Electromagnetic Interference

 Electromagnetic Interference (EMI)

Electromagnetic Interference is defined as: -

        �Any electromagnetic disturbance that interrupts, obstructs, or otherwise degrades or limits the effective performance of electronics and electrical equipment. 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.�

 Examples of EMI:

Some of the simple examples of EMI are familiar to almost everyone. Juicers, blenders, certain electric shavers, fans, or sewing machines cause disruption on TV or radios. The same result is sometimes observed when certain cars pass by and almost definitely observed when tractors and such heavy vehicles pass by.

             But these are mere nuisances. More serious examples of spectrum pollution occur if a heart pace maker is disrupted by external electromagnetic fields. Here the results could be unconsciousness or even death. Similar are the cases for other medical electronic equipment. Hence mobile phones and similar equipments are banned in hospitals.

Another problem, which we often come across, is the �Network not responding� on our mobile phones. This often results in disruption in voice communication and sometimes even the dropping of calls. We may call this a petty problem but for a stockbroker a call may just cost him millions. Similar problems in communication can occur if a field army in limited warfare finds that its communications are jammed. The consequence can be lost of a marine, a battle, or even a country, not to mention the thousands of lives.

Fortunately European governments and US have issued specifications with which the electrical, electromechanical and electronic equipments must comply. Hence the problem now remains in policing and enforcing of these specifications as well as maintaining them.

Intrasystem vs. Intersystem EMI

             The cause of EMI problem may be within the system one is dealing with, in which case the problem is labeled an intrasystem problem, such as coupling of different cables, reception of emissions from within the system and such problems. 

On the other hand EMI may come from outside, in which case it is called intersystem EMI. The common examples such as disruption in transmission all come under this category.

ADDRESSING EMI PROBLEMS :

When electromagnetic interference is suspected, the first step in resolving the problem is to determine the mechanism for energy transfer to the affected device(s): radiation, conduction, or induction.

Radiated electromagnetic energy entering an adjacent device is one of the most difficult to identify and control. The existence of this condition implies that there is insufficient control of both ingress energy in an affected adjacent device (victim) and of emissions from the radiating device (source). In general, when dealing with radiated EMI the facility is faced with three options:

• Remove (or reduce) the source.
• "Harden" the victim.
• Separate the devices.

There are many design conditions that can control this situation, very few of which can be corrected within the hospital. This condition is aggravated when many devices are located in a relatively small area.

      When a problem is recognized in a particular device or among a group of devices, it is essential for the biomedical engineering group to enlist manufacturer participation in resolving the interfering condition. This can be problematic unless the manufacturers of both the emitting devices and the susceptible equipment recognize that the problem lies with both devices. The facility's biomedical engineering department will also play a role in coordinating the efforts of the manufacturers, as well as controlling the physical environment in which the devices are located.

       There are several approaches toward the mitigation of EMC/EMI problems that can be employed by the hospital's biomedical engineering group, although many of the solutions listed below are more directly controlled by the manufacturer of the radiating and affected devices:

• In the case where radiation is the suspected method of propagation, an increase in the shielding of the major units, both internal and external, might be attempted .
• The addition of RF filters to wiring entering and exiting the equipment, from both the radiating source and the susceptible devices, may reduce the intensity or the effect of the received radiation.
• The effectiveness of grounding (ohmic values as well as lengths) should be explored. The length of the connection between the radiating device and an effective radio frequency ground can be critical.
• An increase in separation between the devices may alleviate the problem.
• In extreme cases, failing other solutions, noncritical devices such as television sets can be removed from the immediate area until a more acceptable solution can be found.

When induction is suspected, there are several approaches to mitigation. Cabling can be replaced with more effectively shielded types. In cases where the substitution of special cabling with a shielded type is not possible, rerouting or increasing the distance between cable bundles may offer a solution.

       Interference by conduction can usually be reduced or eliminated by a careful analysis of wiring routing and connections, including potential loops existing between electrically powered beds or other devices and the affected equipment. Adding filters to inputs and power sources is another possible solution.
       As the risks to the proper functioning of patient therapeutic and diagnostic equipment increase due to the effects of electromagnetic radiation, it will become of paramount importance for the engineering department to familiarize itself with the conditions that cause the phenomenon, the methods of detecting EMI, assessment of the potential hazards, and the development and application of techniques to control EMI.

Types of Electromagnetic Interference Sources:

          Sources of emi can be divided into the following broad categories:

• Functional: Emi can originate from any source designed to generate electromagnetic energy and which may create interference as a normal part of its operation. The interference may be unintentional or caused by other on board or adjacent platform systems. This interference also may be intentional or caused by electronic countermeasures (ECM).

• Incidental: Emi can originate from man-made sources. These are sources not designed specifically to generate electromagnetic energy but which do in fact cause interference. Examples of incidental emi sources include power lines, motors, and switches.
• Natural: Emi can be caused by natural phenomena, such as electrical storms, rain particles, and solar and interstellar radiation. It is recognized by the following Audible Noises: �

(i)                  Intermittent impulses of high intensity that are caused by nearby electrical storms.

(ii)                Steady rattling or cracking caused by distant electrical storms.

(iii)               Continuous noise of precipitation static caused by electrically charged rain drops.

(iv)              A steady hiss at high frequencies caused by interstellar noise

 �        Hull-generated: Emi can be caused by the interaction of radiated signals with elements of the hull and rigging of a ship. (The functional signals themselves do not cause interference.) The following are two general methods by which emi is transmitted: -

(i)                 Conduction: Undesired energy, from one equipment, is coupled to interconnecting cables or components of another equipment. This energy is conducted via the wiring in the shielded enclosure that protects sensitive circuits. You will find proper design, adequate isolation, and shielding of cables and equipment can control this problem.

(ii)                Radiation: Energy is beamed directly from the transmitting antenna, or source, to the victim, receiving antenna. When a receiver picks up this interference, you have two solutions. Interfering energy can be eliminated at the source or you can filter, or blank it out at the victim equipment. Filtering is far less desirable. Interference may be on the same frequency as the desired signal and will not be eliminated without affecting the reception of all desired signals. Emi also can degrade overall receiver performance in a less noticeable way. It does this by desensitizing the receiver front end. The noise level is raised and effectively lowers the signal to noise ratio and thus the sensitivity. This causes a decrease in desired signal amplification. The protective devices may include filters, multicouplers, preselectors, and so forth. These devices can minimize interference caused by inadequate frequency separation or poor physical isolation between transmit and receive antennas.

 EMI Coupling Paths

 EMI coupling paths maybe conductive or radiation in nature.

 The radiation paths are:

1.      Antenna to antenna

2.      Box to wire

3.      Antenna to box

4.      Wire to antenna

5.      Antenna to wire

6.      Wire to box

7.      Box to Antenna

8.      Wire to wire

9.      Box to box

The Conduction paths are:

1.      Wire to wire

2.      Common Ground Impedance

3.      Filters

4.      Common Source Impedance

Sources Of Electromagnetic Interference: -

Natural:

�      Terrestrial

�         Atmospheric

�         Precipitation Static

�      Extra-Terrestrial

�         Sun

�         Cosmic

�         Radio Stars

Man-Made:

�     Personal: -

1.      Body Fat Measuring Scales

2.      Cell Phones

3.      Copy Machines

4.      Cordless Phones

5.      Electric Blankets

6.      Electric Razors

7.      Electrolysis (hair removal)

8.      Fax Machines

9.      Hair dryers

10.  Hand-held messengers

11.  Heating Pads

12.  Magnetic Mattresses/chairs

13.  Pagers

14.  Patient alert devices

15.  Personal Computers

16.  Personal Digital Assistants

17.  Power Toothbrushes

18.  Radio Controlled Clocks

19.  Tanning Beds

20.  Tattoos

21.  Thermolysis (hair removal)

�     Kitchen and Household Items

1.      Blenders

2.      Can Openers

3.      Clothes dryers

4.      Convection Ovens

5.      Electric brooms

6.      Electric knives

7.      Electric ovens

8.      Food Processors

9.      Gas Ovens

10.  Induction Ovens

11.  Microwave Ovens

12.  Pest Control Systems

13.  Portable Space Heaters

14.  Sewing Machines

15.  Toasters

16.  Vacuum Cleaners

17.  Washing Machines

�     Do-It-Yourself Items: -

1.      Arc Welding

2.      Battery-Powered equipment, Cordless Power Tools

3.      Car Engine repair

4.      Chain Saws

5.      Drills

6.      Electric Screw Drives

7.      Hedge Clippers

8.      High-Power Generators

9.      Jackhammers

10.  Jigsaws

11.  Lawn Mowers

12.  Leaf Blowers

13.  Running Motors and Alternators

14.  Small Motor Repair

15.  Snow Blowers

16.  Soldering Guns

�     Entertainment Items: -

1.      AM/FM Radios

2.      Bingo Wands

3.      CB/Police Radio Antennas

4.      CD/DVD Players

5.      Laser Tags

6.      Remote Controls with Attenuators

7.      Remote Controls (TV, Stereo, garage Doors, Video Equipment, Cameras)

8.      Scuba Diving

9.      Slot Machines

10.  Stereo Speakers

11.  Televisions

12.  VCR�s

13.  Video Games

�     Travel and Environment:

1.      Amusement Parks/Roller Coasters

2.      Airport Security Systems

3.      Residential Power Generators

4.      High Voltage Lines

5.      Radio Frequency Transmitters

6.      Theft Detection Systems

7.      Theft Tag Deactivation

8.      Transformers

9.      TV or Radio Towers

�     Medical Processes:

1.      CT Scans

2.      Dental Drills

3.      Diagnostic x-rays

4.      Dithermy

5.      Electrocardiograms (ECG)

6.      Electrocautery

7.      High-Energy Radiation

8.      Magnetic Resonance Imaging (MRI)

9.      TENS

10.  Ultrasound

�     Miscellaneous:

1.      Air Purifiers

2.      Electric Invisible Fences

3.      House Arrest Devices

4.      Magnetic Fields

5.      Polygraphs

6.      Stun Guns

EMI Errors in Equipment:

           Analog signals or digital data whose content or context have been altered by the currents produced by the ingress of undesired electromagnetic fields are said to be corrupted. Data corruption by radiated electromagnetic fields can occur in several ways. The two error mechanisms of most concern for medical devices are bit corruption and junction rectification. Bit corruption problems are commonly associated with digital devices, whereas junction rectification is generally recognized as a problem prevalent in analog equipment.

Bit Corruption

          Bit corruption occurs when an undesired artifact of appropriate duration and of sufficient amplitude is introduced into a "byte" of data in a digital system. This introduction can be accomplished via any of the three methods previously described: radiation, induction, or conduction. The introduction of this undesired bit effectively changes the "value" or meaning of the altered byte.

          This type of interference is not uncommon, especially on data transmission lines or long cable runs. In most cases the altered bytes can be recognized with error detection algorithms, which are built in to the affected device. The erroneous data can be rejected and/or retransmitted. However, in situations where devices of common manufacturer and model are located or connected in such a way that they affect each other, the corrupted bit can repeatedly occur within recurrent bytes and might eventually be recognized as 'Invalid" data. Failure of the error correction scheme to trap bad data may subsequently alter the operation of the affected device. If corrupted data bytes are interpreted by a microprocessor as CPU instructions they may cause the instrument to unexpectedly change states or "lock up" by attempting to perform an invalid operation. If they are interpreted as data they may yield inaccurate results.

Junction Rectification

          Junction rectification occurs when a semiconductor device is exposed to a high-frequency alternating current field, such as those produced by radio transmitters in close proximity to a device. This exposure may cause a direct current to be produced within semiconductor junctions through unintentional AC-to-DC rectification. This unintentional rectification occurs as a result of one of the intrinsic characteristics of semiconductor junctions. An example of intentional rectification occurs with semiconductor rectifiers in AC-to-DC power supplies.

          When received as an unwanted interference event, the rectified signals can produce a DC offset voltage or modulated signal, which may be "interpreted" as artifact or invalid data in the device. This artifact can cause many different and "inexplicable" temporary or permanent device malfunctions. For all practical purposes, a relatively high-energy electromagnetic field is required to produce this effect.

           It is important to note that the circumstances that cause bit corruption to affect digital devices can also affect analog devices by appearing as momentary changes or pulses in analog circuits. Likewise, junction rectification can affect digital devices by generating positive DC offset voltages, which appear as digital "1"s (highs) occurring in the digital stream.

Hazards of Electromagnetic Interference

           Radio-frequency (rf) transmitting systems with high-power transmitting tubes and high-gain antennas have increased the possibility of injury to personnel working in the vicinity. An electromagnetic radiation hazard exists when electronic equipment generates a strong enough electromagnetic field to fall in a category listed below:

• Causes harmful or injurious effects to humans and wildlife.

• Induces or otherwise couples currents and/or voltages of magnitudes large enough to initiate electroexplosive devices or other sensitive explosive components of weapons systems, ordnance, or other explosive devices.
• Creates sparks large enough to ignite flammable mixtures or materials that must be handled in the affected areas These hazardous situations can be caused by a transmitter or antenna installation. These generate electromagnetic radiation in the vicinity of personnel, ordnance, or fueling operations in excess of established safe levels. Sometimes the existing electromagnetic radiation levels increase to a hazardous level. When personnel, ordnance, or fueling evolutions are located in an area that can be illuminated by electromagnetic radiation, hazardous situations may occur. Electromagnetic radiation is hazardous to personnel in two ways. It can cause RF burns; and it can cause biological, thermal, and neurological effects to personnel (RAD-HAZ). Because of the differences in characteristics and safety precautions required for each of the two types, they will be discussed separately. An RF burn hazard is a hazardous condition caused by the existence of radio frequency (RF) voltages in places where they are not intended to be. Any ship with high-power HF transmitters is susceptible. Potentially hazardous voltages have been found in many areas. Some of these areas are lifelines, vertical ladders, ASROC launchers, gun mounts, rigging for underway replenishment, and boat davits. Another of these areas is on aircraft tied down on carrier and helicopter flight decks. Whether or not an induced voltage creates an RF burn hazard depends on whether personnel will come into contact with the object being energized. Generally, only the voltage between an object and the deck is important. The RF burn occurs when a person comes into contact with a source of RF voltage in a 3-41 manner that allows RF current to flow through the area of contact. Resistance of the skin to the current flow at the areas of contact causes heat. The effect of the heat on a person at the point of contact ranges from noticeable warmth to a painful burn. The most useful and widespread technique in the reduction of RF burn hazards is the proper bonding and grounding of all metallic objects in the RF radiation field. In some cases, the RF burn hazard can be eliminated only through the use of restrictive operating procedures. These procedures govern the simultaneous use of transmitting and cargo equipment. Techniques such as operation of transmitters at reduced power and the prohibition of simultaneous use of certain combinations of antennas, frequencies, and cargo handling equipment are used. Figure 3-35 shows typical rf radiation hazard warning signs

Screening Out Electromagnetic Interference:-

Electrostatic shields, such as those used in shielded audio cables, have no effect on electromagnetic interference.  Electrostatic shields are electrically conductive, but are often made of non-magnetic material, such as copper or aluminium.

The most effective magnetic screen is mu-metal, a highly magnetically conductive material.  Mu-metal is often used for head shields in magnetic tape recorders, since tape heads are highly sensitive to magnetic fields.

Mu-metal is very expensive and is therefore not used widely in as a means of magnetic screening in audio equipment.  Ferrous metals, such as steel can be used for magnetic screening in sensitive audio equipment.  It is impractical to make magnetically shielded audio cables.  We therefore must rely on other means to reduce the effects of electromagnetic interference. 

Note that using thick wires has no effect on electromagnetic interference.  A 6" square solid copper buss bar is just as susceptible to electromagnetic interference as a hair thickness wire!

Ways to Minimize Electromagnetic Interference:-

            Electromagnetic interference is generated by the presence of alternating current in a conductor causing a corresponding alternating current to flow in an adjacent conductor.

Electromagnetic interference can be minimized by:

1. Enclosing equipment in magnetically conductive shields.  This is normally only done for sensitive items such as tape heads, microphones and audio transformers.  It is not practical to do this with normal audio cables.
2. Keeping devices which radiate electromagnetic interference such as power transformers and video monitors away from your equipment.