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.

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.

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.

The efficiency of the radiation is dependent 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. One cure for this is to use a braid-breaker to reduce the common mode signal.

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.

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.

According to two extensive studies carried out in 2004, one at Massachusetts General Hospital, and another in Australia:

"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.

See also

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• Electrostatic discharge
• Electromagnetic shielding
• Electromagnetic compatibility
• PCB layout guidelines
• TEMPEST

Reference

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External links

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• Glossary of EMI, EMC and Electrical Shielding terms

Radio electronics