hf-protection of electronic circuits thomas losert
TRANSCRIPT
HF-Protection ofElectronic Circuits
Thomas Losert
2
Overview
• Noise Sources (External, Internal)• Basics about Shielding• Examples• Outlook
3
Overview
• Noise Sources (External, Internal)• Basics about Shielding• Examples• Outlook
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External Noise Sources
• External noise is received mostly through I/O-connectors or electromagnetic waves
• Shield I/O connectors (or even the cables) and/or use filters for blocking noise
• Use a star-topology for ground, i.e. connect all ground lines in a common point
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Internal Noise Sources• Internal noise is caused mostly by signals with
high frequency (e.g. Oscillator of Microcontroller) or high currents (e.g. switching power supply)
• Minimize length of circuit paths that could act as antennas for noise signals (e.g., ground-planes)
• Maximize Distance to sensitive signals (e.g. measurement signals)
• Subdivide the system in (nested) „System Zones“ and use filters for blocking noise at the boundaries
• Eliminate noise sources (e.g., no floating input pins)
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Overview
• Noise Sources (External, Internal)• Basics about Shielding• Examples• Outlook
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Basics about Shielding• Due to the „skin effekt“ for higher frequencies
a thinner shield is sufficient(e.g., conductive foil, tape, or paint)
• For higher frequencies (shorter wavelengths) the tolerable gap dimension in the shield decreases
• Noise should be eliminated at the source if possible (e.g., decoupling capacitors)
• Noise sensitive circuits should be separated from noise sources (e.g., ground planes, conductive material, or distance)
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Overview
• Noise Sources (External, Internal)• Basics about Shielding• Examples• Outlook
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Example: Decoupling Capacitors
Power Plane
Micro-control-ler
GroundPlane
Power Plane
Micro-control-ler
GroundPlane
Power Plane
Micro-control-ler
GroundPlane
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Example: Ground Planes• Current can only flow in loops• Larger loops (bigger „antennas“) emit more
noise• Dedicated ground-planes help minimizing the
effective area of the loop• Ground-planes should not be used for routing of
other signals• Adding paths to the ground-plane require a
detailed analysis of every track on the board (otherwise the ground-plane could be wasted)
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Example: Shielded Circuit• Metal case
protects against undesired electro-magnetic waves
• feed-through capacitor for decoupling the power supply
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Example: Subdivision of a System• Further shielding
could be achieved with compartments of conductive material
• For shielding of ventilation openings lattice-like panels are available
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Micro-control-lerLDR
ADC
VCC
GND
Example: Path to Ground
• Separate high noise/current lines and analog/sensitive signals
Micro-control-lerLDR
ADC
AVCC
VCC
GND
AGND
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Overview
• Noise Sources (External, Internal)• Basics about Shielding• Examples• Outlook
15
Outlook
On Saturday, May 1st, 2004same time, same place
„PCB Design“
