Electrical noise generator

A device that produces electrical noise for use in electrical measurements. Electrical noise generators are commonly employed to measure the noise figure or noise temperature of radio receivers. They are also used in various other tests in radar and communications systems. Celestial noise sources are used to calibrate large antennas.

Some standard types of noise generators are hot-wire, diode, gas-discharge tube, hot and cold loads (terminations), and radio star. A hot-wire noise source consists of the filament of a lamp heated by direct current. Thermal noise having spectral density 4 kTR, where k is Boltzmann’s constant and T and R are the temperature and resistance of the filament respectively, is generated across the terminals of the filament. A diode noise generator utilizes the temperature-limited shot effect to generate noise. At frequencies less than the reciprocal transit time of the diode, the noise spectral density is 2eI, where e is the charge on the electron and I is the average anode current. A gas-discharge noise generator, commonly referred to as a noise tube, consists of a fluorescent light tube enclosed in a waveguide. Noise generation is essentially thermal. The noise tube is commonly employed at microwave frequencies. Hot and cold loads consist of well- matched terminations, either transmission line or waveguide, held at a given temperature by using an oven or by applying cryogenic refrigeration. Noise generation is thermal. Common temperatures for noise-generating terminations are nominally 80 and 300 K (minus 316 and plus 80°F).

Celestial radio sources (radio stars) are commonly employed as reference noise sources for evaluating the characteristics of very low-noise, high-gain space communications receiving antennas. There are a number of accurately calibrated sources available -- the choice depending on system parameters (frequency, antenna gain, system noise temperature, elevation angle, and so forth) and physical location. The most common radio sources employed are Cassiopeia A, Taurus A, Cygnus A, and Orion A. The first three are classified as non-thermal sources in which radiation results from relativistic electrons interacting with an interstellar magnetic field. The electrons are rotated in a plane perpendicular to the magnetic field direction, and radiation is characterized by a component polarized parallel to that plane. The polarized component is small, however, and the major portion of the radiation is unpolarized. The non-thermal sources have flux densities which decrease with increasing frequency and , consequently, tend to have a cutoff frequency above which they are not usable. Orion A is a thermal source in which radiation occurs from a hot, ionized cloud. Orion A has a constant flux density at frequencies above 2 GHz. As a point of reference, a typical value of thermal noise received from Cassiopeia A by a 10-m-diameter (33-ft) antenna operating in the C band (4-6 GHz) would be about 150 K (minus 190 F).

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