On the fight against impulse noise. Example of Noise Suppression in AC Power Supplies

pulse control switching. A typical block diagram of the inclusion of a PIP in the channel for receiving a HF transceiver is shown in Fig. 1.

An impulse noise signal received at the input of the noise suppressor is amplified at node A2 and then detected by the impulse detector U2. Adjusting the detector response threshold allows you to optimize the jammer performance. The pointed pulses from the output of the U2 node turn on the square-wave generator G1, which control the operation of the key stage S1 located in the signal path of the receiving device. In fig. 2 shows one of the first published PIP schemes.

Actually the suppressor of impulse noise is made on transistors VT2-VT4 and diodes VD1-VD3. The VT2 stage is an IF amplifier. A pulse detector is assembled on the VD1 diode. The cascade on the VT3 transistor together with the diodes VD2, VD3 forms rectangular pulses that control the electronic key on the VT4 transistor.

Passage in the signal path in this case is interrupted due to the fact that the output of the stage on the transistor VT1 (IF amplifier) \u200b\u200bduring the operation of the PIP turns out to be closed (at high frequency) to the common wire.

For all its simplicity, the assembly assembled according to the diagram in Fig. 2, works well. By changing the data of the oscillatory circuit, this PIP can be used in receivers with an intermediate frequency from 0.5 to 9 MHz.

The transistors indicated in the diagram can be replaced with any of the KP306 (VT1, VT2) and KPZ0Z (VT3, VT4) series. Instead of 1N9I4 diodes, you can use any of the KD522 series, instead of 1N34A from the D311 series.

The cascade in which the signal is interrupted is an important element of the PIP and largely determines the quality of its operation. The attenuation of the signal when passing through this stage should not exceed 3 dB and, at the same time, when the signal path is opened, it should reach 80 dB or more. In addition, the switching control pulses that enter this stage have an amplitude of several volts and should not penetrate into the signal path, since the level of the useful signal can be calculated in microvolts. To this it is necessary to add the following: since the PIP is installed before the filter of the main selection, it must withstand high level signals and not cause nonlinear effects.

This problem was successfully solved by G3PDM

l]. The key stage developed by him for the noise suppressor (Fig. 3) is made on a field-effect transistor VT1. The resistance between its source and drain, depending on the control voltage applied to the gate, varies from 100 Ohm to several megohms. Switching pulses here can penetrate into the signal path through the gate-source capacitance (its value is 5 ... 30 pF). To neutralize its action, a control pulse in antiphase is fed into the output circuit of the stage through a capacitor C3, by adjusting which it is possible to almost completely eliminate switching noise. When manufacturing a cascade, the 2N3823 transistor can be replaced by KPZ0ZA, 2N4289 by KT361A.

Dissatisfaction with the quality of the key cascade in traditional PIPs prompted further searches. The W5QJR suggested that in KB receivers with double frequency conversion, the control pulse should be fed not to the key stage, but to the second local oscillator. If sufficiently narrow band filters are installed in the path of the first and second IF, then a frequency shift of the second local oscillator by several kilohertz will lead to the fact that the signal and noise will no longer fall into the passband of the second filter, i.e., the signal path will be open. Since it is often taken away by only a few kilohertz, the normal operation of the local oscillator is preserved, there are no non-stationary transients, and with them switching noise.

The quality of this PIP is characterized by the following example. When installing a KB radio receiver in a car, reception without a PIP was impossible, since powerful impulse noise from the ignition system completely blocked the signals of amateur stations. When the PIP was turned on, the interference from the ignition system practically did not interfere with the reception. In the W5QJR noise suppressor, a separate pulse superheterodyne receiver for 38.8 MHz is connected to the antenna of the main receiver. An amplified pulse signal at a frequency of 10.7 MHz is detected and enters the delay unit of the pulse controlling the switching and adjusting its duration. A part of this PIP is shown in Fig. 4.

A pulse detector is made on the VD1 diode. Cascades on VTI-VT3 transistors are included in the control signal generation unit. Logic elements DD1.1-DD1.4 form rectangular pulses supplied to the varicap connected to the local oscillator circuit, the frequency of which is taken to the side.

Resistor R13 regulates the delay time of the control pulses, and resistor R14 regulates their duration. VTI-VT3 transistors can be any of the KT316 series, the VD1 diode - any of the KD522 series, VD2 - D814A; DD1 - K561LE5.

Due to the fact that the installation of the PIP developed by the W5QJR is possible only in KB receivers with fixed first and second IF, then, naturally, the search for the most acceptable version of the impulse noise suppressor continued. This was largely due to the appearance on the amateur HF bands of a strong periodic interference, reminiscent of the sound of a woodpecker. Since the strength of this interference is often up to S9 + 20 dB, it causes a lot of trouble for shortwave around the world.

Observations of the “woodpecker” and measurement of its parameters, given by VK1DN, showed that, in contrast to ordinary impulse noise (they have a pulse duration of 0.5 ... 1 μs), this noise is longer (15 ms), the repetition period is 10, sometimes 16 and much less often 20 and 32 Hz, its front and fall are not so steep, and the amplitude of the pulses arriving at a given moment can differ significantly from the previous ones.

This leads to the fact that not all impulse noise arriving at the input of the receiver trigger the PIP

and they freely penetrate into the receiving tract. Knowing the quantitative characteristics of the “woodpecker” pulse, it is easy to conclude: to improve the operation of the noise suppressor, it is necessary to increase the gain in the pulse noise receiving path, as well as to lengthen the switching control pulse to 15 ms.

In fig. 5 depicts a PIP, in the development of which the above considerations are taken into account. The useful signal from the mixer output is fed to the IF amplifier, assembled on field-effect transistors VT2 and VT3, and then through the key stage on pulse diodes VD1-VD4 is fed to a crystal filter.

From the mixer output through the source follower on the VT1 transistor, the IF signal is branched off into the impulse noise amplification path, in which the DA1 microcircuit is used, which is a part of the superheterodyne AM receiver (before the detector).

Its converter lowers the frequency of the incoming signal from 9 MHz to 2 MHz. The detected noise pulse through the source follower on the VT5 transistor comes to the start unit, assembled on the VT6 transistor.

The variable resistor R14 is adjusted during operation, depending on the air situation, the PIP response threshold. The DD1 microcircuit generates a control pulse, which is fed to the key stage through an inverting amplifier on a VT4 transistor. The PIP described by DJ2LR can be installed in a receiver with an IF from 3 to 40 MHz. In this case, you only need to use the appropriate circuits at the input of the DA1 microcircuit. Only the design of the key cascade is critical in manufacturing. It requires careful shielding and symmetrical arrangement of parts for better balancing and decoupling. When repeating the node as elements VT1, VT5, you can use transistors of the KPZOZ series, VT2, VT3 - the KP903 series, VT4 - the KT316 series, VT6 - the KT361 series. DA1 - К174ХА2, DD1 - К155АГЗ.

The measurements given in the data indicate the high parameters of the created node. Signal attenuation at the moment of signal path opening exceeds 80 dB. The value characterizing the upper limit of the dynamic range is +26 dBm. And most importantly, we managed to completely get rid of the impulse noise created by the “woodpecker”, which made it possible to receive even very weak signals from DX stations. The article concludes that the installation of this PIP in high-end receivers will not lead to a deterioration in their dynamic range.

Measurements of the parameters of impulse noises from the “woodpecker”, which were provided by VK1DN, showed that these oscillations are very stable - with an accuracy of 10 ~ 5. This makes it possible to start the control pulse shaping unit not with an incoming noise, but with a signal from a local generator. Naturally, it must be highly stable and be able to adjust the output signal taking into account the phase of the incoming signals.

In fig. 6 shows a part of the VK1DN PIP scheme. Trimmer resistors R3 and R6 adjust the control pulse, achieving the best suppression of interference.

Since the formation of the trigger pulse does not actually depend on the construction of the KB of the receiver, VK1DN considers it possible to include a cascade switch in the low-frequency path of the receiver. Despite the fact that it is not possible to completely get rid of the interference and, in addition, the AGC system “breathes”, there is still a positive effect. In the node, you can use the K555TL2 microcircuit, the KT316 series transistor, the KD522 series diodes.

In fig. 7 shows the key stage of the low-frequency PIP and its triggering unit. Since VK1DN uses a field-effect transistor as a key, it is natural that it faced the problem of “crawling” control pulses into the signal path, which was mentioned at the beginning of the article. He solved it in his own way. It turned out that this interference can be significantly reduced by decreasing the steepness of the leading edge and the decay of the control pulses.

For this, a large capacitor C1 of 33 μF was installed at the output of the buffer stage on the operational amplifier DA1, which separates the generator of these pulses from the rest of the device. Together with the elements C2 and VD1, it forms a triangular pulse with an amplitude of 9 V from a rectangular pulse.Transistor VT1 turns out to be closed at a voltage at its base of 7V (for the MPF102 transistor). In the node, you can use the K140UD7 microcircuit, the KPZ0Z series transistor, the KD522 series diode.

According to VK1DN, it is desirable to power the digital stages from a separate source in order to avoid the penetration of interference into the LF path. The control signal to the low-frequency PIP should be fed from the output of the element DD1.5, and to the high-frequency one from the transistor VT1 (see Fig. 6). This is required in order for the control pulse to have the correct polarity.

Since there is no information in the original source about how the key cascade in the VK1DN RF PIP was performed, you should pay attention to this when repeating or experimenting.

S. Kazakov

Literature:

2. Van Zant F. Solid state noise blanker. - QST, 1971, No. 7, p. 20,

3. Hawker P. Technical topics. - Radio communication, 1978, No. 12, p. 1025.

4. Nicholls D. Blankihg the woob-pecker. Harn Radio, 1982, no. 1, p. 20.

5. Ronde U. Increasing Receiver Dynamie Range. - QST, 1980, no. 5, p. sixteen.

6. Nicholls D. Blanking the woobpecker. - Ham Radio, 1982, no. 3, p. 22.

Switching power supplies, thyristor controllers, switches, high-power radio transmitters, electric motors, substations, any electric discharges near power lines (lightning, welding machines, etc.) generate narrow-band and broad-band interference of various nature and spectral composition. This complicates the operation of low-current sensitive equipment, introduces distortions in the measurement results, causes malfunctions and even failure of both instrument assemblies and entire complexes of equipment.

In symmetrical electrical circuits (ungrounded circuits and circuits with a grounded midpoint), antiphase interference appears in the form of symmetrical voltages (across the load) and is called symmetric, in foreign literature it is called "differential mode interference". Common mode interference in a balanced circuit is called asymmetric or common mode interference.

Symmetrical line interference usually dominates at frequencies up to several hundred kHz. At frequencies above 1 MHz, asymmetric interference prevails.

A rather simple case is narrow-band interference, the elimination of which is reduced to filtering the fundamental (carrier) frequency of the interference and its harmonics. A much more complicated case is high-frequency impulse noise, the spectrum of which covers a range of up to tens of MHz. Dealing with such interference is quite challenging.

Only a systematic approach will help to eliminate strong complex interference, which includes a list of measures to suppress unwanted components of the supply voltage and signal circuits: shielding, grounding, correct installation of supply and signal lines and, of course, filtering. A huge number of filtering devices of various designs, quality factors, applications, etc. produced and used all over the world.

Filter designs differ depending on the type of interference and the area of \u200b\u200bapplication. But, as a rule, the device is a combination of LC-circuits forming filter stages and P-type filters.

An important characteristic of a line filter is the maximum leakage current. In power applications, this current can reach values \u200b\u200bthat are dangerous to humans. Based on the leakage current values, the filters are classified according to safety levels: applications that allow human contact with the device housing and applications where contact with the housing is undesirable. It is important to remember that the filter housing requires mandatory grounding.

TE-Connectivity, building on Corcom's more than 50 years of experience in the design and development of electromagnetic and RF filters, offers the broadest range of devices for use in a variety of industries and instrumentation. A number of popular series from this manufacturer are presented on the Russian market.

B Series General Purpose Filters

Series B filters (Figure 1) are reliable and compact filters at an affordable price. A wide range of operating currents, good quality factor and a wide range of connection types provide a wide range of applications for these devices.

Figure: 1.

Series B includes two modifications - VB and EB, the technical characteristics of which are shown in table 1.

Table 1. Main technical characteristics of B series line filters

The electrical circuit of the filter is shown in Figure 2.

Figure: 2.

The attenuation of the interference signal in dB is shown in Figure 3.

Figure: 3.

T series filters

Filters of this series (Figure 4) are high-performance RF filters for power circuits of switching power supplies. The advantages of the series are excellent suppression of antiphase and common mode noise, compact size. Low leakage currents allow the T-series to be used in devices with low power consumption.

Figure: 4.

The series includes two modifications - ET and VT, the technical characteristics of which are shown in table 2.

Table 2. Main technical characteristics of T series line filters

The electrical diagram of the T series filter is shown in Figure 5.

Figure: five.

The attenuation of the noise signal in dB when the line is loaded on a 50 Ohm terminating resistor is shown in Figure 6.

Figure: 6.

K series filters

K-series filters (Figure 7) are general-purpose RF power filters. They are intended for use in power circuits with high impedance loads. Ideal for applications where pulsed, continuous and / or pulsed RF interference is induced on the line. Models with the EK index meet the requirements of the standards for use in portable devices, medical equipment.

Figure: 7.

Filters with index C are equipped with a choke between the frame and the grounding conductor. The main electrical parameters of K series line filters are shown in Table 3.

Table 3. Main electrical parameters of K series line filters

The electrical circuit of the K series filter is shown in Figure 8.

Figure: 8.

The attenuation of the noise signal in dB when the line is loaded onto a 50 Ohm terminating resistor is shown in Figure 9.

Figure: nine.

EMC Series Filters

Filters in this series (Figure 10) are compact and efficient two-stage RF power filters. They have a number of advantages: a high coefficient of common-mode noise reduction in the low-frequency range, a high coefficient of antiphase noise reduction, and compact dimensions. The EMC series is focused on applications with switching power supplies.

Figure: ten.

The main technical characteristics are shown in table 4.

Table 4. Main electrical parameters of EMC line filters

The EMC filter wiring diagram is shown in Figure 11.

Figure: eleven.

The attenuation of the noise signal in dB when the line is loaded onto a 50 Ohm terminating resistor is shown in Figure 12.

Figure: 12.

EDP \u200b\u200bSeries Filters

2. Corcom Product Guide, General purpose RFI filters for high impedance loads at low current B Series, TE Connectivity, 1654001, 06/2011, p. fifteen

3. Corcom Product Guide, PC board mountable general purpose RFI filters EBP, EDP & EOP series, TE Connectivity, 1654001, 06/2011, p. 21

4. Corcom Product Guide, Compact and cost-effective dual stage RFI power line filters EMC Series, TE Connectivity, 1654001, 06/2011, p. 24

5. Corcom Product Guide, Single phase power line filter for frequency converters FC Series, 1654001, 06/2011, p. thirty

6. Corcom Product Guide, General purpose RFI power line filters - ideal for high-impedance loads K Series, 1654001, 06/2011, p. 49

7. Corcom Product Guide, High performance RFI power line filters for switching power supplies T Series, 1654001, 06/2011, p. 80

8. Corcom Product Guide, Compact low-current 3-phase WYE RFI filters AYO Series, 1654001, 06/2011, p. 111.

Obtaining technical information, ordering samples, delivery - e-mail:

Network and signal EMI / RFI filters from TE Connectivity. From board to industrial installation

Company TE Connectivity is a world leader in the design and manufacture of surge protectors for effective suppression of electromagnetic and radio frequency interference in electronics and industry. The product range includes more than 70 series of devices for filtering both power supply circuits from external and internal sources, and signal circuits in the widest scope of applications.

Filters have the following design options: miniature for installation on a printed circuit board; housings of various sizes and types of connection of supply lines and load lines; in the form of ready-made power connectors and communication connectors for network and telephone equipment; industrial, made in the form of ready-made industrial cabinets.

Mains filters are manufactured for AC and DC applications, single and three-phase networks, cover the range of operating currents 1 ... 1200 A and voltages 120/250/480 VAC, 48 ... 130 VDC. All devices are characterized by a low voltage drop - no more than 1% of the operating voltage. The leakage current, depending on the power and filter design, is 0.2 ... 8.0 mA. The averaged frequency range over the series is 10 kHz… 30 MHz. Series AQ designed for a wider frequency range: 10 kHz… 1 GHz. Expanding the applications of its devices, TE Connectivity produces filters for low and high impedance load circuits. For example, high impedance filters of the series EP, H, Q, R and V for low impedance loads and low impedance series B, EC, ED, EF, G, K, N, Q, S, SK, T, W, X, Y and Z for high impedance loads.

Communication connectors with built-in signal filters are available in shielded, dual and low profile designs.

Each TE Connectivity filter is double tested: at the assembly stage and already as a finished product. All products comply with international quality and safety standards.

EMI suppression filter (10+)

High-frequency electromagnetic interference filter

The reason for the occurrence of high-frequency impulse noise is commonplace. The speed of light is not infinite, and the electromagnetic field travels at the speed of light. When we have a device that somehow converts the mains voltage by frequent switching, we expect that ripple currents directed towards each other will occur in the power wires going to the mains. Through one wire, current flows into the device, and through the other, it flows out. But that's not the case at all. Due to the finiteness of the field propagation velocity, the incoming current pulse is phase shifted relative to the outgoing one. Thus, at a certain frequency, high-frequency currents in the network wires flow in the same direction, in phase.

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If something is not clear, be sure to ask!

The German company Epcos (formerly Siemens' Passive Components Division) has a wide range of products to address electromagnetic compatibility (EMC) issues of electrical or electronic devices.

A significant subgroup of EMC components of Epcos is made up of filters designed to protect devices from high-frequency electromagnetic interference (radio interference).

Electromagnetic interference (EMI) occurs as a result of the operation of devices designed to generate or convert electricity. They represent electromagnetic fields in the space surrounding such technical equipment (TS).

The main sources of high-frequency interference are pulsed power supplies (household electronics, industrial and medical devices, etc.), nonlinear circuits

To combat interference in the circuits of neighboring vehicles, as well as nodes and blocks within individual vehicles, EMI filters are used. In general, EMI filters are usually low-pass filters and can be installed both directly at the source of interference and in front of the receiver of interference (receptor). EMI filters Epcos (mains filters) are designed to suppress noise coming through the wires of a two- or three-phase network to the input of the protected device, that is, these are filters of the "receiving side". This article is devoted to Epcos line filters, each of which is a separate complete node installed in front of the receiver. All filters under consideration pass the 50/60 Hz mains frequency unhindered.

Common-mode interference voltage occurs as a potential difference between the phase (signal) wire, the return wire (the so-called ground or neutral wire) and ground (device case, radiator, etc.). The common mode noise current has the same direction in the forward and return conductors of the network.

In symmetrical electrical circuits (ungrounded circuits and circuits with a grounded midpoint) antiphase interference appears in the form of symmetrical voltages (across the load) and is called symmetric, in foreign literature it is called differential mode interference. Common mode interference in a balanced circuit is called asymmetric or common mode interference.

Symmetrical line noise usually predominates at frequencies up to several hundred kilohertz. At frequencies above 1 MHz, asymmetric interference predominates.

Interference that occurs in unbalanced circuits is called unbalanced. For antiphase interference, an unbalanced circuit is a circuit with a divided (balanced with respect to earth) load.

For power circuits, an asymmetric load is more typical, but, for example, the sources of high-frequency interference themselves (converters on IGBT transistors, etc.) can generate asymmetric (common-mode) interference. On the other hand, common mode noise under certain conditions is converted to antiphase.

EMI filters are characterized by a set of parameters. Let us dwell on the parameters characterizing the Epcos EMI filters:

1. Number of wires in the network: 2, 3 (4).
2. Rated (mains) voltage: 250 (220), 440 (380) V, etc.
3. interference suppression range (barrier frequency band);
4. interference suppression level (standard; with enhanced suppression, etc.);
5. rated current, A;
6. the type of interference suppressed by the filter:
   • general type;
   • differential type;
   • asymmetrical interference;
7. connector type;
8. type of shell;
9. climatic category (temperature range in which the filter meets the requirements (standards) for other technical characteristics).

Filter designs differ depending on the type of interference. So, to compensate for symmetrical interference, when voltage distortions arise between the phase conductors of the network, a so-called du / dt low-frequency filter is used, containing interference suppression X-capacitors. Note that X-capacitors are those capacitors that shunt the line wires together at a high frequency.

Due to the fact that with a low internal resistance of the interference source, its elimination would require excessively large capacitances necessary to ensure a given voltage division, in practice, inductors are connected in series with the capacitor, which increases the resistance in a series circuit. The result is a so-called T-shaped (or U-shaped) low-pass filter.

At high frequencies, in order to limit its own capacity, the choke is often performed in the form of a set of individual inductances (sections or so-called "beads", the English name is beads) connected in series. At high frequencies, ferrite chokes can be used, for example, for frequencies of 30, 50 and 100 MHz, Epcos serially produces chokes / beads of the B8248x series in chip sizes 0603 ... 1806, designed for a current of 0.05 ... 4 A. chokes in output design. At higher frequencies, a low inductance can provide sufficient reactance. In this case, to obtain a choke, it is enough to pass the power cable through a group of ferrite rings.

In fig. 1 shows the equivalent circuit of an EMI du / dt filter. It performs the procedure for subtracting the differentiated signal from the original. As a result, the filter smooths out peaks and eliminates voltage surges caused by symmetrical interference. However, it has almost no effect on the disturbance voltage between the mains conductors and earth ground, as well as on the leakage current.

Figure: 1

Along with X-capacitors and conventional chokes, Epcos EMI filters use two types of inductors connected (with a common core).

Epcos current compensated EMI suppression chokes are usually made on a ring ferrite core. They use two coils (two wires) for a two-wire network, three for a three-wire network, etc. In this case, the opposite winding of wires can be geometrically implemented by co-winding them on two halves of a ferrite ring.

The Epcos Z-shaped choke is made by winding two wires on a ring core made of metal powder and having a high saturation threshold, which linearizes the I - V characteristics of the coils and reduces the risk of distortions associated with their nonlinearity.

Below are some specific examples of Epcos EMI filters with schematic diagrams and explanations of the features.

Example A1: Epcos B84110-B series EMI du / dt EMI filter with common mode rejection (no Y-capacitors).

This filter is used to protect switching power supplies, TVs, computers, industrial and portable equipment. The use of asymmetrical interference filters, in particular, significantly removes the restrictions on the length of the cable supplied to the motor from the inverter in industrial applications.

Example A2: Epcos EMI filter SIFI-D series (part number B84114-D) with common mode rejection and Y-capacitors6 (in addition to X-capacitors filter B84110-B). The resistor at the input (Fig. 3), installed in parallel with the X-capacitor, is designed to discharge it (large capacitor).

To compensate for several types of interference, a combination of chokes (serial, etc.) is installed.

Example A3: Epcos EMI filter of the SIFI-E series (part number B84115-E). It differs from the previous one additionally connected Z-shaped choke for additional attenuation of symmetrical interference (Fig. 4).

In fig. 5 shows the comparative characteristics of the insertion loss (in terms of symmetrical interference) for two series of filters. It shows that the first filter has a much lower level of frequency suppression in the band up to several hundred kilohertz.

Figure: five

In addition to coupled coils, Epcos EMI filters often include a multi-tier (loop-through) capacitor. The intrinsic inductance of such a capacitor is very small. In doing so, it can compensate for both antiphase and common-mode interference.

Epcos offers EMI filters designed to suppress interference in a wide range of high and ultra-high frequencies, from about 10 kHz up to 40 GHz and above. In this case, the average suppression bandwidth of all filters is about 1 MHz. Among the various models of Epcos EMI filters, one can single out, in particular, special ones, with a given leakage current.

The filter parameters leave an imprint on the possible areas of its application. The scope of application of a specific Epcos filter can be more accurately determined from the corporate catalog and on the website www.epcos.com on the Internet. A number of areas (but not all possible) are listed below where the use of Epcos EMI filters is advisable.

1. Modular systems for automated (soft) starting of electric motor drives ("Active terminal" / AFE) using powerful semiconductor switches (IGBT transistors) controlled by constant voltage. The switches are commutated with constant voltage from the output of voltage converters (AC / DC). For instance:

• cNC machines;
• elevators, etc.

2. Voltage converters of electric generators (wind power plants, etc.).

3. Transport, for example:

• converter drives of modern urban rail vehicles, in particular, trams;
• metro, electric trains, etc .;
• vehicles requiring a low leakage current (with a complicated grounding procedure), in particular trolleybuses, etc.
• high-speed trains (long distance).

4. Drives of steel rolling mills (interference with powerful commutation, as well as regulation of the rotation speed of the sheet feed drives).

5. Conveyor (tape) lines.

6. Filters for switching power supplies and UPS.

7. Pumps.

8. Heating, ventilation and air conditioning systems (HVAC systems).

9. Filters for suppression of interference signals in installations / cabinets with a high concentration of electronic equipment units (with a small volume of space).

10. When using power cables as conductors for communication communications (home Internet, as well as security systems with a limited number of wires in the input cable).

11. Filters for data transmission and telephone lines (ISDN, etc.).

Examples of EMI filter applications

Home Internet: data transmission within the home and between the home and the power substation (Fig. 6). Suppression of interference when using power cables as conductors of communication communications. In the absence of an EMI filter, the subscriber's radio-electronic equipment is noisy with interference from the mains voltage.

Figure: 6

Shown in Fig. 7 the circuit is used for voltage converters of electric generators. The converter itself is necessary due to the fact that the parameters of the signal, for example, the amplitude of the voltage generated at the output of the generator, usually do not correspond to the parameters of the network. EMI filters protect a generator (for example, a wind farm) from the penetration of high-frequency interference from the voltage converter.

Figure: 7

Modular systems of automated soft start of electric motor drives "Active terminal" / AFE (Fig. 8).

Figure: 8

IGBT transistors, activated by a simple DC voltage from the output of the inverter, allow quick connection or disconnection of high power motor drives. At the input of the converter there is a three-phase sinusoidal mains voltage, and at the output there is a constant voltage. However, fast switching of the power circuit is a source of high frequency interference. As a result of the penetration of interference to the input, the voltage between the phases of the network is distorted (a symmetrical type of interference occurs). The level of asymmetrical interference can also be significant due to the lengthy cable from the voltage converter to the external network. EMI filter 8 Epcos, installed at the input of the converter, compensates almost completely both disturbances, "decoupling" the converter and the external network.

Municipal rail transport (trams). The EMI filter is installed between the voltage converter of the electric motor and the supply (contact) line (Fig. 9).

Figure: nine

In conclusion, we can state the wide and varied capabilities of Epcos EMI filters for solving EMC problems of power equipment.

Shevkoplyas B.V. “Microprocessor structures. Engineering solutions. " Moscow, publishing house "Radio", 1990. Chapter 4

4.1. Primary Line Noise Suppression

The waveform of the alternating voltage of an industrial power supply network (~ "220 V, 50 Hz) for short periods of time can differ greatly from the sinusoidal one - surges or" insertions "are possible, a decrease in the amplitude of one or several half-waves, etc. The reasons for the occurrence of such distortions are related usually with a sharp change in the mains load, for example, when a powerful electric motor, furnace, welding machine is turned on. Therefore, it is necessary, if possible, to isolate from such sources of interference through the network (Fig. 4.1).

Figure: 4.1 Variants of connecting a digital device to a primary supply network

In addition to this measure, it may be necessary to introduce a power filter at the power input of the device in order to suppress short-term interference. The resonant frequency of the filter can be in the range of 0.1.5-300 MHz; broadband filters provide interference suppression over the entire specified range.

Figure 4.2 shows an example of a mains filter circuit. This filter has dimensions of 30 XZOX20 mm and is mounted directly on the mains input block into the device. Filters should use high frequency capacitors and inductors, either coreless or high frequency cores.

In some cases, it is obligatory to introduce an electrostatic shield (an ordinary water pipe connected to a grounded power switchboard housing) for laying the wires of the primary supply network inside it. As noted in, the short-wave transmitter of the taxi fleet, located on the opposite side of the street, is capable, with a certain relative orientation, to induce signals with an amplitude of several hundred volts on a piece of wire. The same wire, placed in an electrostatic screen, will be reliably protected from such interference.

Figure: 4.2. Example of a network filter circuit

Consider the methods of suppressing network noise directly in the device's power supply. If the primary and secondary windings of the power transformer are located on the same coil (Fig. 4.3, a), then due to the capacitive coupling between the windings, impulse noise can pass from the primary circuit to the secondary. According to the recommended four methods of suppression of such interference (in order of increasing efficiency).

1. The primary and secondary windings of the power transformer are performed on different coils (Figure 4.3, b). The throughput capacity C decreases, but the efficiency decreases, since not all of the magnetic flux from the primary winding region enters the secondary winding region due to scattering through the surrounding space.
2. The primary and secondary windings are made on the same coil, but separated by a copper foil screen with a thickness of at least 0.2 mm. The shield should not be a short-circuited loop. It connects to the frame ground of the device (Figure 4.3, c)
3. The primary winding is completely enclosed in a screen that is not a short-circuited turn. The screen is grounded (Fig.4.3, d).
4. The primary and secondary windings are enclosed in individual screens, between which a dividing screen is laid. The entire transformer is enclosed in a metal case (Fig.4.3,<Э). Экраны и корпус заземляются. Этот тип трансформатора в силу предельной защищенности от прохождения помех получил название «ультраизолятор».

For all of the above methods of suppression of interference, the wiring of the mains wires inside the device should be carried out with a shielded wire, connecting the shield to the frame ground. Invalid uk
tie into one bundle of network and other (power supply, signal, etc.) wires "even in the case of shielding both.

It is recommended to install a capacitor with a capacity of about 0.1 μF in parallel with the primary winding of the power transformer in the immediate vicinity of the winding terminals and, in series with it, a current-limiting resistor with a resistance of about 100 Ohm. This makes it possible to “close” the energy stored in the core of the power transformer at the moment the mains switch is opened.

Figure: 4.3. Options for protecting a power transformer from the transmission of impulse noise from the network to the secondary circuit (and vice versa):
a - no protection; b - separation of the primary and secondary windings; at-laying the screen between the windings; r - complete shielding of the primary winding; d - complete shielding of all elements of the transformer

Figure: 4.4. Simplified power supply diagram (and) and diagrams (b, c),explaining the operation of a full-wave rectifier.

The power supply is the greater the source of impulse noise over the network, the larger the capacitance of the capacitor C

Note that with an increase in the capacitance C of the filter (Fig. 4.4, a) of the power supply of our device, the probability of failures of neighboring devices increases, since the power consumption from the network by our device increasingly acquires the character of blows. Indeed, the voltage at the output of the rectifier also increases in those time intervals when energy is taken from the network (Fig. 4.4, b). These intervals in Fig. 4.4 are shaded.

With an increase in the capacitance of the capacitor C, the periods of its charge become less and less (Fig. 4.4, c), and the current taken in a pulse from the network becomes more and more large. Thus, an outwardly "harmless" device can create interference in the network that is "not inferior" to interference from a welding machine.

4.2. Grounding Rules Providing Protection Against Ground Interference

In devices made in the form of structurally completed blocks, there are at least two types of "ground" buses - housing and circuit. In accordance with safety requirements, the frame bus is necessarily connected to the grounding bus laid in the room. The circuit bus (relative to which the signal voltage levels are measured) should not be connected to the chassis inside the unit — a separate clamp must be brought out for it, isolated from the chassis.

Figure: 4.5. Improper and correct grounding of digital devices. Shown is a ground bus that is typically found indoors

In fig. 4.5 shows options for incorrect and correct grounding of a group of devices, which are interconnected by information lines. (these lines are not shown). Circuit buses "ground" are connected by individual wires at point A, and frame - at point B, as close as possible to point A. Point A may not be connected to the ground bus in the premises, but this creates inconvenience, for example, when working with an oscilloscope, which The probe ground is connected to the case.

In case of improper grounding (see Fig. 4.5), impulse voltages generated by the equalizing currents along the ground bus will actually be applied to the inputs of the receiving trunk elements, which can cause their false operation. It should be noted that the choice of the best grounding option depends on the specific “local” conditions and is often done after a series of careful experiments. However, the general rule (see Figure 4.5) always holds true.

4.3. Suppression of interference on secondary power supply circuits

Due to the finite inductance of the power and ground rails, the impulse currents cause impulse voltages of both positive and negative polarity, which are applied between the power and ground pins of the microcircuits. If the power and ground buses are made with thin printed or other conductors, and high-frequency decoupling capacitors are either completely absent or their number is not enough, then when several TTL microcircuits are simultaneously switched at the “far” end of the printed circuit board, the amplitude of impulse noise on the power supply (voltage surges acting between the power pin and the ground of the microcircuit) can be 2 V or more. Therefore, when designing a printed circuit board, the following guidelines should be followed.

1. The supply and ground rails must have minimum inductance. To do this, they are made in the form of lattice structures that cover the entire area of \u200b\u200bthe printed circuit board. It is unacceptable to connect TTL microcircuits to the bus, which is a "branch", because as it approaches its end, the inductance of the power supply circuits accumulates. The power and ground rails should, if possible, cover the entire free area of \u200b\u200bthe PCB. Particular attention should be paid to the design of accumulative heap matrices on K565RU5, RU7, etc. The matrix should be a square so that the address and control lines have a minimum length. Each microcircuit must be located in an individual cell of the lattice structure formed by the power and ground buses (two independent grids). The power and ground buses of the storage matrix should not be loaded with "foreign" currents flowing from address drivers, control signal amplifiers, etc.
2. Connection of external power and ground buses to the board through the connector should be made through several contacts, evenly spaced along the length of the connector, so that the input into the lattice structures of the power and ground buses is made from several points at once.
3. Power supply interference should be suppressed close to where it occurs. Therefore, a high-frequency capacitor with a capacity of at least 0.02 μF must be located near the power pins of each TTL microcircuit. This also applies especially to the aforementioned heap memory chips. To filter low-frequency interference, it is necessary to use electrolytic capacitors, for example, with a capacity of 100 μF. When using dynamic memory microcircuits, electrolytic capacitors are installed, for example, in the corners of the storage matrix or in another place, but near these microcircuits.

Accordingly, instead of high-frequency capacitors, special power buses BUS-BAR, CAP-BUS are used, which are laid under or between microcircuit lines, without disrupting the usual automated technology of installing elements on a board with subsequent "wave" soldering. These buses are distributed capacitors with a linear capacitance of about 0.02 μF / cm. With the same total capacitance as discrete capacitors, buses provide significantly better noise rejection at higher wiring densities.

Figure: 4.6. Variants of connection of P1-PZ boards to the power supply unit

In fig. 4.6 recommendations are given for connecting devices made on printed circuit boards P1 — PZ to the output of the power supply. A high-current device made on the PZ board creates more noise on the power and ground buses, so it should be physically brought closer to the power supply, or even better, it should be powered using individual buses.

4.4. Rules for working with agreed communication lines

In fig. 4.7 shows the waveform of signals transmitted through the cable, depending on the ratio of the resistance of the load resistor R and the characteristic impedance of the cable p. Signals are transmitted without distortion when R \u003d p. The characteristic impedance of a particular type of coaxial cable is known (for example, 50, 75, 100 ohms). The characteristic impedance of flat cables and twisted pairs is usually close to 110-130 ohms; its exact value can be obtained experimentally by selecting a resistor K, when connected, distortions are minimal (see Fig. 4.7). When conducting an experiment, do not use variable wire resistances, since they have a large inductance and can distort the waveform.

Communication line of the "open collector" type (Fig. 4.8). For transmission of each trunk signal with a rise time of about 10 ns at distances exceeding 30 cm, a separate twisted pair is used or one pair of conductors in a flat cable is separated. In the passive state, all transmitters are turned off. When any transmitter or group of transmitters is triggered, the line voltage drops from more than 3 V to about 0.4 V.

With a line length of 15 m and with its correct matching, the duration of transients in it does not exceed 75 ns. The line implements the Wiring OR function in relation to signals represented by low voltage levels.

Figure: 4.7. Signal transmission via cable. О - voltage pulse generator

Communication line of the "open emitter" type (Fig. 4.9 "). This example shows an example of a line using a flat cable. Signal wires alternate with earth wires. Ideally, each signal wire is bordered on both sides by its own earth wires, but this is usually not particularly necessary. In Figure 4.9, each signal wire is adjacent to its own and foreign ground, which is usually quite acceptable. A flat cable and a set of twisted pairs are essentially the same thing, and yet the latter is preferable in conditions of an increased level of external interference. An open emitter line provides a Wired OR function to signals represented by high voltage levels. The timing characteristics are approximately the same as those of an "open collector" line.

Communication line of the "differential pair" type (Fig. 4.10). The line is used for unidirectional signal transmission and is characterized by increased noise immunity, since the receiver reacts to the difference in signals, and the interference induced from the outside acts on both wires in approximately the same way. The length of the line is practically limited by the ohmic resistance of the wires and can reach several hundred meters.

Rice, 4.8. Open collector communication line

Figure: 4.9. Open emitter communication line

Figure: 4.10. Differential pair communication line

All of the lines discussed should use receivers with high input impedance, low input capacitance, and preferably with a hysteresis transfer characteristic to increase noise immunity.

The physical implementation of the highway (Fig. 4. II), Each device connected to the trunk contains two connectors. A circuit similar to that shown in Fig. 4.11, was considered earlier (see Fig. 3.3), so we will dwell only on the rules that must be observed when designing matching units (SB).

Transmission of trunk signals through connectors. The best options for wiring connectors are shown in Fig. .4.12. In these cases, the front of the pulse traveling along the line almost does not "feel" the connector, since the inhomogeneity introduced into the cable line is insignificant. This, however, requires taking 50% of the contacts used underground.

If this condition is impracticable for some reason, then it is possible, to the detriment of noise immunity, to accept the second, more economical (but the number of contacts) option for wiring the connectors, shown in Fig. 4.13. This option is often used in practice. Twisted pair earths (or flat cable earths) are collected on metal strips of the largest possible cross-section, for example 5 mm2.

The wiring of these lands is carried out evenly along the length of the bar, as the corresponding signal wires are wired. Both strips are connected via a connector using a series of jumpers of minimum length and maximum cross-section, and the jumpers are evenly spaced along the length of the strips. Each earthing jumper must correspond to no more than four signal lines, but the total number of jumpers must not be less than three (one in the center and two at the edges).

Figure: 4.13. Valid option for signal transmission through the connector. H- \u003d 5 mm2 — strip section, 5 ^ 0.5 mm2 — earth wire section

Figure: 4.14. Options for performing branches from the trunk

Execution of branches from the highway. In fig. 4.14 shows options for incorrect and correct execution of a branch from the trunk. The path of one line is traced, the earth wire is shown conditionally. The first option (a typical mistake of novice circuit engineers!) Is characterized by splitting the wave energy into two parts,

Figure: 4.15. Options for connecting receivers to the trunk
coming from line A. One part goes to the charge of line B, the other goes to the charge of line C. After the charge of line C, the "full" wave begins to propagate along line B, trying to catch up with the wave with half the energy that left earlier. The signal front thus has a stepped shape.

If the branch is performed correctly, the segments of the lines A, C and B turn out to be connected in series, therefore the wave is practically not split and the signal fronts are not distorted. The transmitters and receivers located on the board should be as close to its edge as possible to reduce the inhomogeneity introduced at the point where the line segments B and C.

One or two-way transceivers can be used to decouple the receiver beams from the backbone (see Fig. 3.18. 3.19). When branching a line into several directions, a separate transmitter should be allocated for each (Fig.4.15, at).

For line transmission, it is better to use trapezoidal rather than rectangular pulses. Signals with gentle edges, as noted, propagate along the line with less distortion. In principle, in the absence of external interference for any arbitrarily long and even inconsistent line, you can choose such a slow rise rate of the signal that the transmitted and received signals will differ by an arbitrarily small amount.

To obtain trapezoidal pulses, the transmitter is designed as a differential amplifier with an integrating feedback loop. At the input of the main receiver, also made in the form of a differential amplifier, an integrating circuit is installed to filter high-frequency interference.

When transmitting signals within the board, when the number of receivers is large, "serial termination" is often used. It consists in the fact that in series with the output of the transmitter, in the immediate vicinity of this output, a resistor with a resistance of 20-50 Ohm is connected. This allows you to damp oscillatory processes at the signal edges. This technique is often used when transmitting control signals (KA5, SAZ, \\ UE) from amplifiers to LSI dynamic memory.

4.5. About the protective properties of cables

In fig. 4.16, a shows the simplest scheme for transmitting signals over a coaxial cable, which in some cases can be considered quite satisfactory. Its main disadvantage is that in the presence of pulsed equalizing currents between the frame earths (potential equalization is the main function of the frame earth system), part of these currents 1 can flow through the cable braid and cause a voltage drop (mainly due to the inductance of the braid), which ultimately acts on the load K.

Moreover, in this sense, the diagram shown in Fig. 4.16, a, turns out to be preferable, and with an increase in the number of points of contact of the cable sheath with the frame ground, the possibility of the induced charges to drain from the braid improves. The use of a cable with an additional braid (Fig. 4.16, c) allows you to protect yourself from both capacitive pickups and equalizing currents, which in this case flow through the outer braid and practically do not affect the signal circuit.

Connecting a cable with an additional braid according to the diagram shown in Fig. 4.16, d, allows you to improve the frequency properties of the line by reducing its linear capacity. Ideally, the potential of any elementary section of the central core coincides with the potential of the elementary cylinder of the inner braid surrounding this section.

Lines of this type are used in local computer networks to increase the speed of information transfer. The outer sheath of the cable is part of the signal circuit, and therefore this circuit is equivalent to the circuit shown in fig. 4.16.6.

Figure: 4.16. Cable Uses

Neither the copper nor the aluminum braid of a simple coaxial cable protects it from low frequency magnetic fields. These fields induce an EMF both on the braid section and on the corresponding section of the central core.

Although these EMFs are of the same name in sign, they do not compensate each other in magnitude due to the different geometry of the corresponding conductors - the central core and the braid. The differential EMF is ultimately applied to load K. Additional braid (Fig. 4. 16, c, d) also unable to prevent the penetration of a low-frequency magnetic field into its inner region

Protection from low-frequency magnetic fields is provided by a cable containing a twisted pair of wires, enclosed in a braid (Fig.4.16, e). In this case, the EMF induced by an external magnetic field on the wires that make up the twisted pair completely compensate each other both in sign and in absolute value.

This is all the more true, the smaller the pitch of the twisting of the wires in comparison with the zone of action of the field and the more carefully (symmetrically) the twisting is performed. The disadvantage of such a line is its relatively low frequency "ceiling" —about 15 MHz — due to the large energy losses of the useful signal at higher frequencies.

The diagram shown in Fig. 4.16, e, provides the best protection against all types of interference (capacitive pickup, equalizing currents, low-frequency magnetic fields, high-frequency electromagnetic fields).

It is recommended to connect the inner braid to the “radio engineering” or “true” (literally grounded) ground, and the outer braid to the “system” (circuit or frame) ground. In the absence of a "true" ground, you can use the connection diagram shown in fig. 4. 16, g.

The outer braid connects to the system ground at both ends, while the inner braid only connects to the source side. In cases where there is no need for protection from low-frequency magnetic fields and it is possible to transmit information without using paraphase signals, one of the twisted pair wires can serve as a signal wire, and the other as a shield. In these cases, the circuits shown in Fig. 4.16, c, f, can be thought of as coaxial cables with three shields — a twisted pair ground wire, an inner and outer braided cable.

4.6. Using optocouplers to suppress interference

If the devices of the system are separated by a considerable distance, for example, by 500 m, then it is difficult to count on the fact that their lands always have the same potential. As noted, the equalizing currents along the earth conductors create impulse noise on these conductors due to their inductance. This interference is ultimately applied to the inputs of the receivers and can cause false alarms.

The use of differential pair lines (see § 4.4) allows only common mode noise to be suppressed and therefore does not always produce positive results. In fig. 4.17 shows the diagrams of optocouplers between two devices remote from each other.

Figure: 4.17. Optocoupler isolation schemes between devices remote from each other:
a - with an active receiver, b - with active transmitter

The scheme with an "active receiver" (Fig. 4.17, a) contains a transmitting optocoupler VI and a receiving optocoupler V2. When pulsed signals are applied to the input X, the LED of the VI optocoupler periodically emits light, as a result, the output transistor of this optocoupler periodically saturates and the resistance between points a and b drops from several hundred kilo-ohms to several tens of ohms.

When the output transistor of the transmitting optocoupler is turned on, the current from the positive pole of the source U2 passes through the LED of the optocoupler V2, line (points a and b) and returns to the negative pole of this source. Source U2 is isolated from source U3.

If the output transistor of the transmitting optocoupler is off, no current flows through the source circuit U2. The signal X "at the output of the optocoupler V2 is close to zero if its LED is on, and close to +4 V if this LED is off. Thus, when X \u003d\u003d 0, the LEDs of the transmitting and receiving optocouplers are on and, therefore, X" \u003d\u003d 0. When X \u003d\u003d 1 both LEDs are off and X "\u003d\u003d 1.

Optocoupler isolation can significantly increase the noise immunity of the communication channel and ensure the transmission of information over distances of the order of hundreds of meters. Diodes connected to the transmitting and receiving optocouplers serve to protect them from reverse voltage surges. The resistor circuit connected to the source U2 serves to set the current in the line and limit the current through the LED of the receiving optocoupler.

The current in the line according to the IRPS interface can be selected equal to 20 or 40 mA. When choosing the resistor values, the ohmic resistance of the communication line must be taken into account. Scheme with an "active transmitter" (Fig. 4.17, b) differs from the previous one in that the power supply of the U2 line is located on the side of the transmitter. This has no advantage - both circuits are essentially the same and are so-called "current loops".

The recommendations given in this chapter may seem too harsh for a beginner circuit designer. The fight against interference appears to him as a "battle with the windmill", and the lack of experience in the design of devices of increased complexity creates the illusion that it is possible to create a workable device without following any of the recommendations given.

Indeed, sometimes this is possible. There are even known cases of serial production of such devices. However, in informal reviews of their work, you can hear many interesting non-technical expressions, such as visit effect and some others, simpler and more understandable.