Chelmsford Amateur Radio Society Advanced Course (7) EMC Part-1 - The Causes

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Chelmsford Amateur Radio Society Advanced
Course(7) EMC Part-1 - The Causes
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EMC What is it?

• Nothing has changed from the Foundation Course or
  Intermediate Course.
• It just gets more explained in-depth at Advanced
  Level.
• You can now have 400W to use.
• This may lead to EMC Issues for you and your
  Neighbours.
• Diplomacy at all times please with neighbours.

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ElectroMagnetic Compatibility

• All electronic equipment is capable of radiating
  and absorbing radio frequency (RF) energy.
• The principle behind ElectroMagnetic
  Compatibility is that equipment should limit
  radiation to below a specified level, and be
  able to withstand a certain level of incident RF
  radiation.
• The levels are given in the EMC regulations.
• BS EN 55022 (Computers)
• BS EN 55020 (Radio TV)
• These are only a guide as circumstances vary for
  each location.

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EMC Considerations-1

• Good RF house keeping is vital . . .
• Spurious outputs from transmitters
• Along with unintended leakage wanted RF.
• Too much power radiated.
• Leading to excessive field strength.
• Only use as much power necessary to make the
  contact.
• This will reduce the EMC potential
• Your set-up
• Mode used, Antenna location, Antenna type used
  (Next session).

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EMC Considerations-2

• Considerations . . .
• Filters used (EMC session-2)
• Ferrite ring, High Pass, Low Pass, Band Pass
  Notch.
• Poor immunity of affected device
• Age, construction and use of equipment..
• Proximity for affected item.
• Coupling / Connections, Location.
• Good Quality Coax
• Quality connectors, soldered correctly and water
  tight.

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RF Earths

• Do not use the Mains Earth
• Provide a separate earth point consisting of
  several copper rods in the ground and a thick
  copper wire to the equipment (e.g ATU /
  Transmitter).
• Earth outer of coax cable as it enters any
  building.
• Do not use water / gas pipes as they may not be
  truly earthed.
• BUT REMEMBER - PME from the safety talk . . .
• Requirements as per Safety Slides
• PME cross-bonding can use several Ferrite rings
  to maintain RF Isolation

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Transmitter Field Strength Power Flux Density
(PFD)

• There are two ways to express the strength of RF
  Fields
• Power Flux Density. In a one square metre window
  as Watts per square meter (W/m²)
• Field Strength. In Volts per Meter (V/m)
• To use either you must know the Effective
  Radiated Power (ERP) of the system under test.
• ERP Power to antenna x Gain of antenna
• For gains quoted with reference to a Dipole in
  dBdthere is a formula to remember . . .

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Field Strength Example-1

• Field Strength (V/m) 7x ?ERP / d - where d
  is the distance in metres
• Example
• Antenna 6dBd (4 times), Transmitting 50W
• At a distance of 30m
• Field Strength 7 x ? (50 x 4) / 30 7 x
  14.14 / 30 98.98 / 30
• 3.3 Volts/metre (V/m)

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Field Strength Example-2

• Yagi Antenna 9dBd (8 times), Transmitting 400W
• At a distance of 10m
• Field Strength 7 x ? (400 x 8) / 10 7 x
  48.99 / 10 395.9 / 10
• 39.59 V/m
• As can be seen the field strength in V/m can get
  high.
• These values may exceed recommended EMC levels
  for domestic appliances
• (see graph in EMC Chapter in book, Fig 8.5)

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Interference Pickup - 1

• Direct Pickup
• Interfering signal unaffected by tuning of
  affected equipment.
• May be interfering with any IF stage.
• Antenna may be disconnected and still be
  susceptible to interference.
• More of a problem for VHF and UHF rather than HF
  due to size of equipment (near wavelength size).
• Including the TV antenna (near 70cm in size)
• Move transmitting antenna / equipment further
  apart. Lower Power.

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Interference Pickup - 2

• Conducted Pickup
• Occurs on any interconnecting leads.
• TV, VCR, HiFi, CD, DVD, PC, Phones etc
• Fitting a Ferrite ring on each may help (Next
  Session).
• Pickup on outer (screen) of TV coax down lead.
• Antenna Pickup
• Different to conducted pickup.
• Occurs on centre conductor, from the antenna.
• Fitting a Ferrite ring on each may help (Next
  Session).

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Interference Pickup - 3

• Pickup in the RF Stage
• Direct result of RF on the equipment.
• Harmonics of the transmitter at the wanted
  received frequency.
• Example 2nd harmonic 51.51MHz (FM calling on 6m)
  is in the VHF FM
  Broadcast Band at 103.02MHz.

Long Wave 148.5 to 255 kHz Medium Wave 526.5
to 1606.5 kHz Short Wave 3.95 to 26.1 MHz VHF
FM (Band 2) 87.5 to 108 MHz DAB digital
broadcasting 217.5 to 230 MHz TV channels 21
35 (band 4) 470 to 590 MHz TV channels 37 68
(band 5) 598 to 854 MHz Broadcast
Satellite 11.7 to 12.5 GHz
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RF Overloading

• Strong interfering signal enters via the
  receivers RF stage.
• Filtering before first amplifier is basic (very
  wide in bandwidth)
• Cure by better filtering before affected RF
  stage.
• Interference will be on all channels.
• Strong out of band signals will get in resulting
  in the device being driven out of its linear
  region.
• Distortion is created in the affected amplifier
  resulting in harmonics of the unwanted signal or
  other intermod products.
• Note that harmonics etc are generated internally.
  They are not present on the transmitted
  interfering signal source.

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Cross-Modulation

• A specific form of overloading
• Strong interfering signal enters via the
  receivers RF stage.
• Causes the receiver AGC to vary with the
  amplitude of the strong interfering signal.
• Cure - Better filtering is required before
  affected RF stage.
• SSB, AM and CW are the worst offenders.
• May cause light and dark horizontal lines.
• May affect sound.
• FM may go unnoticed (no amplitude variations)
  but . . . .

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Blocking Interference

• Another form of overloading is Blocking also
  known as De-Sense
• Strong interfering signal enters via the
  receivers RF stage on FM.
• Affects AGC which turns down the gain of IF amp
  leading to reduced sensitivity or picture
  quality.
• Net result - Blank Screen...
• Cure - Better filtering is required before
  affected RF stage.
• Also strong SSB, AM may cause blocking.

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Intermodulation

• InterModulation Products - I.M.P
• Intermodulation is another overloading effect
  where non-linear mixing processes occur.
• A Receiver problem caused by low dynamic range.
• Strong received signals will mix with other
  signals in the radio/TV to create many new
  receivable frequencies, some in range of the
  receiver leading to sound / vision interference .
• Cure- Better filtering is required before
  affected RF stage.
• More on mixing in transmitters interference and
  receivers.
• Cure- filtering is required.
• To reduce the Intermodulation generated in the
  receiver.
• Reducing the receivers input signal bandwidth
  thus eliminating / attenuating frequencies
  responsible for the blocking.

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Masthead TV Amplifiers

• Usually very wide band devices
• Prone to out of band signals
• Common Design Specification 87MHz to 850MHz -
  wider than UHF TV
• Some are even wider - 40MHz to 850MHz.
• Strong Signals cause overloading
• Resulting in Harmonics and Intermodulation in the
  preamplifier
• Can overload the TV causing Cross Modulation and
  Intermodulation.
• Fit Notch Filter
• Watch how the power is fed to this device
• Via coax - power may be lost to device.
• Use a Band Specific Mast Head Preamplifier 470MHz
  to 850MHz

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Passive Intermodulation

• Not a common effect - sometimes called the Rusty
  Bolt Effect or PIM
• Based on nearby conductors picking up strong RF
  and rectifying due to corrosion of the conductor.
  . .
• That then re-radiates a new RF signal.
• Leading to Intermodulation Products
• Conductor could be anything (close by).
• Wire fence, Other antennas, Guttering etc..
• Badly fitted connectors also can cause this
  effect
• Common on 75? TV connectors, also poorly fitted
  PL259s
• Ferrite cores can also be overloaded become
  non-linear
• Resulting in harmonics being generated.

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Image Frequencies

• The image frequency (or second channel) of a
  receiver is (2x IF) from the wanted frequency
  with the local oscillator in the middle.
• More on this in the receiver session
• If the image is picking up the interference the a
  suitable filter may remove it.
• Note that the image frequency will vary as the
  receiver is tuned to other frequencies

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Entry Routes into TV
Simplified TV Block Diagram Each part will be
discussed in the following slides Remedies in
EMC session 2
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Entry via the IF Stage - 1

• Interfering signal is at the IF frequency.
• TV IFs are 33.5 to 39.5MHz.
• A strong signal on 18.1MHz could double (in the
  TV IF amp due to overloading)
• Resulting in on screen patterning, loss of
  colour, picture rolling.
• Could have been a high second harmonic in the
  transmission.
• Typical IF Frequencies on Broadcast Radios
• FM Radio uses 10.7MHz IF. Signals at 10.1MHz may
  cause interference (Direct Pick-up) or a 3.5MHz
  transmission may cause interference from the 3rd
  Harmonic.
• AM Radios have IF at 455 to 500kHz.

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TV Baseband Pickup

• For TV any strong signal up to 6 to 7MHz may
  cause directly problems in baseband video
  circuits.
• Affects the Video as diagonal patterning and
  colour on screen
• Similar the the VHF-UHF FM patterning.
• Sound can also be affected.
• Especially on SSB any PN junction in the audio
  amp may recover the distorted sound of the SSB
  transmission.
• If the volume control has no effect - then the
  point of entry is after the volume control.

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TV Ghosting

• Not a real EMC issue - we just get the blame
  sometimes.
• Named after the visual effect that appears on
  screen
• Double image like that of a ghost a short
  distance to the right
• Caused by multi path signals being reflected off
  large buildings of other man made / natural
  objects
• Delaying the signal so a second image is present.
• One TV line takes 64µS - equating to 1cm of
  picture per 1µS
• Each cm of distance to the ghost image works out
  to 300m of additional path length.

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Coupling to other devices

• Any electronic devices leads may act as
  unintentional antenna
• The most common effect is any PN junction (Diode
  / Transistor) may rectify RF.
• Can lead to an interference problem on the device
  
• Examples TV, VCR, HiFi, CD, DVD, PC, Phones,
  Burglar alarms,
• Radio operated Car locking/alarm systems . . .
  (esp. on 433MHz)
• The item may meet the EMC Directive limit on its
  own.
• But not when installed with long power /
  interconnecting leads.
• A ferrite ring could be used to cure the
  interference.