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EMC problem (excessive field strength)
Encyclopedia
An EMC problem occurs when an electronic or electromagnetic system is adversely affected by the high field strength
s produced by a radio transmitter. EMC problems are not due to defects in the transmitter, and so do not necessarily require improvements in the radio transmitter design
, such as reducing its radiated harmonic
s. EMC problems can have a range of effects on equipment, and there are ways to mitigate or eliminate them in practice. Effective EMC mitigation techniques may differ by the type of equipment that malfunction
s, and by the nature of the strong radio frequency
field.
. This is a situation where the output of a transmitter is clean, the receiver is able to reject the transmitter's frequency but some object energised by the RF field is then able to generate harmonics or act as a non-linear mixer which generates intermodulation
products.
To solve this the best thing to do is to locate the rusty bolt and either remove it or render it harmless. Possible examples of rusty bolts include the pole holding an aerial, cast iron
drainpipe
s, loose manhole
covers and guy wires on radio mast
s.
The best method of curing this defect is to improve the RF shielding and the RF filtering of all power, audio and control leads entering the affected unit.
s which protect the receivers in many VHF rigs lacking an electromechanical relay to switch between transmit and receive (such as the 144 MHz DJ-F1E) from the transmitter's RF output. In the case of the DJ-F1E, these dioides can create intermodulation products when the radio set is overloaded.
stage, and if it falls within the passband of the stage, it can cause an unwanted effect. In an AM system, it is possible that the unwanted signal will be amplified by the linear stage and appear finally to the detector as if it were a sideband of the carrier, it is possible sometimes for harmonics of an HF transmitter generated within a TV set to enter the video IF (which is at about 30 MHz). These harmonics which enter the TV IF could give similar herring bone pattern effects to those seen when harmonics of the HF transmitter are generated in the front end.
In FM systems where the IF stage gain is very high and the stage is designed to be linear, it is likely that if the unwanted signal is at least 10dB weaker than the wanted signal that the capture effect
will result in the receiver ignoring the unwanted signal. If however the unwanted signal is stronger than the wanted signal then it is likely that the receiver will be unable to demodulate the wanted signal. This is related to the normal method of jamming FM systems, to jam an FM signal it is normal to transmit a signal which is 10 dB stronger (at the front end of the target receiver) and is within the plateau of most narrow IF stage.
transmitter then the braid of the coaxial should be considered as a path of entry of RF into the affected device. One additional feature which can assist a braid breaker is the use of an RF ground, a quarter wave length of wire will act as an RF ground even when it goes nowhere. This is because the quarterwave transforms the infinite impedance into a zero impedance. The quarterwave length of wire can be regarded as an artificial earth for RF purposes.
By adding a braid-breaker
filter to the system common mode signals will be attenuated while unbalanced signals will be allowed to pass unhindered. This is also something which is added to long leads in military
electronic equipment which is intended to continue to function even after being subjected to a nuclear electromagnetic pulse (EMP
) and also would increase the likelihood of an electronic device surviving a near-by lightning strike (direct lightning hits are very hard to protect against).
For UHF TV a wide range of good designs exist, these include a ferrite RF choke on the downlead and some networks of passive components.
are a well used method for filtering many leads on communication equipment. In common with the braid breaker the intention is to suppress common mode signals. One popular method of making an RF choke is to use a large ferrite bead
where the cable passes through the hole. Such chokes can often be seen on the power connection leads of laptops and on the video signal leads of computer monitors.
The addition of RF chokes can often reduce greatly the RF current flowing through a TV set and hence improve the immunity of the installation.
One design for VHF/UHF TV which also has the ability to filter out 144 MHz, 432 MHz and 1296 MHz signals has been created by a Russian (UA9FAD).http://www.df9cy.de/df9cy_tvistop.htm. A design for UHF TV which is designed for the six meter band (50 MHz) has been published by the UK six meter group. http://www.uksmg.org/filter.htm One design for FM radios (band II) has been created by a German (DF9CY) http://www.df9cy.de/tech-mat/df9cy_bcbpf.htm
transmitters a click may be heard as the RF field appears and also when it disappears.
In the case of amplitude modulation
it is possible for the original audio to be recovered by the affected device. AM is still used for longwave
and mediumwave
broadcasting, and sometimes in communications equipment used by services such as the fire brigade, military
or police
.
A weird animal quacking or an unintelligible human voice is a likely sign that the transmitter is using single-sideband modulation
. SSB (single-sideband) tends to be used in applications where long range communication is needed. Here, the non-linear device is mixing frequencies, but because the carrier has been suppressed, it is not possible to restore the original audio with such a simple demodulator.
system with external speaker attached by long leads to the main unit (holding the audio electronics) can be troubled by EMC problems which are caused by the speaker leads acting as a dipole
aerial.
One method of curing this is to add a ferrite choke to the speaker leads close to where they enter the main unit.
handbook one common way in which stereo systems are affected is when the RF current picked up by the speaker cable, and then flows through the feedback circuit into an early stage in the audio amplifier. The RF is then rectified
when it flows through the base/emitter junction of a bipolar transistor.
Field strength
In physics, the field strength of a field is the magnitude of its vector value.In theoretical physics, field strength is another name for the curvature form...
s produced by a radio transmitter. EMC problems are not due to defects in the transmitter, and so do not necessarily require improvements in the radio transmitter design
Radio transmitter design
Radio transmitter design is a complex topic which can be broken down into a series of smaller topics. A radio communication system requires two tuned circuits each at the transmitter and receiver, all four tuned to the same frequency...
, such as reducing its radiated harmonic
Harmonic
A harmonic of a wave is a component frequency of the signal that is an integer multiple of the fundamental frequency, i.e. if the fundamental frequency is f, the harmonics have frequencies 2f, 3f, 4f, . . . etc. The harmonics have the property that they are all periodic at the fundamental...
s. EMC problems can have a range of effects on equipment, and there are ways to mitigate or eliminate them in practice. Effective EMC mitigation techniques may differ by the type of equipment that malfunction
Malfunction
A malfunction is when something functions wrongly or does not function at all.Some types of malfunctions are:*Malfunction , malfunction of a parachute*Sexual malfunction, also called "sexual dysfunction"**See also dyspareunia...
s, and by the nature of the strong radio frequency
Radio frequency
Radio frequency is a rate of oscillation in the range of about 3 kHz to 300 GHz, which corresponds to the frequency of radio waves, and the alternating currents which carry radio signals...
field.
Singing sparks and allied effects
The singing spark can be thought of as the ultimate form of the rusty bolt effectRusty bolt effect
The rusty bolt effect is a description of radio interference due to interactions with dirty connections or corroded parts. It is more properly known as passive intermodulation, and can result from a variety of different causes such as ferromagnetic conduction metals, nonlinear microwave absorbers...
. This is a situation where the output of a transmitter is clean, the receiver is able to reject the transmitter's frequency but some object energised by the RF field is then able to generate harmonics or act as a non-linear mixer which generates intermodulation
Intermodulation
Intermodulation or intermodulation distortion is the amplitude modulation of signals containing two or more different frequencies in a system with nonlinearities...
products.
To solve this the best thing to do is to locate the rusty bolt and either remove it or render it harmless. Possible examples of rusty bolts include the pole holding an aerial, cast iron
Cast iron
Cast iron is derived from pig iron, and while it usually refers to gray iron, it also identifies a large group of ferrous alloys which solidify with a eutectic. The color of a fractured surface can be used to identify an alloy. White cast iron is named after its white surface when fractured, due...
drainpipe
Rain gutter
A rain gutter is a narrow channel, or trough, forming the component of a roof system which collects and diverts rainwater shed by the roof....
s, loose manhole
Manhole
A manhole is an opening used to gain access to sewers or other underground structures, usually for maintenance.Manhole may also refer to:* Manhole , a metal band from Los Angeles* The Manhole, a computer game...
covers and guy wires on radio mast
Radio masts and towers
Radio masts and towers are, typically, tall structures designed to support antennas for telecommunications and broadcasting, including television. They are among the tallest man-made structures...
s.
Motorboating
Motorboating can be seen in radio equipment, it can be regarded as a self inflicted EMC problem. While this idea of self imposed EMC effects could include all manner of unwanted feed back and crosstalk effects within an item of equipment, for reasons of brevity no discussion of internal feed back (which does not involve a signal leaving an item of equipment) will be included here.The best method of curing this defect is to improve the RF shielding and the RF filtering of all power, audio and control leads entering the affected unit.
Harmonics generated in situ
In situ harmonics occur when an RF current flows through a device, and harmonics are generated due to the non-linear nature of the device. This can be regarded as a rusty bolt effect which exists within the malfunctioning device. A good example of this is the pair of diodeDiode
In electronics, a diode is a type of two-terminal electronic component with a nonlinear current–voltage characteristic. A semiconductor diode, the most common type today, is a crystalline piece of semiconductor material connected to two electrical terminals...
s which protect the receivers in many VHF rigs lacking an electromechanical relay to switch between transmit and receive (such as the 144 MHz DJ-F1E) from the transmitter's RF output. In the case of the DJ-F1E, these dioides can create intermodulation products when the radio set is overloaded.
Reproduction of the signal in an IF stage
It is possible for the unwanted signal to enter an Intermediate frequencyIntermediate frequency
In communications and electronic engineering, an intermediate frequency is a frequency to which a carrier frequency is shifted as an intermediate step in transmission or reception. The intermediate frequency is created by mixing the carrier signal with a local oscillator signal in a process called...
stage, and if it falls within the passband of the stage, it can cause an unwanted effect. In an AM system, it is possible that the unwanted signal will be amplified by the linear stage and appear finally to the detector as if it were a sideband of the carrier, it is possible sometimes for harmonics of an HF transmitter generated within a TV set to enter the video IF (which is at about 30 MHz). These harmonics which enter the TV IF could give similar herring bone pattern effects to those seen when harmonics of the HF transmitter are generated in the front end.
In FM systems where the IF stage gain is very high and the stage is designed to be linear, it is likely that if the unwanted signal is at least 10dB weaker than the wanted signal that the capture effect
Capture effect
In telecommunication, the capture effect, or FM capture effect, is a phenomenon associated with FM reception in which only the stronger of two signals at, or near, the same frequency will be demodulated....
will result in the receiver ignoring the unwanted signal. If however the unwanted signal is stronger than the wanted signal then it is likely that the receiver will be unable to demodulate the wanted signal. This is related to the normal method of jamming FM systems, to jam an FM signal it is normal to transmit a signal which is 10 dB stronger (at the front end of the target receiver) and is within the plateau of most narrow IF stage.
Braid-breakers
If an electronic device with a long coaxial aerial lead is disturbed by a signal from a HFHigh frequency
High frequency radio frequencies are between 3 and 30 MHz. Also known as the decameter band or decameter wave as the wavelengths range from one to ten decameters . Frequencies immediately below HF are denoted Medium-frequency , and the next higher frequencies are known as Very high frequency...
transmitter then the braid of the coaxial should be considered as a path of entry of RF into the affected device. One additional feature which can assist a braid breaker is the use of an RF ground, a quarter wave length of wire will act as an RF ground even when it goes nowhere. This is because the quarterwave transforms the infinite impedance into a zero impedance. The quarterwave length of wire can be regarded as an artificial earth for RF purposes.
By adding a braid-breaker
Braid-breaker
A braid-breaker is a filter that prevents television interference . In many cases of TVI, caused by a high field strength of a nearby high frequency transmitter, the aerial down lead plugged into the back of the TV acts as a longwire antenna or as a simple vertical element...
filter to the system common mode signals will be attenuated while unbalanced signals will be allowed to pass unhindered. This is also something which is added to long leads in military
Military
A military is an organization authorized by its greater society to use lethal force, usually including use of weapons, in defending its country by combating actual or perceived threats. The military may have additional functions of use to its greater society, such as advancing a political agenda e.g...
electronic equipment which is intended to continue to function even after being subjected to a nuclear electromagnetic pulse (EMP
EMP
EMP or Emp may refer to:Science and technology*Electromagnetic propulsion*Electromagnetic pulse, a burst of electromagnetic radiation*Electron microprobe microanalyzer, a tool for chemical composition determination...
) and also would increase the likelihood of an electronic device surviving a near-by lightning strike (direct lightning hits are very hard to protect against).
For UHF TV a wide range of good designs exist, these include a ferrite RF choke on the downlead and some networks of passive components.
RF chokes on leads
RF chokesChoke (electronics)
A choke is a coil of insulated wire, often wound on a magnetic core, used as a passive inductor which blocks higher-frequency alternating current in an electrical circuit while passing signals of much lower frequency and direct current by having an impedance largely determined by reactance, which...
are a well used method for filtering many leads on communication equipment. In common with the braid breaker the intention is to suppress common mode signals. One popular method of making an RF choke is to use a large ferrite bead
Ferrite bead
A ferrite bead is a passive electric component used to suppress high frequency noise in electronic circuits. It is a specific type of electronic choke. Ferrite beads employ the mechanism of high dissipation of high frequency currents in a ferrite to build high frequency noise suppression devices...
where the cable passes through the hole. Such chokes can often be seen on the power connection leads of laptops and on the video signal leads of computer monitors.
The addition of RF chokes can often reduce greatly the RF current flowing through a TV set and hence improve the immunity of the installation.
Filters in the aerial lead
If the receiver's input is routed via a device which rejects the unwanted signals then this additional filtering can improve the immunity of the installation. It is the case that the additional filter will improve the situation. When a transmitter needs to be colocated with a receiver where the frequency difference is very small (for example 145.000 and 145.600 MHz) the cavity filters can be used on both the transmitter and the receiver to improve the purity of the transmitter and the immunity of the receiver.One design for VHF/UHF TV which also has the ability to filter out 144 MHz, 432 MHz and 1296 MHz signals has been created by a Russian (UA9FAD).http://www.df9cy.de/df9cy_tvistop.htm. A design for UHF TV which is designed for the six meter band (50 MHz) has been published by the UK six meter group. http://www.uksmg.org/filter.htm One design for FM radios (band II) has been created by a German (DF9CY) http://www.df9cy.de/tech-mat/df9cy_bcbpf.htm
Different transmitter modes
It is possible for the non-linear circuits within audio equipment to act as an inadvertent radio receiver. In the case of FMFM broadcasting
FM broadcasting is a broadcasting technology pioneered by Edwin Howard Armstrong which uses frequency modulation to provide high-fidelity sound over broadcast radio. The term "FM band" describes the "frequency band in which FM is used for broadcasting"...
transmitters a click may be heard as the RF field appears and also when it disappears.
In the case of amplitude modulation
Amplitude modulation
Amplitude modulation is a technique used in electronic communication, most commonly for transmitting information via a radio carrier wave. AM works by varying the strength of the transmitted signal in relation to the information being sent...
it is possible for the original audio to be recovered by the affected device. AM is still used for longwave
Longwave
In radio, longwave refers to parts of radio spectrum with relatively long wavelengths. The term is a historic one dating from the early 20th century, when the radio spectrum was considered to consist of long, medium and short wavelengths...
and mediumwave
Mediumwave
Medium wave is the part of the medium frequency radio band used mainly for AM radio broadcasting. For Europe the MW band ranges from 526.5 kHz to 1606.5 kHz...
broadcasting, and sometimes in communications equipment used by services such as the fire brigade, military
Military
A military is an organization authorized by its greater society to use lethal force, usually including use of weapons, in defending its country by combating actual or perceived threats. The military may have additional functions of use to its greater society, such as advancing a political agenda e.g...
or police
Police
The police is a personification of the state designated to put in practice the enforced law, protect property and reduce civil disorder in civilian matters. Their powers include the legitimized use of force...
.
A weird animal quacking or an unintelligible human voice is a likely sign that the transmitter is using single-sideband modulation
Single-sideband modulation
Single-sideband modulation or Single-sideband suppressed-carrier is a refinement of amplitude modulation that more efficiently uses electrical power and bandwidth....
. SSB (single-sideband) tends to be used in applications where long range communication is needed. Here, the non-linear device is mixing frequencies, but because the carrier has been suppressed, it is not possible to restore the original audio with such a simple demodulator.
Stereo leads as a dipole
It is the case that a stereoSTEREO
STEREO is a solar observation mission. Two nearly identical spacecraft were launched into orbits that cause them to respectively pull farther ahead of and fall gradually behind the Earth...
system with external speaker attached by long leads to the main unit (holding the audio electronics) can be troubled by EMC problems which are caused by the speaker leads acting as a dipole
Dipole
In physics, there are several kinds of dipoles:*An electric dipole is a separation of positive and negative charges. The simplest example of this is a pair of electric charges of equal magnitude but opposite sign, separated by some distance. A permanent electric dipole is called an electret.*A...
aerial.
One method of curing this is to add a ferrite choke to the speaker leads close to where they enter the main unit.
Rectification in the audio equipment
According to the RSGBRSGB
-Organizations:* Radio Society of Great Britain, a radio operators organization established in 1913* Russian Soviet Government Bureau, informal diplomatic organization in the USA from 1919 to 1921-Videogames:...
handbook one common way in which stereo systems are affected is when the RF current picked up by the speaker cable, and then flows through the feedback circuit into an early stage in the audio amplifier. The RF is then rectified
Rectifier
A rectifier is an electrical device that converts alternating current , which periodically reverses direction, to direct current , which flows in only one direction. The process is known as rectification...
when it flows through the base/emitter junction of a bipolar transistor.