A quantity used to transmit information is called a "signal." The signal that enters a measuring instrument is called an "input signal," and since it is an electrical quantity, it is called a "voltage signal." Generally, signals from sensors are a continuous quantity of magnitude, so they are called "analog signals." The signal voltage or signal power at any point in time is called the "signal level."
The term "level" does not refer to the voltage or power values themselves, but rather to a value expressed logarithmically (dB) as a ratio to a set reference value. Terms like "noise level" and "vibration level" each have their own defined reference values. Here, "signal level" refers to the magnitude of the signal, and is used, for example, in phrases like "electrical conditions: signal level ±5V or less."
"Noise" refers to interference that is superimposed on a signal and obscures the measured or supplied value. All noise generated inside measuring instruments and other equipment is called "internal noise," noise with a flat frequency spectrum in the frequency band to which it is associated is called "white noise," the average magnitude of the noise is called the "noise level," and the ratio of signal power to noise power is called the "S/N ratio."
"Common-mode voltage" refers to the voltage applied between each of two input terminals (such as the ± terminals of a signal) and a third terminal (for example, the device case or ground terminal). The change in output information caused by the presence of this voltage is called "common-mode interference." Common-mode interference can occur when connecting multiple devices such as sensors and measuring instruments, and this can be prevented by using twisted-pair shielded wires. Also, if the signal shield wire is connected to the case of each device, a loop (ground loop) can be created between the cases and their respective grounds, and unwanted current can flow through this loop, which can affect the signal as noise. Care must be taken to prevent ground loops by grounding only one of the device sides of the signal line's shield wire.
Furthermore, an undesirable input voltage superimposed on the measured voltage is called a "series-mode (normal-mode) voltage," and the resulting change in output information is called "series-mode (normal-mode) interference." This is why, when power lines and weak signal lines are wired together in parallel, noise is superimposed on the weak signal due to electromagnetic induction (interference). This can be prevented by running the wires through iron pipes or by using twisted-pair shielded wire.
When an inverter motor or other equipment is in operation, power lines or control lines from inside the equipment can be accessed.
Interference caused by undesirable currents transmitted through a circuit is called "conducted interference," and "power supply noise" is an example of this. Interference caused by undesirable electromagnetic fields radiated from inside equipment with incomplete shielding is called "radiated interference," the phenomenon in which the operation of equipment is affected by electromagnetic fields is called "electromagnetic interference," and conducted interference and radiated interference are collectively called "unwanted radiation" or "leakage interference."
Heavy electrical equipment such as large motors are prone to generating multiple interferences, and for safety reasons, grounding is performed.
When measuring instruments encounter various types of interference, we often lump them together and say "noise has been added to the signal," but the countermeasures required vary depending on the type of interference, and in reality, implementing these countermeasures is quite difficult.
At the very least, measuring instruments and computers require proper grounding to effectively prevent noise. Ideally, the noise source should implement these measures...
(Excerpt from the email newsletter issued on July 19, 2002)
