Questions and Answers
What is a common issue with mains rectifiers?
Up to what frequency can a 5-kVA power supply fail conducted emissions limits?
What type of devices are known to have similar emissions to mains rectifiers?
What is a common frequency range for switch-mode power converters?
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How many times the switching frequency can a switch-mode convertor have significant levels of emissions?
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What is the primary function of conductors in terms of electromagnetic fields?
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What can be interfered with by emissions from a 70-kHz switching power supply of a personal computer?
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At what distance does the transition to full electromagnetic fields occur?
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What is the frequency of clocks used in some personal computers?
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What happens when the wavelength is comparable with conductor length?
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Why are all conductors considered antennas?
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What is the effect of increasing frequencies on treating conductors as electric or magnetic field emitters and receivers?
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What is the purpose of providing a means to safely transfer lightning currents to the airframe in antenna mounting?
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What happens when the length of a whip antenna equals one fourth of its wavelength?
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What type of devices are banned outright on commercial airline flights to protect avionics systems from interference?
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What is the result of conductors behaving as resonant antennas?
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Why are conductors important in terms of signal contamination?
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What is the typical frequency range of signals produced by Personal Electronic Devices (PED)?
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What is the purpose of bonding between airframe and flight control surfaces?
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What is the relationship between frequency and wavelength in terms of the transition to full electromagnetic fields?
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Which of the following is NOT a source of Electromagnetic Interference (EMI)?
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Why are most airlines extending the ban on portable radios and television receivers?
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What is the primary means of achieving bonding between airframe and flight control surfaces?
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What type of devices are considered intentional emitters of Electromagnetic Interference (EMI)?
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What is a major concern for passengers sitting near critical wiring inside the aircraft?
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What is the typical material used for the inside of the passenger compartment?
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How many miles of electrical wiring can be found in a commercial passenger jet?
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What makes personal electronic devices a source of EMI?
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What is a characteristic of external RF sources?
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How can the airframe affect EMI?
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What is a similarity between the airframe and a satellite dish?
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Why is the airframe a significant contributor to EMI?
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What is the purpose of shielding in EMI management?
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What is the effective frequency range for absorbing low-frequency magnetic waves?
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How does a shield act on EMI?
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What happens to the EMI after passing through a shield?
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Why is effective termination of shields to the connector backshell important?
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What is the purpose of building up the density of shield material?
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What is the effective way to shield high-frequency signals?
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What is the primary purpose of plating the skins of cases and cable shields?
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Study Notes
Mains Rectifiers and Switching Noise
- Mains rectifiers emit switching noise at harmonics of the fundamental frequency, generating considerable frequencies depending on their power.
- A 5-kVA power supply can fail conducted emissions limits up to several megahertz due to the switching noise of its 50- or 60-Hz bridge rectifier.
- Thyristor-based DC motor drives and phase-angle AC power control have similar emissions, which can interfere with long- and medium-wave broadcasting and part of the short-wave band.
Switch-Mode Power Converters
- Switch-mode power converters operate at fundamental frequencies between 2 and 500 kHz.
- They can have significant emissions at 1000 times their switching frequency, interfering with radio communications up to and including the FM broadcast band.
Conductors and Electromagnetic Fields
- All conductors are antennas, converting conducted electricity into electromagnetic fields, which can then leak out into the wider environment.
- Conductors can also convert electromagnetic fields in their locality to conducted electrical signals, with no exceptions to this rule in our universe.
- At distances greater than one-sixth of the wavelength of the frequencies of concern, E and M fields develop into full electromagnetic (EM) fields with both electric and magnetic components.
Resonance and Bonding
- When wavelength is comparable with conductor length, resonances occur, allowing signals to be converted to fields (and vice versa) at almost 100%.
- Bonding is crucial to prevent hazardous surges from being conducted into the airframe via antenna cables or wire harnesses, especially for externally mounted parts such as control surfaces, engine nacelles, and antennas.
Man-Made Sources of EMI
- Intentional radio frequency (RF) emitters like CB radios, remote-controlled toys, and walkie-talkies are banned on commercial airline flights to protect avionics systems from interference.
- Personal Electronic Devices (PED) such as laptop computers, handheld scanners, and game players can produce signals in the 1-MHz range, affecting avionics equipment performance.
- External sources like radio and radar transmitters on the ground or radar from a passing military plane can be even more disruptive due to their high power and frequency.
Effect of Airframe and Shielding
- The aluminium airframe can act as a resonant cavity, compounding the effects of both internal and external EMI by concentrating transient signals and broadcasting interference into nearby equipment.
- Shielding is critical to eliminate EMI, and the frequency of the interfering signal is a crucial concern when designing an effective shielding solution.
- Low-frequency magnetic waves are most effectively shielded by absorbing signals in highly permeable material, while high-frequency signals are most effectively shielded by reducing entry windows and ensuring adequate surface conductivity to ground.
Cable Shielding
- Cable shielding involves placing a conductive material between the cable conductor and its outer jacket, or covering individual conductors within a cable with shielding material.
- The shield can reflect energy or pick up noise and conduct it to ground, preventing EMI from reaching the conductors.
- Shields must be effectively terminated to the connector backshell to prevent radiation entering the system at the backshell/connector/shield interface.
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Description
This quiz covers the emissions of switching noise in power supplies, including linear and switch-mode types, and their impact on conducted emissions limits.