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

Chelmsford Amateur Radio Society Advanced Course (7) EMC Part-1 - The Causes. EMC: What is it?. Nothing has changed from the Foundation Course or the Intermediate Course. It just gets more explained in-depth at Advanced Level. You can now have 400W to use.

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

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

  2. EMC: What is it? • Nothing has changed from the Foundation Course or the 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.

  3. 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.

  4. 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 (More in EMC Session-2).

  5. 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 watertight.

  6. 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 (eg. 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

  7. 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 metre (W/m²) • Field Strength.In Volts per Metre (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 . . .

  8. 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)

  9. Field Strength Example-2 Yagi Antenna = 9dBd (8 times), Transmitting 400W At a distance of 10m: Field Strength = 7 x  (400x 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 levelsfor domestic appliances (see graph in EMC Chapter in book, Fig 12.5)

  10. 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.

  11. 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).

  12. 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

  13. 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.

  14. AM / SSB - Cross Modulation on screen 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 . . . .

  15. AM / SSB - Blocking on screen 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.

  16. 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.

  17. 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

  18. 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.

  19. 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

  20. TVAntenna TV IF: 33.5-39.5MHz Video Amp UHF TV Tuner IF Amp & Detector CRT Preamp Video: 0 - 5MHz Chroma: 4.43MHz Sound: 6 MHz Nicam: 6.5 MHz UHF: 470-860MHz 6 MHz Sound IF & FM Detector VolumeControl Audio Amp Entry Routes into TV Simplified TV Block Diagram: Each part will be discussed in the following slides Remedies in EMC Session-2

  21. 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 amplifier 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.

  22. TV Baseband Pickup • For TV any strong signal up to 6 to 7MHz may directly cause 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.

  23. 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 multipath 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 300metres of additional path length.

  24. Coupling to other devices • Any electronic device’s leads may act as an 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.

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