Transcript Advanced Licence Course EMC (2)

Chelmsford Amateur Radio Society
Advanced Course
(7) EMC
Part-2 - The Cures
Chelmsford Amateur Radio Society
Advanced Licence Course
Christopher Chapman G0IPU
Slide Set 19: v1.3, 2-Oct-2007
(7) EMC-2: The Cures
1
Good EMC Station Layout
Balanced Antenna
Morse Key
Use Key Click Filter
Balun (if Coax Feed)
Good Quality Coax
and Connectors
Coax or
Balanced
Feeder
ATU
Filter:LPF for HF or
BPF for VHF/UHF
Do Not
Over-Modulate
Transmitter
VSWR
Meter
Good RF Earth
Do Not Use Mains Earth
Do Not Join Mains & RF Earths
Filter Mains
13.8v
• Screen All Units Well
Especially the Transmitter
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Advanced Licence Course
Restrict Mic
Audio Bandwidth
Christopher Chapman G0IPU
PSU
Slide Set 19: v1.3, 2-Oct-2007
(7) EMC-2: The Cures
2
Choice of Antenna Type
Dipole / Balanced System
EMC Correct Antenna System
• Antenna sited as far away and
as high as possible.
• Good Quality coax run under
ground where possible.
• Coax earthed at point of entry
to house
Antenna as high as possible
and far from House
Balun
Coax drops at
90°to antenna
Station
Coax Underground
• Balun used.
Bond Coax Outer
to RF Earth
• Balanced Antenna System
Chelmsford Amateur Radio Society
Advanced Licence Course
Christopher Chapman G0IPU
Slide Set 19: v1.3, 2-Oct-2007
(7) EMC-2: The Cures
3
Choice of Antenna Type
Long Wire System
Poor EMC Antenna System
• Unbalanced Antenna System.
Strong RF Field
• Strong RF fields near
the house.
Station
• Poor earth system.
Strong RF Field
• Near TV antenna.
RF Earth
Chelmsford Amateur Radio Society
Advanced Licence Course
Christopher Chapman G0IPU
Slide Set 19: v1.3, 2-Oct-2007
(7) EMC-2: The Cures
4
Choice of Antenna Type
Better EMC - Long Wire System
Better EMC Antenna System
• Unbalanced Antenna System.
• Coax fed to
remote ATU.
• Earths at each
end of Coax.
• 7MHz trap keeps
higher frequencies (HF)
away from house.
7MHz Trap
Kept as far as
possible
Good Spacing
Prevent access to
antenna at ground level
Station
ATU
Earth
Earth
Earth
• LF still too close though.
Chelmsford Amateur Radio Society
Advanced Licence Course
Christopher Chapman G0IPU
Slide Set 19: v1.3, 2-Oct-2007
(7) EMC-2: The Cures
5
Choice of Antenna Type
Best EMC - Balanced System
Excellent EMC Antenna System
• Antenna balanced system.
• Well located.
Balanced Antenna as far
as possible from house
• Well Earthed.
Balanced
Twin Feeder
• Away from house.
Balun
• Needs a lot of garden
Coax underground
Earth Rod
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Advanced Licence Course
Station
Christopher Chapman G0IPU
Earth Rod
Slide Set 19: v1.3, 2-Oct-2007
(7) EMC-2: The Cures
6
Resistor Capacitor Filters
RC = Resistor Capacitor
RC High Pass Filter
•
•
•
•
High Pass Filter
Passes high frequencies
Blocks low frequencies (attenuates)
50% Power Cut-off point is at
1/2 (0.707) of the voltage
RC Low Pass Filter
• Low Pass Filter
• Passes low frequencies
• Blocks high frequencies (attenuates)
• 50% Power Cut-off point is at
1/2 (0.707) of the voltage
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C
R
Vin
Vout
R
Vin
Christopher Chapman G0IPU
C
Vout
Slide Set 19: v1.3, 2-Oct-2007
(7) EMC-2: The Cures
7
Response Graph
High Pass Filter
Vout
Vout = Vin
0.707 Vin
Corner of cut-off frequency
Frequency
Chelmsford Amateur Radio Society
Advanced Licence Course
Christopher Chapman G0IPU
Slide Set 19: v1.3, 2-Oct-2007
(7) EMC-2: The Cures
8
Response Graph
Low Pass Filter
Vout
0.707 Vin
Corner of cut-off frequency
Frequency
Chelmsford Amateur Radio Society
Advanced Licence Course
Christopher Chapman G0IPU
Slide Set 19: v1.3, 2-Oct-2007
(7) EMC-2: The Cures
9
LC Cascade  (pi) Filters
• Inductors are used as they
vary in reactance with
frequency (unlike resistors). Vin
• There are three frequency
sensitive elements.
C
L
Vout
C
Vout
LC High Pass Filter
• They roll off three times
faster than RC Filters
• Little attenuation at
pass band
L
L
Vin
C
LC Low Pass Filter
Chelmsford Amateur Radio Society
Advanced Licence Course
Christopher Chapman G0IPU
Slide Set 19: v1.3, 2-Oct-2007
(7) EMC-2: The Cures
10
Band Pass LC Filters
Series
• High Input Impedance.
• Blocking frequencies outside
the pass band.
• Little attenuation at pass band.
Parallel
• Low Input Impedance.
• Blocking frequencies outside
the pass band.
• Shorts unwanted signals to earth.
• Little attenuation at pass band.
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Advanced Licence Course
L
C
Vin
Vin
Christopher Chapman G0IPU
Vout
L
C
Vout
Slide Set 19: v1.3, 2-Oct-2007
(7) EMC-2: The Cures
11
Response Graph
Band Stop / Notch Filter
Pass Band
Loss
Stop Band
0
Loss (dB)
10
20
Pass Band
Pass Band
Frequency
fc
Chelmsford Amateur Radio Society
Advanced Licence Course
Christopher Chapman G0IPU
Slide Set 19: v1.3, 2-Oct-2007
(7) EMC-2: The Cures
12
Cascade Bandpass Filters
Three LC Stages:
L
High Input Impedance
• The input / output
impedance is
governed by the
type of LC circuit
used.
Vin
C
L
Vin
Low Input Impedance
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L
C
C
L
Christopher Chapman G0IPU
Vout
L
C
Vout
C
L
Slide Set 19: v1.3, 2-Oct-2007
(7) EMC-2: The Cures
13
Band Stop / Notch Filters
Series
• Low Impedance at resonance.
• Stops a given band of frequencies
at resonance.
• Passing others.
L
Vin
C
Parallel
• High Impedance at resonance.
• Blocks the unwanted signal.
• Passing others outside of resonance.
Notch Filter
• When response is sharp they are notch
filters, removing a particular frequency.
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Vout
C
Vin
Christopher Chapman G0IPU
L
Slide Set 19: v1.3, 2-Oct-2007
(7) EMC-2: The Cures
Vout
14
Mains Filter
High Voltage Capacitors *
To Equipment
Mains
Voltage Dependent Resistor
*Do not use ordinary Capacitors
• Some Mains Filters/Suppressors will have
three VDR’s across L-N, L-E, N-E
Chelmsford Amateur Radio Society
Advanced Licence Course
Christopher Chapman G0IPU
Slide Set 19: v1.3, 2-Oct-2007
(7) EMC-2: The Cures
15
Ferrite Ring Filters
• Using two Ferrite Rings will
increase inductance.
• Common Mode (Interference)
• Differential Mode (Mains Current)
One or more
ferrite rings
Approx 8 to 15
turns of cable
Mains or
Coax Cable
Common Mode
Differential Mode
(RF Interference)
(Mains)
Unwanted
in-phase
currents
blocked by
choke
Wind only two-thirds
of ferrite
Wanted
antiphase
currents
Chelmsford Amateur Radio Society
Advanced Licence Course
Christopher Chapman G0IPU
Slide Set 19: v1.3, 2-Oct-2007
(7) EMC-2: The Cures
16
Braid Braking Filters
• Signals picked up
by the aerial are in
Differential Mode in
the coax
• Signals picked up
by the braid will be
Common Mode and
will be attenuated.
High Pass Filter & Braid Breaker
C1 4p7
L1
L2
C2 4p7
R1 1M5
L1, L2: 4 turns of 20SWG 6mm Dia x 6mm Long
Chelmsford Amateur Radio Society
Advanced Licence Course
Christopher Chapman G0IPU
Slide Set 19: v1.3, 2-Oct-2007
(7) EMC-2: The Cures
17
Coaxial Trap/Stub Filters
Coaxial Trap/Stub Filter
•
Open circuit
on end
Coax cut to /4 of the
interfering signal
Comprises a /4 of coax,
resonant on the frequency
to be reduced connected in
parallel (Tee’d on to) the
main feeder and
left open circuit.
taking in account the velocity of propagation
in coax (0.66)
FM Broadcast
Receiver
Chelmsford Amateur Radio Society
Advanced Licence Course
Christopher Chapman G0IPU
Slide Set 19: v1.3, 2-Oct-2007
(7) EMC-2: The Cures
18
Ferrite Beads
Audio & Non RF Circuits
(A) has a capacitor across the
base-emitter.
Base-emitter is forward biased.
Behaves like a diode.
Rectifies RF to a Voltage.
Upsetting operation of transistor.
Or adds modulation to wanted signal
(B) has a ferrite bead acting as
a RF choke.
(C) has a ferrite bead in the track added
(if there is space)
(D) has both methods.
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Advanced Licence Course
Christopher Chapman G0IPU
Capacitor is 1nF low
inductance disk ceramic type
Slide Set 19: v1.3, 2-Oct-2007
(7) EMC-2: The Cures
19
Mobile Installations
Considerations
Installations
• Care should be taken
about the installation
of radio equipment in
vehicles.
Out of Field of vision
“Hands Free” Mic
Secure wiring
at all times
30A Main
Fuse
• Any EMC issue can
have SERIOUS
safety implications.
12v 30A Filter
• Fit filters to any power
supply where possible
to radio.
• Keep car cable looms,
Engine Management
Units and RF cables
separate at all times.
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Advanced Licence Course
Radio
Fuses
Easy Access
Remote PTT
Secure
Antenna
Mounts
Easy See / Access for Radios
Christopher Chapman G0IPU
Slide Set 19: v1.3, 2-Oct-2007
(7) EMC-2: The Cures
20
Good EMC House Keeping
• The key thing to remember is:
• Keep your station in good working order.
– Do checks from time to time on the station equipment.
• This means “the emitted frequency of the apparatus comprised in the
Radio Equipment is stable and free from Unwanted Emissions as the state of
technical development of amateur radio apparatus reasonably permits”
• And that
“the bandwidth occupied by the emission is such that not more
than 1% of the mean power of the transmission falls outside the
nominal modulated carrier bandwidth”
• You will need to show this using a wavemeter or the station receiver.
• Always treat any reported EMC issue.
– If you can help a neighbour without invalidating any of their warranties
(try not to modify their equipment).
– Use a quality filter (not a birds nest construction).
Chelmsford Amateur Radio Society
Advanced Licence Course
Christopher Chapman G0IPU
Slide Set 19: v1.3, 2-Oct-2007
(7) EMC-2: The Cures
21
Social Issues
• Be diplomatic
- even if you can’t cure the EMC problem.
– You may have to modify your operating practices.
– Only use as much power necessary to make
the contact - This will reduce the EMC potential
• Remember that the station log may be of
considerable assistance in confirming sources of
interference to neighbours.
– The Complainant should also keep a log, of the
instances of interference.
– If Ofcom becomes involved they may well expect
both parties to keep a log.
• Conduct tests in co-operation with the
complainant in instances of interference.
Chelmsford Amateur Radio Society
Advanced Licence Course
Christopher Chapman G0IPU
Slide Set 19: v1.3, 2-Oct-2007
(7) EMC-2: The Cures
22
Advice
• Advice is available from the RSGB EMC Committee
http://www.rsgb.org.uk/emc
• Local Ofcom officers can also help in cases of interference.
THE END
Chelmsford Amateur Radio Society
Advanced Licence Course
Christopher Chapman G0IPU
Slide Set 19: v1.3, 2-Oct-2007
(7) EMC-2: The Cures
23