In
electrical engineering
Electrical engineering is an engineering discipline concerned with the study, design, and application of equipment, devices, and systems which use electricity, electronics, and electromagnetism. It emerged as an identifiable occupation in the l ...
, noise margin is the maximum voltage amplitude of extraneous
signal
In signal processing, a signal is a function that conveys information about a phenomenon. Any quantity that can vary over space or time can be used as a signal to share messages between observers. The ''IEEE Transactions on Signal Processing'' ...
that can be algebraically added to the noise-free worst-case input level without causing the output voltage to deviate from the allowable logic voltage level. It is commonly used in at least two contexts as follows:
*In
communications system engineering
Telecommunications Engineering is a subfield of electrical engineering which seeks to design and devise systems of communication at a distance. The work ranges from basic circuit design to strategic mass developments. A telecommunication engin ...
, noise margin is the ratio by which the signal exceeds the minimum acceptable amount. It is normally measured in
decibel
The decibel (symbol: dB) is a relative unit of measurement equal to one tenth of a bel (B). It expresses the ratio of two values of a power or root-power quantity on a logarithmic scale. Two signals whose levels differ by one decibel have a po ...
s.
*In a
digital circuit In theoretical computer science, a circuit is a model of computation in which input values proceed through a sequence of gates, each of which computes a function. Circuits of this kind provide a generalization of Boolean circuits and a mathematical ...
, the noise margin is the amount by which the signal exceeds the threshold for a proper '0' or '1'. For example, a digital circuit might be designed to swing between 0.0 and 1.2
volt
The volt (symbol: V) is the unit of electric potential, electric potential difference (voltage), and electromotive force in the International System of Units (SI). It is named after the Italian physicist Alessandro Volta (1745–1827).
Defi ...
s, with anything below 0.2 volts considered a '0', and anything above 1.0 volts considered a '1'. Then the noise margin for a '0' would be the amount that a signal is below 0.2 volts, and the noise margin for a '1' would be the amount by which a signal exceeds 1.0 volt. In this case noise margins are measured as an absolute voltage, not a ratio. Noise margins for CMOS chips are usually much greater than those for TTL because the V
OH min is closer to the power supply voltage and V
OL max is closer to zero.
**Real digital inverters do not instantaneously switch from a logic high (1) to a logic low (0), there is some capacitance. While an inverter is transitioning from a logic high to low, there is an undefined region where the voltage cannot be considered high or low. This is considered a noise margin. There are two noise margins to consider: Noise margin high (N
MH) and noise margin low (N
ML). N
MH is the amount of voltage between an inverter transitioning from a logic high (1) to a logic low (0) and vice versa for N
ML. The equations are as follows: N
MH ≡ V
OH - V
IH and N
ML ≡ V
IL - V
OL.
Typically, in a CMOS inverter V
OH will equal V
DD and V
OL will equal the ground potential, as mentioned above.
***V
IH is defined as the highest input voltage at which the slope of the
voltage transfer characteristic (VTC) is equal to -1,
[{{Cite book, url=https://archive.org/details/introductiontodi00gopa, title=Introduction to digital electronic circuits, last=Gopal., first=Gopalan, K., date=1996, publisher=Irwin, isbn=0256120897, location=Chicago, oclc=33664747, url-access=registration] where the VTC is the plot of all valid output voltages vs. input voltages. Similarly, V
IL is defined as the lowest input voltage where slope of the VTC is equal to -1.
In practice, noise margins are the amount of noise, that a logic circuit can withstand.
Noise margins are generally defined so that positive values ensure proper operation, and negative margins result in compromised operation, or outright failure.
See also
*
Digital circuit In theoretical computer science, a circuit is a model of computation in which input values proceed through a sequence of gates, each of which computes a function. Circuits of this kind provide a generalization of Boolean circuits and a mathematical ...
*
Signal integrity
Signal integrity or SI is a set of measures of the quality of an electrical signal. In digital electronics, a stream of binary values is represented by a voltage (or current) waveform. However, digital signals are fundamentally analog in nature, ...
*
Substrate coupling In an integrated circuit, a signal can couple from one node to another via the substrate. This phenomenon is referred to as substrate coupling or substrate noise coupling.
The push for reduced cost, more compact circuit boards, and added customer f ...
*
ITU G.992.1
*
signal-to-noise ratio
Signal-to-noise ratio (SNR or S/N) is a measure used in science and engineering that compares the level of a desired signal to the level of background noise. SNR is defined as the ratio of signal power to the noise power, often expressed in deci ...
*
signal
In signal processing, a signal is a function that conveys information about a phenomenon. Any quantity that can vary over space or time can be used as a signal to share messages between observers. The ''IEEE Transactions on Signal Processing'' ...
References
External links
DMT a DSL monitoring and downstream noise margin tweaking program.
MIT PDF of a PowerPoint Presentation on for Digital Noise Margin.
Electronic engineering
Electronic design
Electronic design automation
Integrated circuits