Every electrical circuit has noise greater or lesser extent. This becomes undesirable when the signal to noise ratio becomes low enough to adversely affect circuit operation. This is clearly applicable to any electronic device today.
Mechanical or electromechanical devices that cause major changes in current or voltage are common sources of noise. The radio frequency noise can come from walkie-talkies, wireless computer systems and other radio based systems. Typical sources of noise include switching power lines, switching of inductive loads, fluorescent lights, arcs, welding machines, inadequate separation of conductors at different levels, lightning, static discharge, harmonics and ground loops. Noise can occur in both supply lines as in the controls.
Coupled noise signal
NOISE COUPLING METHODS
Noise is often described as a function-like. Is engaged in a circuit. There are five basic types of noise coupling: capacitive, inductive, radio frequency, common impedance conductive.
Electromagnetic interference (RFI by its initials in English) or radiated noise is coupled into a circuit depending on how close is the radiant power of the receiver. In general, if the receiver is less than one sixth of the wavelength of the source, the noise coupling mechanism is dominated by capacitive and inductive effects, but if that distance is greater than one sixth of the wavelength The radiated noise is a plane wave and will be coupled radio frequency effects. This is commonly known as radio frequency interference (RFI).
Capacitiwo or electrostatic noise is coupled to a circuit through a capacitive effect and is based on voltage. A voltage difference between two conductors separated by air or other insulating material, creates a capacitor through which noise is coupled.
Or inductive noise reaches a magnetically coupled circuit via an inductive effect and is based on the current. The current flowing through a circuit induces a current in another circuit noisy. The portion of the circuit in which inductive noise coupling can be seen as a simple loop or a coil inductively coupled coil noise (circuit).
The complex mechanism of coupling of the RFI is based on reflection, absorption and effects of antenna. The effectiveness of RFI coupling in a system is a function of the radiation source, its strength, the characteristics of the transmission path. The distance involved and the receiver sensitivity.
The common impedance noise occurs when there are separate circuits that share common wires (impedance). The ground wires, extensive common neutral and shared return paths can cause common-impedance coupling.
Conducted noise is coupled into a circuit through the transmission of noise due to wiring or other conductive materials. Sooner or later all other sounds become noise driven. Common mode noise is defined between the leads and the circuit reference and common mode noise is defined between circuit conductors and ground.
In general, the factors that determine the noise level are:
- The output impedance of the signal.
- The load impedance of the signal source (input impedance of the receiver circuit).
- Cable length, type of shielding and grounding.
- The proximity to noise sources.
- The amplitude of the signal and noise.
GENERAL GUIDELINES TO DETERMINE THE LENGTH OF THE WIRING
Depending on the type of signals to circulate around them, consider that the length of the cables must adhere to the following:
- Signs of analogic current source to: For a 4-20 mA, shielded cable, bandwidth limited to 10 Hz, required accuracy of 0.5% and average levels of industrial noise. Cable length: 300 to 1500 meters.
- Signs of analog voltage source: For a signal of + / -1 to + / -10 V, shielded cable, bandwidth limited to 10 Hz, required accuracy of 0.5% and average levels of industrial noise. Cable length: 15 to 90 meters.
- Signs of analog voltage source: For a signal 10 MVA 1 V, shielded cable, bandwidth limited to 10 Hz, required accuracy of 0.5% and average levels of industrial noise. Cable length: 1.5 to 30 meters.
- TTL digital signals: With a flat-type cable and average levels of industrial noise. Cable length: 3 to 30 meters.
GROUND RULES
- The shielding at frequencies below 1-10 MHz, should be grounded at one point. The shielding at frequencies above 1-10 MHz can be grounded at several points.
Nonmagnetic materials are only effective against electrostatic noise (capacitive coupling).
- Magnetic materials are effective against electrostatic and magnetic noise.
- The twisted wires are effective against magnetic noise and are generally good application.
- The inductive effects are more significant when the cable length is greater than 1 / 20 of the frequency in question.
- In a loop of instruments without grounding, you should follow the first rule
- The vulnerability of the circuit is coupled noise on the circuit loop exposed to noise. The reduction of the loop, reducing noise.
