Interference
What causes the interference signals?
Malfunction can be caused by:
- an external interference signal, for example, caused by common sources of interference, such as electricity cables, microwaves, fluorescent tubes, wireless video cameras, WiFi, Bluetooth mouse, labtop and cordless telephones.
- an internal interference signal caused by the electronic circuit itself such as the logic family used or its switching power supply.
In the case of malfunctions there is electrical energy transfer, this is possible in four ways.
- Galvanic connection via a common line.
- Capacitive coupling via electrostatic fields
- Inductive coupling via magnetic fields
- Electromagnetic coupling via electromagnetic fields and waves
The electrical basics .
| Resistance | Capacity | Induction | Frequency | |
| symbol | R | C | L | f |
| unity | ohm | farad | Henri | Hertz |
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impedance |
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ω = 2πƒ |
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The impedance Z (alternating current resistance) of the R is independent of the frequency, but the ZC and the ZL are frequency dependent.
Alsot the impedance of C decreases at higher frequencies and the impedance of L increases at higher frequencies.
| A magnetic field B is created around a conductor through which a current I flows. If the current changes direction, the magnetic field also changes direction. |
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| If the current changes direction with a high frequency, an E-M (electromagnetic field) is created. |  |
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An electrical signal connection often consists of a signal wire and a return wire. The magnetic field is created by the changing current in the wire. To reduce the magnitude of the magnetic field, B, we can use a twisted pair
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To reduce the magnitude of the magnetic field, B, we can also use a shielded cable. |
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If the electrical signal is a block-shaped signal with a frequency f, then higher frequencies also have an effect, because according to the analysis of fourier, a block-shaped signal is composed of several sinusoidal signals with a higher frequency (the higher harmonics) |
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| Er is een directe relatie tussen de golflengte λ (in m), de frequentie f (in Hz) en de voortplantingssnelheid v van de golf (in m/s) in het betrokken medium.
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How can interference be prevented or limited?
- The starting point is that tackling the source of interference is more efficient than fighting the consequences.
- External interference can be combated by using the Faraday cage principle, placing the electronics in a metal housing and shielding the signal cables
- When designing the circuit take into account the occurrence of possible interference signals, choose the right logic family that fits the circuit not the fastest.
- a suppression capacitor across the power supply connection of an IC, for high frequencies the C forms a short circuit.
- use twisted pair or shielded cables for connections.