Sept. 29, 2014 – Every signal and network is going to have some noise, but good design practices can help minimize some of that “sound.”
The first step in achieving a better design is understanding where network noise comes from.
A typical digital communications network has multiple devices connected to a twisted wire pair, and terminators at both ends. The purpose of these terminators is to match the network impedance.
Impedance (Z) is measured in ohms. It is the ratio of voltage to the flow of current allowed, and is defined as the total opposition, combined effect of capacitance, inductance and resistance that a circuit offers a signal at a given frequency of a device or circuit to the flow of an alternating current (AC) at a given frequency. The key word for networks is frequency, which for each network type is related to the baud rate.
When network impedances are matched, the source and load impedance are equal, resulting in the network’s ability to operate at its peak efficiency. To achieve maximum power in a network, the transfer line impedance must match the source and load impedances. If the impedances are not matched, standing waves resulting from signal reflections will develop along the line. A signal reflection is similar to the behaviour of waves as they pass between mediums of different densities or a wave of water going against a mossy shore where it is largely absorbed versus a rocky pond shore or cliff where we see a lot of reflected ripples. Not having matching impedance (terminators) on a digital network results in an impedance discontinuity or mismatch that will degrade the amplitude and phase accuracy, and results in reflections on the network.
Any cable can become a transmission line when it has a length greater than λ/8 at an operating frequency where λ=300/fMHZ. And all transmission lines have a characteristic impedance Zo that is a function of the line’s inductance and capacitance Zo=(L/C)0.5. Therefore, each spur or terminal on a network is a potential source for these reflections. And because these reflections are of different amplitudes and frequencies, there can quickly be a lot of signals at different amplitudes and wavelengths (frequencies) on any wire pair. The most reliable way to minimize/eliminate these reflections is to be sure the source and load impedances equal the characteristic impedance of the transmission line, as this minimizes signal reflections to eliminate one source of noise.
Though not an impedance problem, another source of noise on networks is electromagnetic interference (EMI) coupling caused by inductance of nearby power lines. This is one reason to use conduit, ground the shield and maintain minimum distances between different cable trays. The IEC has a standard — IEC 61000-5-2:1997 Electromagnetic Compatibility (EMC) Part 5: Installation and Mitigation Guidelines — in which “Section 2: Earthing and Cabling” addresses tray spacing by assigning different classes to different types of cables based on the energy levels they contain. The classes defined in the standard are as follows:
• Class 1 is for cables carrying very sensitive signals.
• Class 2 is for cables carrying slightly sensitive signals.
• Class 3 is for cables carrying slightly interfering signals.
• Class 4 is reserved for cables carrying strongly interfering signals.
• Class 5 and 6 are reserved for medium- and high-voltage supply distribution cables respectively.
Hopefully you now have a better understanding of two common sources of noise in a facility and how to prevent these problems in the first place.
This column originally appeared in the September 2014 issue of Manufacturing AUTOMATION.