EMI
Large electric motors create powerful electromagnetic fields and cause Electromagnetic interference (EMI) problems. Electric motors, which may cause EMI, are found in various kinds of equipment-refrigerators, air conditioners, washing machines, furnaces, copier, elevators, and machine tools, etc.
RFI
Radio-frequency interference (RFI) is in general caused by radio or television sources. A weaker source, such as in-house wireless telephone, fluorescent lamp and lamp dimmer can also interfere with the electronic device if they are placed too close.
Harmonic Distortion
It is a form of pollution in the electric plant that can cause problems if the sum of the harmonic currents increases above certain limits. All power electronic converters used in different types of electronic systems can increase harmonic disturbances by injecting harmonic currents directly into the grid. Common non-linear loads include motor starters, variable speed drives, computers and other electronic devices, electronic lighting, welding supplies and uninterrupted power supplies.
The effects of harmonics can be overheating of transformers, cables, motors, generators and capacitors connected to the same power supply with the devices generating the harmonics. Electronic displays and lighting may flicker, circuit breakers can trip, computers may fail and metering can give false readings.
Figure shows how the current harmonics (ih) in the input current (is) of a power electronic converter affect the supply voltage (ut).
In a theoretical case where output current can be estimated as clean DC current, the harmonic current frequencies of a 6-pulse three phase rectifier are n times the fundamental frequency (50 or 60 Hz), where n = 6k ± 1 and k=1,2,... .
Total harmonic distortion (THD) indicates the strength of electromagnetic noise generated. THD in the current is the root mean square (rms) of all the harmonic currents as a percentage of the fundamental current.
Radio Frequencies
Radio frequency
3 - 30Hz Extremely low frequency ELF
30 - 300Hz Super low frequency SLF
300 - 3000Hz Ultra low frequency ULF
3 - 30kHz Very low frequency VLF
30 - 300kHz Low frequency LF
300kHz - 3MHz Medium frequency MF
3 - 30MHz High frequency HF
30 - 300MHz Very high frequency VHF
300MHz - 3GHz Ultra high frequency UHF
3 - 30GHz Super high frequency SHF
30 - 300GHz Extremely high frequency EHF
ELF
The United States Navy utilized extremely low frequencies (ELFs) as radio band and radio communications. Because of the electrical conductivity of seawater, submarines are shielded from most electromagnetic communications. The Submarine Integrated Antenna System (SIAS) was a research and development effort to communicate with submerged submarines.
SLF
This frequency range includes the frequencies of AC power grids (50 hertz and 60 hertz). The radio services Seafarer (USA) on 76 hertz and ZEVS (Russia) on 82 hertz operate in this range. They both provide communication services for submarines at a certain depth.
ULF
Many types of waves in the ULF frequency band can be observed in the magnetosphere and on the ground. These waves represent important physical processes in the near-Earth plasma environment. This band is used for communications in mines, as it can penetrate the earth. The speed of the ULF waves is often associated with the Alfven velocity that depends on the ambient magnetic field and plasma mass density.
VLF
VLF waves can penetrate water to a depth of roughly 10 to 40 metres (30 to 130 feet), depending on the frequency employed and the salinity of the water. VLF is used to communicate with submarines near the surface (for example using the transmitter DHO38), while ELF is used for deeply-submerged vessels. VLF is also used for radio navigation beacons (alpha) and time signals (beta), and electromagnetic geophysical surveys.
LF
In the western hemisphere, its main use is for aircraft beacon, navigation (LORAN), information, and weather systems. In Europe, and parts of Northern Africa and of Asia, part of the LF spectrum is used for AM broadcasting as the longwave band. Some radio frequency identification (RFID) tags utilize LF. These tags are commonly known as LFID's or LowFID's (Low Frequency Identification).
Electronic ballasts for fluorescent lamp employ transistors to alter mains voltage frequency into high-frequency AC, typically from 25–50kHz, while also regulating the current flow in the lamp. EMI at the electronic ballast fundamental frequency and its harmonics propagate from the ballast’s electronic circuits to the line conductors. This EMI may interfere with other electrical equipment on the same distribution network. Most electromagnetic ballasts have THD between 18% and 35%. Most of them are below 20%.
Switching-mode power supply (SMPS) is an electronic power supply that incorporates a switching regulator. Unlike a linear power supply, the pass transistor of a switching mode supply switches very quickly (typically between 50 kHz and 1 MHz) between full-on and full-off states, which minimizes wasted energy. Voltage regulation is provided by varying the ratio of on to off time. In contrast, a linear power supply must dissipate the excess voltage to regulate the output. Output current flow depends on the input power signal, the storage elements and circuit topologies used, and also on the pattern used to drive the switching elements. Disadvantages of SMPS, include greater complexity, the generation of high-amplitude, high-frequency energy that the low-pass filter must block to avoid electromagnetic interference (EMI) - (properly earthed) EMI/RFI filter is connected between the input terminals and the bridge rectifier, and a ripple voltage at the switching frequency and the harmonic frequencies thereof.
MF
Radio stations are allocated an AM broadcast band from 526.5 kHz to 1606.5 kHz in Europe; in North America this extends from 535 kHz to 1705 kHz.
There is an amateur radio band known as 160 meters or 'top-band' between 1800 and 2000 kHz (allocation depends on country and starts at 1810 kHz outside the Americas). Amateur operators transmit CW morse code, digital signals and SSB voice signals on this band. 2182kHz is the international calling and distress frequency for SSB maritime voice communication (radiotelephony). It is analogous to Channel 16 on the marine VHF band. There are a number of coast guard and other ship-to-shore frequencies in use between 1600 and 2850 kHz.
HF
Also known as the decameter band or decameter wave. Since the ionosphere often refracts HF radio waves quite well (a phenomenon known as skywave propagation), this range is extensively used for medium and long range radio communication. Some radio frequency identification (RFID) tags utilize HF. These tags are commonly known as HFID's or HighFID's (High Frequency Identification).
Noise, especially man-made interference from electronic devices, tends to have a great effect on the HF bands. For example, frequencies on which broadband over power lines (BPL) operates (typically corresponding with the HF band) and the tendency for the BPL "signal" to leak from power lines. Some BPL providers have installed "notch filters" to block out certain portions of the spectrum (namely the amateur radio bands), but a great amount of controversy over the deployment of this access method remains.
VHF
Common uses for VHF are FM radio broadcast, television broadcast, land mobile stations (emergency, business, and military), long range data communication with radio modems, Amateur Radio, marine communications, air traffic control communications and air navigation systems (e.g. VOR, DME & ILS). In most of the world, FM radio stations broadcast in the band goes from 87.5 to 108.0 MHz (band II).
British television originally used VHF band I and band III. Television on VHF was in black and white with 405-line format. British colour television was broadcast on UHF (channels 21–69), beginning in the late 1960s. From then on, TV was broadcast on both VHF and UHF. The last British VHF TV transmitters closed down on January 3, 1985. VHF band III is now used in the UK for digital audio broadcasting, and VHF band II is used for FM radio, as it is in most of the world.
(Band I ranges from 47 to 88 MHz, and it is primarily used for radio and television broadcasting. Channel spacings vary from country to country, with spacings of 6, 7 and 8 MHz being common. Band III ranges from 174 to 230 MHz.)
UHF
It is widely used in two-way radio systems and cordless telephones, whose transmission and reception antennas are closely spaced. UHF signals travel over line-of-sight distances. A repeater propagates UHF signals when a distance greater than the line of sight is required. Civilian applications, such as GMRS, PMR446, UHF CB, 802.11b ("WiFi") and the widely adapted GSM and UMTS cellular networks, also use UHF frequencies.
Universal Mobile Telecommunications System (UMTS) is a third generation mobile cellular technology for networks based on the GSM standard. UMTS employs wideband code division multiple access (W-CDMA) radio access technology to offer greater spectral efficiency and bandwidth to mobile network operators. Developed by the 3GPP (3rd Generation Partnership Project), UMTS is a component of the International Telecommunications Union IMT-2000 standard set and compares with the cdma2000 standard set for networks based on the competing cdmaOne technology. The specific frequency bands originally defined by the UMTS standard are 1885–2025 MHz for the mobile-to-base (uplink) and 2110–2200 MHz for the base-to-mobile (downlink).
A microwave oven works by passing non-ionizing microwave radiation through the food, usually at a frequency of 2.45 gigahertz (GHz)—a wavelength of 122 millimetres (4.80 in).
Since 2006, UMTS networks in many countries have been or are in the process of being upgraded with High Speed Downlink Packet Access (HSDPA), sometimes known as 3.5G. Currently, HSDPA enables downlink transfer speeds of up to 21 Mbit/s. Work is also progressing on improving the uplink transfer speed with the High-Speed Uplink Packet Access (HSUPA). Longer term, the 3GPP Long Term Evolution project plans to move UMTS to 4G speeds of 100 Mbit/s down and 50 Mbit/s up, using a next generation air interface technology based upon Orthogonal frequency-division multiplexing.
SHF
Also known as the centimeter band or centimeter wave as the wavelengths range from ten to one centimeters. This frequency is used for microwave devices, WLAN, most modern radars. The commencing Wireless USB technology will be using approximately 1/3 of this spectrum.
EHF
This band is commonly used in radio astronomy and remote sensing. This band has a wavelength of ten to one millimetre, giving it the name millimeter band or millimetre wave, sometimes abbreviated MMW or mmW.
Signals in the 57–64 GHz region are subject to a resonance of the oxygen molecule and are severely attenuated. Satellite-based remote sensing near 60 GHz can determine temperature in the upper atmosphere by measuring radiation emitted from oxygen molecules that is a function of temperature and pressure.
