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The limiting noise source in a receiver depends on the frequency range in use: At frequencies below about 40 MHz but above about 20 MHz, nearby radio frequency interference caused by human-made devices is the main issue, and lightning in occasional storms that pass within line-of-sight of the antenna.
Electromagnetic interference (EMI), also called radio-frequency interference (RFI) when in the radio frequency spectrum, is a disturbance generated by an external source that affects an electrical circuit by electromagnetic induction, electrostatic coupling, or conduction. [1]
Electromagnetic interference divides into several categories according to the source and signal characteristics. The origin of interference, often called "noise" in this context, can be man-made (artificial) or natural. Continuous, or continuous wave (CW), interference comprises a given range of frequencies.
Jansky recognized three sources of radio noise: [5] (pp 404–405) The first (and strongest) source was local thunderstorms. The second source was weaker noise from more distant thunderstorms. The third source was a still weaker hiss that turned out to be galactic noise from the center of the Milky Way.
In RF applications, noise power is defined using the relationship P noise = kTB, where k is the Boltzmann constant, T is the noise temperature, and B is the noise bandwidth. Typically the noise bandwidth is determined by the bandwidth of the intermediate frequency (IF) filter of the radio receiver. Thus, we can define the noise temperature as:
The noise figure can also be seen as the decrease in signal-to-noise ratio (SNR) caused by passing a signal through a system if the original signal had a noise temperature of 290 K. This is a common way of expressing the noise contributed by a radio frequency amplifier regardless of the amplifier's gain.
A radio noise source is a device that emits radio waves at a certain frequency, used to calibrate radio telescopes, [1] such that received data may be compared to a known value, as well as to find the focal point of a telescope soon after construction, so that the wave guide and front end may be properly located.
A long list of noise measures have been defined to measure noise in signal processing: in absolute terms, relative to some standard noise level, or relative to the desired signal level. They include: Dynamic range, often defined by inherent noise level; Signal-to-noise ratio (SNR), ratio of noise power to signal power
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