Noise-figure meter

Noise-figure meter

A noise figure meter is an instrument for measuring the noise figure of an amplifier, mixer, or similar device. An example instrument is the 1983-era Agilent 8970A.


Measurement methods

Describe automated Y-factor noise measurement

A gated broadband noise source (such as an avalanche diode) drives the device under test. A measurement is made with the noise source on; another measurement with the noise source off. From those measurements and the characteristics of the noise source, the noise figure can be calculated.

Noise source

Some noise figure meters need a calibrated broadband noise source. Several methods are used to generate broadband noise. Some methods require two sources: a "hot" and "cold" source. For high frequency measurements, the noise source will be embedded in a transmission line.

Thermal noise

Noise (electronics)#Thermal noise

Thermal noise in a resistor. Resistor in liquid nitrogen. Resistor in boiling water.

Shot noise

Noise (electronics)#Shot noise

Electrons crossing a gap make discrete arrivals. Impulse. White noise. Compare to thermal electrons.

Motchenbacher & Fitchen (1973, p. 292) describe using a forward biased diode as a calibrated noise source. They also describe a generator made from a low-noise amplifier with a shorted input. Its noise voltage is determined by the shot noise of the amplifier's input transistor.

Vacuum tube

Random noise generators can be made from temperature-limited vacuum tube diodes. (Motchenbacher & Fitchen 1973, pp. 289–291) The vacuum tube's anode (plate) is high enough to collect all the electrons emitted from the hot cathode. The operating conditions are set to avoid a space charge around the filament/cathode that would affect the electron emission. The anode current exhibits shot noise.

The noise current is set by the filament temperature. The current is an exponential function of filament temperature.

At low frequencies, there is 1/f noise. At high frequencies, the transit time of the electron becomes an issue.

Ott (1976, pp. 218–219) describes using a noise diode to measure noise factor.

Zener and avalanche diodes

Voltage breakdown diodes are often used as noise generators. (Motchenbacher & Fitchen 1973, pp. 180–182) There are two breakdown mechanisms: Zener and avalanche. Diodes with the corresponding effects are known as Zener diodes and avalanche diodes. The two mechanisms have different noise behaviors.

The Zener effect (or internal field emission effect) dominates below 7 volts.[1] The junction is thin, and the electric field is large enough that electrons jump the energy gap. The primary noise is shot noise. There is little other noise (excess noise).

Avalanche breakdown is noisier. A carrier traversing the semiconductor junction is accelerated by the reverse-bias field, and it can generate new electron-hole pairs in a collision. Those new carriers can also generate more carriers in a subsequent collisions. The carriers don't arrive singly but rather in bunches. The result is avalanche multiplication of what would have been just shot noise. The spectrum, like shot noise, is white.

Avalanche breakdown can also exhibit multi-state noise. The generated output noise appears to switch between two or more distinct levels. This noise has a 1/f characteristic. The effect can be minimized.

Motchenbacher & Fitchen (1973, pp. 291–292) describe a noise source using a Zener diode (and also suitable for an avalanche diode).

Some commercial microwave noise generators use avalanche diodes to create a large excess noise figure that can be turned off and on. The impedance of the diode is different during the two states, so an output attenuator is used. The attenuator reduces the noise source output, but it minimizes mismatch loss. (Swain & Cox 1983, p. 26)

See also


  1. ^ Motchenbacher & Fitchen (1973, p. 182) graphs noise voltage versus diode breakdown voltage at 250 μA for a family of diodes. At 3 V, the noise volage is about 1 μV per root Hz. At 7 V, the noise is about 28 μV per root Hz.



  • HP Application notes
  • Newer generation manuals
  • Ailtech fixed IF

External links

Wikimedia Foundation. 2010.

Look at other dictionaries:

  • Noise figure — (NF) is a measure of degradation of the signal to noise ratio (SNR), caused by components in a radio frequency (RF) signal chain. The noise figure is defined as the ratio of the output noise power of a device to the portion thereof attributable… …   Wikipedia

  • Noise floor — This article is about physics term. For the Bright Eyes album, see Noise Floor (Rarities: 1998 2005). In signal theory, the noise floor is the measure of the signal created from the sum of all the noise sources and unwanted signals within a… …   Wikipedia

  • Noise generator — A Noise generator is a piece of equipment used to produce random electrical noise, in order to test how electrical equipment may react to noise, or to measure the intensity of noise. Tube noise generators were used for true random number… …   Wikipedia

  • Noise (electronics) — Electronic noise [1] is a random fluctuation in an electrical signal, a characteristic of all electronic circuits. Noise generated by electronic devices varies greatly, as it can be produced by several different effects. Thermal noise is… …   Wikipedia

  • Noise dosimeter — A noise dosimeter (American) or noise dosemeter (British) is a specialized sound level meter intended specifically to measure the noise exposure of a person integrated over a period of time; usually to comply with Health and Safety regulations… …   Wikipedia

  • Meter (music) — Musical and lyric metre. See also: Hymn meter and Poetic meter. Meter or metre is a term that music has inherited from the rhythmic element of poetry (Scholes 1977; Latham 2002) where it means the number of lines in a verse, the number of… …   Wikipedia

  • Jitter — For other meanings of this word, see Jitter (disambiguation). Jitter is the undesired deviation from true periodicity of an assumed periodic signal in electronics and telecommunications, often in relation to a reference clock source. Jitter may… …   Wikipedia

  • List of electronics topics — Alphabetization has been neglected in some parts of this article (the b section in particular). You can help by editing it. This is a list of communications, computers, electronic circuits, fiberoptics, microelectronics, medical electronics,… …   Wikipedia

  • AN/FPQ-6 — The AN/FPQ 6 is a fixed, land based C band radar system used for long range, small target tracking. The AN/FPQ 6 Instrumentation Radar located at the NASA Kennedy Space Center was the principal C Band tracking radar system for Apollo… …   Wikipedia

  • Distortion — This article is about technology, especially electrical engineering. For other uses, see Distortion (disambiguation). Distort redirects here. For other uses, see Distort (disambiguation). A distortion is the alteration of the original shape (or… …   Wikipedia

Share the article and excerpts

Direct link
Do a right-click on the link above
and select “Copy Link”