Radio interference suppression
Radio interference suppression is the limitation of radio interference to a permissible level that is specified in standards or, in specific individual cases, until the interference has been eliminated.
Radio interference suppression is a sub-area of EMC . Radio interference suppression is regulated by law together with EMC for the civil sector in Germany in the law on the electromagnetic compatibility of equipment . Compliance is monitored by the radio measurement service department of the Federal Network Agency , which maintains the corresponding measurement points and measurement vehicles.
Origin and classification of radio interference
- Broadband interference
- With rapid changes in current, electromagnetic harmonics arise that can reach very high frequencies . Disturbances such as these occur particularly during rapid on / off switching processes that span a wide frequency band and extend well into the megahertz range. They interfere with radio traffic and are audible or visible in everyday radio and television broadcasts as interference. Typical sources of such disturbances are e.g. B. electric motors with collectors or the ignition systems of internal combustion engines . It is characteristic of broadband interference that a broad frequency band is swept almost completely without any gaps and no individual constant interference frequencies occur.
- Narrow band disorders
- These include high-frequency oscillators, e.g. B. for clocking a microprocessor in a PC and their harmonics. The frequency constancy and the large distance between the harmonics in relation to the measurement bandwidth are characteristic. Harmonics can also occur with narrowband interference.
- Conducted radio interference
- They mainly spread along lines, especially the power supply network, and penetrate other devices through them. Example: PC switched-mode power supply without interference suppression , which couples into the power supply of a medium-wave radio via the power supply network and thereby interferes with reception.
- Field-bound radio interference
- Spread mainly as radiation. Example: PC with poorly shielded connection cables. Here, the connecting cables act as transmitting antennas and couple the internal clock frequencies of the PC as radio interference directly into the receiving antenna of a nearby VHF radio.
- Permanent disturbances
- This includes all radio interference that is continuously transmitted. The usual limit values apply to these faults.
- Discontinuous disturbances (cracks)
- Individual short switching clicks (<200 ms), e.g. B. by a hand-operated light switch. For short-term malfunctions, there are relief in the limit values.
In practice there are of course mixed forms between these types of radio interference.
Evaluation of radio interference
In the civil sector it has been customary for decades to evaluate radio interference not only according to its absolute level, but also according to the frequency of its occurrence. For this purpose, the perception of annoyance of the human hearing was simulated in studies and incorporated into an evaluation curve , the so-called CISPR quasipeak evaluation. This means that individual short clicks are not taken into account, but with increasing frequency, ever stricter limit values apply until the strictest limit values for continuous disturbances are reached.
This is intended to ensure that the effort that has to be made for radio interference suppression remains in a technically and economically justifiable relationship to the interference effect.
In the military sector, on the other hand, this assessment based on frequency is not common; the peak value is always measured here.
Practical examples
- A hand-operated light switch for room lighting
- This is normally only activated two to four times a day and only generates a short click lasting a few milliseconds. It would be uneconomical to equip every light switch with a noise filter. For such simple devices that only generate brief disturbances (<200 ms) when operated manually, no limit values apply to the level of disturbances.
- Fan heater with collector motor and bimetal switch as thermostat
- Here, the motor must be suppressed according to the stricter permanent interference limit values, since it can run for a long time. The switching clicks of the bimetal switch for regulating the heating coil, on the other hand, are evaluated separately and must not exceed a certain frequency, but then lie above the limit value for the continuous disturbances.
- computer
- A PC that is usually in operation for several hours and generates narrow-band continuous interference must comply with the stricter limit values for this narrow-band continuous interference, because if an interference frequency is in a usable frequency range, radio reception on this frequency is permanently disturbed.
Radio interference suppression
There are various ways to reduce radio interference:
- Reduction of radio interference already at the source, e.g. B. by smoothing the increase in current during switching operations using series resistors, capacitors and chokes. In ignition systems, resistors are often used in the ignition cables to limit the increase in current; in triacs in phase control systems , saturation chokes are connected in series with the triac. The disadvantage here is the increased power loss in the case of semiconductor switches due to the slower rise in current, which must be dissipated as heat and reduce the efficiency.
- Shielding the source of interference, e.g. B. by shielded spark plug connector . Electronic assemblies with vibration generators in the high frequency range are often shielded by high frequency sealed metal housings.
- Filtering of lines from and to the source of interference, e.g. B. Power supply via feedthrough capacitors in which the supply lines are led through a coaxial capacitor which is soldered to the housing. In household appliances and tools with collector motors, so-called interference suppression elements are used, which consist of interference suppression capacitors and / or chokes . Such interference suppression elements must be as close as possible to the interference source in order to switch off the antenna effect of the feed lines. There are different versions, but they all go back to one of the four basic circuits. It is advantageous if the housing of the devices is made of metal, because this provides additional shielding.
Basic circuits of suppressors
The basic principle when designing interference suppression elements is always based on the greatest possible mismatch to the interference source in the HF technical sense, so that the interference is reflected back to the source. If the interference source has a high impedance , a capacitance must first be used for interference suppression, because this represents a low impedance; conversely, with a low-impedance interference source, an inductance must first be connected upstream. Since the impedance of a real source of interference is normally unknown and frequency-dependent, interference suppression elements must always be adapted in practice in a real measurement setup and their effectiveness checked, see EMC measurement . Usually a combination of capacitors and chokes is required, the absolute values and frequency response of which have to be adapted to the impedance of the interference source. In addition, safety regulations must be observed (leakage current, contact voltages, dielectric strength, maximum permissible temperatures, etc.) so that the development of suppressors requires a lot of experience.
Interference suppression with parallel capacitor
With the simplest interference suppression method, an interference suppression capacitor is connected in parallel to the interference source. For the high-frequency interfering vibrations, this capacitance represents a kind of short circuit, which is why these are strongly damped.
Interference suppression with XY capacitors
In addition to the parallel capacitor , two interference suppression capacitors , which can also be installed in a housing, are connected to the protective conductor (earth). This brings additional damping and draws the potential of the harmonics to earth level.
Interference suppression with choke coils
Choke coils in the supply lines to the device represent a high resistance for high-frequency signals, since the inductive reactance increases with the size of the frequency. In this way, the interference signals are attenuated so that they cannot be emitted via the feed line. The mains current, on the other hand, can flow through the coils with almost no loss.
Interference suppression via capacitor-choke combinations
This high-quality interference suppression circuit attenuates the interference signals through the capacitors. The weak residual signals are largely separated from the supply lines by the choke coils. With this type of circuit, depending on the required quality of interference suppression, several stages can be arranged one after the other. In devices susceptible to interference, for example, “network interference suppression” is often built into the power supply lines, with chokes upstream and downstream of the capacitors. This grid interference suppression is intended to suppress signals from other sources of interference that superimpose the grid voltage.
Special properties of interference suppression capacitors
In addition to its capacitive resistance, every capacitor also has a small inductive component. Since this inductive resistance worsens the effectiveness of the capacitor with increasing frequency, special capacitors with a very small inductive resistance are manufactured for interference suppression purposes.
Safety aspects for interference suppression measures
In devices with Y capacitors, a small current always flows through the protective conductor, the so-called leakage current. If the protective conductor is interrupted in the event of a fault, Under certain circumstances, a voltage on conductive parts of the device can cause electric shocks. Therefore the leakage current and thus the size of the Y capacitors is limited in mains operated devices.