capacitor has a filtering effect close to that of an ideal capacitor. In the DFI00kWPSM transmitter, a through-hole capacitor is used to power the high-end electronic tube cabinet and curtain poles. It can effectively filter out electromagnetic interference in a high-frequency environment and provide a stable power supply guarantee for the power supply of each pole of the electronic tube. The application of through-hole capacitor plays a vital role in this process.

1.4 Suppressing High-frequency Interference by Low-pass Filters

The low-pass filter allows signals below the cutoff frequency to pass through, but signals above the cutoff frequency cannot pass through the electronic filtering device. The simplest filter is the RC filter.

The passive RC low-pass filter is composed of resistor R and capacitor, and it is a first-order system. The passive RC low-pass filter is shown in Figure 1. Since the resistor R is independent of the power, the RC low-pass filter is simpler in design and device selection than the LC low-pass filter. Therefore, it is not suitable for high-power output and can only be used in weak signal processing and micro-power applications. Similarly, the RC low-pass filter can be cascaded to form a multi-order system. If the filtering effect is to be achieved during use, the impedance of the front-stage output must be small enough and the impedance of the rear-stage input must be large enough. When forming an active RC filter, an amplifier needs to be added.

Figure 1 Passive RC low-pass filter

Figure 2 Relationship between the amplitude and frequency of the output signal of the low-pass filter circuit

The DF100kWPSM transmitter uses a passive LC filter. The inductive reactance increases with the increase of frequency, while the capacitive reactance decreases with the increase of frequency, as shown in Figure 2. Therefore, the LC low-pass filter is connected to the inductor in series and the capacitor in parallel, while the capacitor and inductor of the high-pass filter are connected in the opposite way. The transmitter uses an LC low-pass filter, which can form a low-impedance bypass for the main harmonics (3, 5, 7). Passive LC circuits are not easy to integrate and are usually used in filter circuits after power rectification. When used in the extremely high voltage power supply of the transmitter’s electronic tube screen, it can effectively filter out the step waveform on the power supply and make the waveform smoother. When the filter is mainly used to filter out voltage ripple or harmonics, a series connection is adopted; when the filter is mainly used to filter out current harmonics, a parallel connection is adopted. The passive LC filter in the transmitter is both in series and in parallel. In this way, voltage and current can be filtered at the same time.

In addition, the application of through-hole capacitor can also effectively suppress high-frequency electromagnetic interference to a certain extent, especially in power supply circuits and signal transmission. The low ground inductance and high-efficiency filtering performance of feedthrough capacitors make it an ideal choice, especially for electronic equipment that requires high stability.

2 Conclusion

During the operation of the DF100kWPSM transmitter, a large number of anti-electromagnetic interference measures are adopted, which enables the transmitter to work stably in a complex environment. This article mainly understands the working principle of the anti-electromagnetic interference components in the transmitter, which is of great help to the daily maintenance of the transmitter.