1 chelmsford amateur radio society advanced licence course christopher chapman g0ipu slide set 18:...

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Chelmsford Amateur Radio SocietyAdvanced Licence Course

Christopher Chapman G0IPU Slide Set 18: v1.0, 8-Sep-2004(7) EMC-1: The Causes

Chelmsford Amateur Radio Chelmsford Amateur Radio Society Society

Advanced CourseAdvanced Course(7) EMC (7) EMC

Part-1 - The CausesPart-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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EElectroMMagnetic CCompatibility

• All electronic equipment is capable of radiating and absorbing radio frequency (RF) energy.

• The The principle behind ElectroMMagnetic CCompatibility 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 EMCEMC 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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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)

Field Strength Example-2

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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.

Interference Pickup - 1Interference Pickup - 1

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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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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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• 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.

RF OverloadingRF Overloading

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• 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 . . . .

AM / SSB - Cross Modulation on screen

Cross-Modulation

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• 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.

AM / SSB - Blocking on screen

Blocking Interference

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• 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.

Intermodulation

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• 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

Masthead TV Amplifiers

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• 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.

Passive Intermodulation

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• 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

Image Frequencies

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Entry Routes into TV

Video Amp

CRT

TVAntenna

UHF TV Tuner

6 MHz Sound IF & FM Detector

IF Amp & Detector

Audio Amp

Preamp

VolumeControl

Simplified TV Block Diagram:Each part will be discussed in the following slidesRemedies in EMC session 2

TV IF: 33.5-39.5MHz

UHF: 470-860MHzVideo: 0 - 5MHz

Chroma: 4.43MHz

Sound: 6 MHz

Nicam: 6.5 MHz

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• 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.

Entry via the IF Stage - 1

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TV Baseband TV Baseband PickupPickup

• 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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• 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.

TV Ghosting

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• Any electronic device’s 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.

Coupling to other devices

1 chelmsford amateur radio society advanced licence course christopher chapman g0ipu slide set 18:...

Documents

causes slide

causes emc considerations

emc session

emc potential

emc regulations

emc issues

rf isolation slide

advanced level