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Encyclopedia > Radio frequency interference

Radio Frequency Interference (RFI) is electromagnetic radiation which is emitted by electrical circuits carrying rapidly changing signals, as a by-product of their normal operation, and which causes unwanted signals (interference or noise) to be induced in other circuits. This interrupts, obstructs, or otherwise degrades or limits the effective performance of those other circuits. It can be induced intentionally, as in some forms of electronic warfare, or unintentionally, as a result of spurious emissions and responses, intermodulation products, and the like. It is also known as Electromagnetic Interference or EMI. Electromagnetism is the physics of the electromagnetic field: a field, encompassing all of space, composed of the electric field and the magnetic field. ... An electrical network or electrical circuit is an interconnection of analog electrical elements such as resistors, inductors, capacitors, diodes, switches and transistors. ... Electronic warfare (EW) has three main components: Electronic Attack (EA) This is the active use of the electromagnetic spectrum to deny its use by an adversary. ... It has been suggested that intermodulation distortion be merged into this article or section. ...


RFI frequently affects the reception of AM radio in urban areas. It can also affect FM radio and television reception, although to a lesser extent. Mediumwave radio transmissions (sometimes called Medium frequency or MF) are those between the frequencies of 300 kHz and 3000 kHz. ... FM radio is a broadcast technology invented by Edwin Howard Armstrong that uses frequency modulation to provide high-fidelity broadcast radio sound. ...


The most important means of reducing RFI are: use of bypass or "decoupling" capacitors on each active device (connected across the power supply, as close to the device as possible), risetime control of high speed signals using series resistors and VCC filtering. Shielding is usually a last resort after other techniques have failed because of the added expense of RF gaskets and the like. Various types of capacitors A capacitor is a device that stores energy in the electric field created between a pair of conductors on which equal but opposite electric charges have been placed. ... Rough plot of Earths atmospheric transmittance (or opacity) to various wavelengths of electromagnetic radiation, including radio waves. ...


The efficiency of the radiation is dependant on the height above the ground or power plane (at RF one is as good as the other) and the length of the conductor in relationship to the wavelength of the signal component (fundamental, harmonic or transient (overshoot, undershoot or ringing)). At lower frequencies, such as 133 MHz, radiation is almost exclusively via I/O cables; RF noise gets onto the power planes and is coupled to the line drivers via the VCC and ground pins. The Rf is then coupled to the cable through the line driver as common node noise. Since the noise is common mode, shielding has very little effect, even with differential pairs. The RF energy is capacitively coupled from the signal pair to the shield and the shield itself does the radiating. A megahertz (MHz) is one million (106) hertz, a measure of frequency. ...


At higher frequencies, usually above 500 MHz, traces get electrically longer and higher above the plane. Two techniques are used at these frequencies: wave shaping with series resistors and embedding the traces between the two planes. If all these measures still leave too much RFI, shielding such as RF gaskets and copper tape can be used. Most digital equipment is designed with metal, or coated plastic, cases. A megahertz (MHz) is one million (106) hertz, a measure of frequency. ...


Switching power supplies can be a source of RFI, but have become less of a problem as design techniques have improved.


Most countries have legal requirements that electronic and electrical hardware must still work correctly when subjected to certain amounts of RFI, and should not emit RFI which could interfere with other equipment (such as radios).


Mobile phones and interference

A well documented hazard of using mobile phones is electrical interference with some types of medical equipment, such as ventilators, defribillators, bedside monitoring equipment, in Intensive Care Units. Such interference could theoretically cause the death of a patient dependent on the equipment. For this reason, using any kind of wireless devices, including mobile phones, inside some areas of hospitals has been forbidden in many places. It has been suggested that this article or section be merged with Medical devices. ... A medical ventilator is a device designed to provide mechanical ventilation to a patient. ... Wireless is an old-fashioned term for a radio receiver, referring to its use as a wireless telegraph. ... A physician visiting the sick in a hospital. ...


According to two extensive studies carried out in 2004, one at Massachusetts General Hospital, and another in Australia: Main entrance of Massachusetts General Hospital Massachusetts General Hospital (MGH) is a teaching hospital of Harvard Medical School and biomedical research facility in Boston, Massachusetts. ...

"Cellular phones placed in close proximity to some commercially available intensive care ventilators can cause malfunctions, including irrecoverable cessation of ventilation. This is most likely to occur if the cellular phone is <30 cm from the device and ringing. Based on our data and the available literature, we believe it is reasonably safe to permit the use of cellular phones in the intensive care unit, as long as they are kept > or =3 feet from all medical devices. The current electromagnetic compatibility standards for mechanical ventilators are inadequate to prevent malfunction."

and

"Clinically relevant electromagnetic interference (EMI) secondary to mobile phones potentially endangering patients occurred in 45 of 479 devices tested at 900 MHz and 14 of 457 devices tested at 1800 MHz. However, in the largest studies, the prevalence of clinically relevant EMI was low. Most clinically relevant EMI occurred when mobile phones were used within 1 m of medical equipment. Although testing was not standardised between studies and equipment tested was not identical, it is of concern that at least 4% of devices tested in any study were susceptible to clinically relevant EMI. All studies recommend some type of restriction of mobile phone use in hospitals, with use greater than 1 m from equipment and restrictions in clinical areas being the most common."


Electrical interference with implanted cardiac and neural pacemakers was also observed, particularly with early analog models of handsets, which irradiated with more power. However, digital handsets may also interfere, albeit pacemaker models manufactured after 1995 have been protected with filters for the most used frequencies in mobile communication. In a study of 276 models of 61 manufacturers, ca. 1,5% of all tested devices suffered interference from cell phones when placed a few centimeters from the body area where the pacemaker was implanted. The consequences of interference were usually minor. Some authors recommend that wearers of pacemakers should abstain totally from using mobile phones. Others advise that the mobile phone should be used on the side of the head opposite to that of the implant, and that they should never be carried in a pocket in the side of the implant. The term pacemaker has multiple meanings: In sports, a pacemaker or pacer is a competitor who enters an athletics race with little or no intention of winning, but purely to set a fast pace for other competitors to follow. ...



See also: Electrostatic discharge, Electromagnetic Compatibility, PCB layout guidelines An electrostatic discharge (ESD) is an electric current driven by the excess electric charge stored on an insulating object. ... Electromagnetic Compatibility (EMC) is the branch of electrical sciences which studies the unintentional generation, propagation and reception of electromagnetical energy with reference to the unwanted effects that such an energy may induce. ... Many practical circuits can be laid out easily if the lumped element model holds for the particular circuit. ...

This article was originally based on material from the Free On-line Dictionary of Computing, which is licensed under the GFDL.

[[Category:Warfare] The Free On-line Dictionary of Computing (FOLDOC) is an on-line, searchable encyclopedic dictionary of computing subjects. ... GNU logo The GNU Free Documentation License (GNU FDL or simply GFDL) is a copyleft license for free content, designed by the Free Software Foundation (FSF) for the GNU project. ...


  Results from FactBites:
 
eEngineer -- Radio Frequency Interference (RFI) (931 words)
This is defined as undesired signals with frequency components that fall within the receiver’s RF passband and are translated into the Intermediate Frequency (IF) passband via the mixer stage.
This is defined as undesired signals with frequency components which fall within or near the receiver’s RF passband and are translated outside of the IF passband via the mixer stage.
This is defined as undesired signals with frequency components that are significantly removed from the receiver’s RF passband.
Emergency radio frequency warning device - Patent 3949300 (1552 words)
The radio frequency safety device of claim 6 further comprising a bandpass shaping filter for limiting the bandwidth of the output of said signal generator to substantially the AM and FM broadcast bands.
The radio frequency safety device of claim 8 wherein said carrier signal has a frequency lower than the lowest frequency in the FM broadcast band and wherein said audio signal is a representation of an audible warning signal.
Radio receivers in the vehicles located in the vicinity of the transmitter detect the modulated noise signal and the received signal accordingly interferes with the station or channel being received by the vehicle radio.
  More results at FactBites »

 
 

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