Noise suppression products. recommendations for selection and use.
Blandova Ekaterina Sergeevna, Doctor of Technical Sciences
NOISE SUPPRESSION PRODUCTS. RECOMMENDATIONS FOR SELECTION AND USE.
The most important condition for protecting information in technical means is the creation of a specialized base of technological components of interference suppression products, necessary for the adoption of circuit design measures to minimize parasitic generations and side emissions at the development stage of any electronic device.
Spurious emissions are caused by the fact that parasitic generations and interference may occur in generator, amplifier and other functional stages of electronic devices. If measures to suppress the said processes directly at the points of their occurrence are not taken during the development of equipment, conditions are created for stable generation, amplification and occurrence of spurious emissions, the level of which may exceed the permissible radio interference standards. The presence of parasitic signals in the device leads to an increase in through currents, an increase in power consumption and, ultimately, to failure of microelectronic products.
Emissions from electronic computing devices are modulated by a useful signal, exist in the form of useful harmonics in a wide frequency range, are distributed both conductively and in the form of radiated electromagnetic interference, and carry a signal with the same information content as the processed signals. Such emissions can be received and displayed on the monitor screen of the interception equipment. Computer devices can be both a source and a receptor — a device susceptible to external electromagnetic interference, and can serve as a re-emitter of this interference.
Spurious emissions and conductive interference create channels for leakage of information processed in technical means.
Technical measures to combat electromagnetic interference include measures to suppress parasitic generations – sources of spurious emissions, shielding equipment from external electromagnetic fields and filtering conductive interference.
Suppression of the interference sourceis carried out by optimal design of electrical circuits and layout of printed circuit boards taking into account the requirements for minimizing parasitic generations created by internal elements of the device and circuitry. These measures include reducing the number of grounded circuits, decoupling power supply circuits, eliminating radiating conductors, reconstructing or eliminating particularly noisy (generating) circuits.
Shieldingis a constructive means of attenuating any radiation and is of great importance both in terms of requirements for susceptibility to interference and in preventing parasitic generation radiation. Shielding can be accomplished by using metal screens, spraying conductive material onto the inner surface of plastic housings, shielding wires, local shielding of noisy circuits and units, and practically consists of localizing the electromagnetic energy generated by the field source. However, technical solutions using continuous shielding by accessible acceptable methods are practically impossible to implement, just as it is impossible to eliminate conductive interference in conductors that are connected to external sources without creating additional conditions for attenuating interference at both ends of the power cable, interface signal circuits, at the input and output contacts of the electrical circuit and inside that part of the signal circuit that can serve as an antenna for receiving (or emitting) interference signals.
Filteringis the main and effective means of suppressing (attenuating) conductive interference in power supply circuits, in interface signal circuits and on printed circuit boards, in ground wires. Noise suppression filters reduce conductive interference from both external and internal interference sources (Fig. 1).
Fig. 1. Filtering interference with a low-pass filter.
1. Electromagnetic interference (EMI) from various electronic devices.
2. High-voltage short-term pulsations.
3. Noise suppression filter.
4. After filtering, the signal (supply voltage) goes to the protected device.
5. Protected device (receptor).
6. The high-frequency component of the interference that does not pass through the filter passes to the ground.
7. The high-voltage pulsation passes to the ground.
The use of noise suppression elements allows optimizing circuitry and design and technological solutions in order to minimize or completely eliminate parasitic generations and side emissions, reduce the susceptibility of equipment to external electromagnetic fields and pulsed signals, and eliminate possible information leakage channels. The reliability and noise immunity of equipment increases, its metal consumption decreases, and its weight, size, and cost indicators improve.
Basic information about noise suppression filters
According to the location of the filter passband relative to the noise suppression band in the frequency spectrum, there are four classes of noise suppression filters (Fig. 2):
— low-pass filters;
— high-pass filters;
— band-pass filters;
— notch filters.
To solve specific problems of ensuring the reliability of operation, compatibility, noise immunity of equipment and other traditional problems of electromagnetic compatibility (EMC), bandpass and notch filters are most often used.
In order to ensure noise immunity of information signals and protect information processed in technical means from leakage through channels of side electromagnetic radiation and interference, as a rule, broadband LC low-pass filters are used, as well as ferrite noise-suppressing products, complete cable products with protection elements and protection elements of information display means (transmissive electromagnetic filters — screens), etc.
It is possible to use active filters based on microcircuits (operational amplifiers). This may be advisable in cases where passive LC filters become very bulky when the cutoff frequency is lowered to audio frequencies, when even when choosing a relatively small capacity (for example, 0.01 μF), the choke becomes disproportionately large in size and weight. In an active filter, the operational amplifier transforms the impedance of the RC circuit connected to it so that the device behaves like an inductor.
Fig. 2. Amplitude-frequency characteristics of interference suppression filters.
Choosing a filter type
The choice of the required filter type depends on the electrical characteristics of the system in which it is to be installed, the requirements for the efficiency of interference suppression, including the cutoff frequency and the upper limit frequency of attenuation, i.e. the frequency characteristics of the filtered circuit, as well as the requirements determined by the operating conditions and the actual limitations of installing the filter in the equipment. All these factors are related to the electrical characteristics of the filter. The main criteria for selecting an interference suppression filter are shown in Fig. 3.
The configuration of the filter's electrical circuit is selected based on the following considerations.
C-type filteris a low-inductance filter that acts as a feedthrough capacitor, shunting interference to ground. It works well at high source and load impedances. Above the cutoff frequency, the slope of the insertion loss characteristic is 20 dB per decade. This filter should be avoided in circuits where overvoltages or transient processes are possible.
The G-type filter should be used where the source and load impedances are significantly different. The inductance should be facing the low-impedance circuit. Above the cutoff frequency, the slope of the insertion loss characteristic is 40 dB per decade.
The P-type filter has two feedthrough capacitors that shunt interference to ground, and an inductance between them. This filter presents a high AC resistance to both the source and the load. It is best suited for use in circuits with high, relatively equal source and load impedances. Above the cutoff frequency, the slope of the insertion loss characteristic is 60 dB per decade.
2P-type, 2T-type filtersand others are used in conditions similar to those of P- and T-type filters, but where higher demands are placed on filter characteristics or effective interference suppression is required in the lower part of the operating frequency range up to 10 kHz. Multi-element compositions of 5 or more inductors and feedthrough capacitors are used. A high steepness of the insertion loss characteristic in such filters is required to prevent insertion loss at power supply network frequencies, as well as in linear filters designed for telephone lines and data transmission lines.
Structures of type C, P and 2P make it possible to achieve higher insertion loss in cases where the source and load resistance are more than 50 ohms. Structures T and 2T make it possible to achieve higher insertion loss in cases where the source and load resistance are less than 50 ohms.
If necessary, elements for suppressing non-stationary processes can be included in the electrical circuit of network filters.
If the filter is to be used primarily in an AC network, there are requirements for the maximum permissible leakage current. If the filter is to be used primarily in a DC circuit, it is selected to match the DC voltage. If there is a possibility of overvoltage, current surges and other non-stationary processes on power supply cables, it is recommended to install an inductance (G or T link) at the filter input, which will to some extent attenuate possible voltage surges, providing a certain degree of protection for the capacitor, as an element that is more sensitive to non-stationary processes.
|
Source impedance |
Steepness of the insertion loss characteristic |
|||
|
High |
Low |
|||
| Source impedance | High (> 50 Ohm) |
 |
20 dB per decade | |
 |
40 dB per decade | |||
 |
60 dB per decade | |||
 |
80 dB per decade | |||
 |
100 dB per decade | |||
| Low (< 50 Ohm) |
 |
20 dB per decade | ||
 |
40 dB per decade | |||
 |
60 dB per decade | |||
 |
80 dB per decade | |||
 |
100 dB per decade | |||
Fig. 3. Criteria for selecting a noise suppression filter circuit
Noise suppression filters are produced by both foreign companies and domestic industrial enterprises. Foreign companies produce noise suppression products across the entire existing range: by load current (0.5-100 A), operating frequency range (0.01 MHz-10 GHz), attenuation (20-100 dB), ambient temperature (-25°C-+85°C), etc. (see Table 1). Moreover, filters manufactured by foreign companies (Siemens, TDK, Corcom, Sprague, Timonta, Murata and many others) are distinguished by the design diversity of housings (cylindrical and rectangular) and terminals (grounding in the form of a journal, with a separate ground or planar terminal, as well as with a terminal in the form of a connector).
Standard interference suppression filters manufactured by foreign companies for a 50 Hz 250 v network
Table 1
|
No. Item |
Name filter | Current, A, not more | Frequency range, MHz | Insertion loss, dB |
Overall dimensions, mm | Weight, kg, not more than |
| 1. | Schaffner FR 102 line filter | 4 | 0.1…300 | 40…60 | 200х10х50 | 1.8 |
| 2. | Filter type 60-SPL-030-3-3 from Spectrum Control Inc | 3 | 0.1.-.50 | 20…60 | 41х35х32 | 0.2 |
| 3. | Power filter series 62-MMF-050-6-13 from the company Spectrum Control Inc | 5 | 0.1…50 | 60…90 | 63х50х32 | 0.3 |
| 4. | Nagano filters | 30 | 0.1…20 | 30…50 | 180х130х100 | 3.0 |
| 5. | Filters from Silden Telec | 25 | 0.15…300 | 40…80 | 273х191х76 | 3.0 |
| 6. | Schaffner FR 501 line filter | 6 | 0.1…500 | 40…80 | 190х65х60 | 2.0 |
Russian electronics industry enterprises produce:
— case-type line noise suppression filters;
— ceramic signal pass-through noise suppression filters;
— ferrite noise suppression products and elements;
— electrical connectors, shielded and with interference-suppressing filter contacts.
Among the network noise suppression filters (NSF) produced by the domestic industry, the filters whose parameters are given in Table 2 have become widespread. These filters are n-link passive LC filters made in sealed metal housings. The connection of the filter input-output to the power grid and the load is carried out using feed-through contacts consisting of a terminal pressed into an insulating bushing. The external metal parts of the filter are protected from corrosion by galvanic coating.
Almost all types of filters listed in Table 2 are filled with epoxy compound and are designed for harsh operating conditions with a guaranteed service life of at least 5 years from the date of manufacture. Unlike previously developed filters (types FP, FPHF, FPS, etc.), parasitic parameters of elements and chokes on composite magnetic circuits were used in these filters when synthesizing their frequency characteristics, which made it possible to significantly improve their specific volume and specific weight characteristics.
In addition, developed for a single-phase two-wire network, they have found wide application in other networks. Fig. 4 and 5 show the connection diagrams of filters of the FPBM type in a three-phase network with both grounded and isolated neutral. The operating conditions section of the technical specifications permits the use of these filters on both alternating (50 Hz, 220 V) and direct (12-120 V) current, and their satisfactory operation in parallel mode allows the current range to be expanded to 100 A.
Fig. 4. Connection diagram of FPBM type filters in a three-phase network with isolated neutral.
Figure 5. Connection diagram of FPBM type filters in a three-phase network with a grounded neutral.
The specified filters have undergone special studies in accordance with the regulatory documents of the State Technical Commission of Russia, meet the special requirements and can be used as auxiliary technical means in designated rooms of categories I, II and III, where classified information circulates in speech form, and technical devices processing classified information are installed.
Domestic-made network noise suppression filters
Table 2
| No. Item | Filter name | Current, A no more | Frequency range, MHz | Insertion loss, dB | Overall dimensions, mm | Weight, kg, no more than |
| 1. | FPBM-1/2/3 | 5/10/20 | 0.01… 10000 | 60…90 | 240х75х55 | 1.8 |
| 2. | FTMA | 0.5 | 0…4 0.01… 1000 | 2 25…70 | 45x40x25 | 0.1 |
| 3. | FSGA | 6 | 0.01…500 | 40…60 | 180х140х50 | 1.7 |
| 4. | FPPS | 3 | 0.1… 1000 | 40…60 | 62х52х42 | 0.35 |
| 5. | FSBS-2/4/7 | 1/2/5 | 0.01…500 | 15…50 | 104х90х60 | 0.6 |
| 6. | FSSHK-1/FSSHK-2 | 3/6 | 0.1…1000 | 40…70 | 62х52х42 | 0.25 |
| 7. | FPBD | 15 | 0.01… ; 1000 | 30…60 | 104х94х52 | 0.6 |
| 8. | FSMA | 30 | 0.01…1000 | 30…60 | 104х94х52 | 0,7 |
| 9. | FSBSh-9 | 10 | 0,01… 1000 | 15…50 | 104х78х30 | 0,26 |
Conclusions
1. Network noise suppression filters are one of the main methods of suppressing conductive noise in power supply circuits, in interface signal circuits, on printed circuit boards, in ground wires.
2. Current and type of load, attenuation value, operating conditions are the main parameters when selecting line filters.
3. Among domestic line noise suppression filters, passive LC filters of the FPBM, FSShK, FSMA type have recently become widespread. They meet the requirements of the State Technical Commission of Russia for protection against leakage of classified information due to side electromagnetic radiation and interference.
Literature
1. Catalog EEM, 1990 (p. 70)
2. Catalog of noise suppression filters
3. Blandova E.S., Meshcheryakov Yu.I., Serezhenko I.I. Noise-suppressing products of electronic equipment//Electronic Industry, No. 2, 1997, pp. 44 – 48
4. Blandova E.S., Serezhenko I.I. Noise-suppressing products manufactured by the electronic industry of Russia//Electronic Industry, No. 4, 2000
5. Catalogs of Siemens (p. 400), Schaffher (p. 48), TDK (p. 17), Spectrum Control Inc (p. 43), Corcom (p. 120), Curtis (p. 30).