Noise generator

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Zener diode based noise source Zener diode noise source.svg
Zener diode based noise source

A noise generator is a circuit that produces electrical noise (i.e., a random signal). Noise generators are used to test signals for measuring noise figure, frequency response, and other parameters. Noise generators are also used for generating random numbers. [1]

Contents

Theory

There are several circuits used for noise generation. For example, temperature-controlled resistors, temperature-limited vacuum diodes, zener diodes, and gas discharge tubes. [2] A source that can be switched on and off ("gated") is beneficial for some test methods.

Noise generators usually rely on a fundamental noise process such as thermal noise or shot noise.

Thermal noise generator

Thermal noise can be a fundamental standard. A resistor at a certain temperature has a thermal noise associated with it. A noise generator might have two resistors at different temperatures and switch between the two resistors. The resulting output power is low. (For a 1 kΩ resistor at room temperature and a 10 kHz bandwidth, the RMS noise voltage is 400 nV. [3] )

Shot noise generator

If electrons flow across a barrier, then they have discrete arrival times. Those discrete arrivals exhibit shot noise. The output noise level of a shot noise generator is easily set by the DC bias current. Typically, the barrier in a diode is used. [4]

Different noise generator circuits use different methods of setting the DC bias current.

Vacuum diode

One common noise source was a thermally-limited (saturated-emission) hot-cathode vacuum tube diode. These sources could serve as white noise generators from a few kilohertz through UHF and were available in normal radio tube glass envelopes. Flicker (1/f) noise limited application at lower frequencies; electron transit time limited application at higher frequencies. The basic design was a diode vacuum tube with a heated filament. The temperature of the cathode (filament) sets the anode (plate) current that determines the shot noise; see Richardson equation. The anode voltage is set large enough to collect all the electrons emitted by the filament. [5] [6] If the plate voltage were too low, then there would be space charge near the filament that would affect the noise output. For a calibrated generator, care must be taken so that the shot noise dominates the thermal noise of the tube's plate resistance and other circuit elements.

Gas-discharge tubes

Long, thin, hot-cathode gas-discharge glass tubes fitted with a normal bayonet light bulb mount for the filament and an anode top cap, were used for SHF frequencies and diagonal insertion into a waveguide. [7] They were filled with a pure inert gas such as neon because mixtures made the output temperature-dependent. Their burning voltage was under 200 V, but they needed optical priming (pre-ionizing) by a 2-Watt incandescent lamp prior to ignition by an anode voltage spike in the 5-kV range.

For lower frequency noise bands glow lamps filled with neon have been used. The circuit was similar to the one for spike/needle pulses.

One miniature thyratron found an additional use as a noise source, when operated as a diode (grid tied to cathode) in a transverse magnetic field. [8]

Forward-biased semiconductor diode

Another possibility is using the collector current in a transistor.[ clarification needed ]

Reverse-biased semiconductor diode

Reverse-biased diodes in breakdown can also be used as shot noise sources. Voltage regulator diodes are common, but there are two different breakdown mechanisms, and they have different noise characteristics. The mechanisms are the Zener effect and avalanche breakdown. [9]

Zener diode

Reverse-biased diodes and bipolar transistor base-emitter junctions that breakdown below about 7 volts primarily exhibit the Zener effect; the breakdown is due to internal field emission. The junctions are thin, and the electric field is high. Zener breakdown is shot noise. The flicker (1/f) noise corner can be below 10 Hz. [10]

The noise generated by zener diodes is a simple shot noise.

Avalanche diode

For breakdown voltages greater than 7 volts, the semiconductor junction width is thicker and primary breakdown mechanism is an avalanche. The noise output is more complicated. [10] There is excess noise (i.e., noise over and above the simple shot noise) because there is avalanche multiplication.

For higher power output noise generators, amplification is needed. For broadband noise generators, that amplification can be difficult to achieve. One method uses avalanche multiplication within the same barrier that generates the noise. In an avalanche, one carrier collides with other atoms and knocks free new carriers. The result is that for each carrier that starts across a barrier, several carriers synchronously arrive. The result is a wide-bandwidth high-power source. Conventional diodes can be used in breakdown.

The avalanche breakdown also has multistate noise. The noise output power randomly switches among several output levels. Multistate noise looks somewhat like flicker (1/f) noise. The effect is process dependent, but it can be minimized. Diodes may also be selected for low multistate noise. [10]

A commercial example of an avalanche diode noise generator is the Agilent 346C that covers 10 MHz to 26.5 GHz. [11]

See also

Related Research Articles

A cathode is the electrode from which a conventional current leaves a polarized electrical device. This definition can be recalled by using the mnemonic CCD for Cathode Current Departs. A conventional current describes the direction in which positive charges move. Electrons have a negative electrical charge, so the movement of electrons is opposite to that of the conventional current flow. Consequently, the mnemonic cathode current departs also means that electrons flow into the device's cathode from the external circuit.

Diode Electronic component that only allows current to flow in one direction

A diode is a two-terminal electronic component that conducts current primarily in one direction ; it has low resistance in one direction, and high resistance in the other. A diode vacuum tube or thermionic diode is a vacuum tube with two electrodes, a heated cathode and a plate, in which electrons can flow in only one direction, from cathode to plate. A semiconductor diode, the most commonly used type today, is a crystalline piece of semiconductor material with a p–n junction connected to two electrical terminals. Semiconductor diodes were the first semiconductor electronic devices. The discovery of asymmetric electrical conduction across the contact between a crystalline mineral and a metal was made by German physicist Ferdinand Braun in 1874. Today, most diodes are made of silicon, but other semiconducting materials such as gallium arsenide and germanium are also used.

Vacuum tube Device that controls electric current between electrodes in an evacuated container

A vacuum tube, electron tube, valve or tube is a device that controls electric current flow in a high vacuum between electrodes to which an electric potential difference has been applied.

Rectifier Electrical device that converts AC to DC

A rectifier is an electrical device that converts alternating current (AC), which periodically reverses direction, to direct current (DC), which flows in only one direction. The reverse operation is performed by the inverter.

Zener diode

A Zener diode is a special type of diode designed to reliably allow current to flow "backwards" when a certain set reverse voltage, known as the Zener voltage, is reached.

Thyristor Type of solid state switch

A thyristor is a solid-state semiconductor device with four layers of alternating P- and N-type materials. It acts exclusively as a bistable switch, conducting when the gate receives a current trigger, and continuing to conduct until the voltage across the device is reversed biased, or until the voltage is removed. There are two designs, differing in what triggers the conducting state. In a three-lead thyristor, a small current on its Gate lead controls the larger current of the Anode to Cathode path. In a two-lead thyristor, conduction begins when the potential difference between the Anode and Cathode themselves is sufficiently large.

Varicap Varicap (varactor diode) is a semiconductor device can change its depletion region capacitance according to reverse biasing voltage

In electronics, a varicap diode, varactor diode, variable capacitance diode, variable reactance diode or tuning diode is a type of diode designed to exploit the voltage-dependent capacitance of a reverse-biased p–n junction.

In electronics, an avalanche diode is a diode that is designed to experience avalanche breakdown at a specified reverse bias voltage. The junction of an avalanche diode is designed to prevent current concentration and resulting hot spots, so that the diode is undamaged by the breakdown. The avalanche breakdown is due to minority carriers accelerated enough to create ionization in the crystal lattice, producing more carriers which in turn create more ionization. Because the avalanche breakdown is uniform across the whole junction, the breakdown voltage is nearly constant with changing current when compared to a non-avalanche diode.

Silicon controlled rectifier Four-layer solid-state current-controlling device

A silicon controlled rectifier or semiconductor controlled rectifier is a four-layer solid-state current-controlling device. The name "silicon controlled rectifier" is General Electric's trade name for a type of thyristor. The principle of four-layer p–n–p–n switching was developed by Moll, Tanenbaum, Goldey and Holonyak of Bell Laboratories in 1956. The practical demonstration of silicon controlled switching and detailed theoretical behavior of a device in agreement with the experimental results was presented by Dr Ian M. Mackintosh of Bell Laboratories in January 1958. The SCR was developed by a team of power engineers led by Gordon Hall and commercialized by Frank W. "Bill" Gutzwiller in 1957.

Thyratron Gas filled tube, electrical switch, rectifier

A thyratron is a type of gas-filled tube used as a high-power electrical switch and controlled rectifier. Thyratrons can handle much greater currents than similar hard-vacuum tubes. Electron multiplication occurs when the gas becomes ionized, producing a phenomenon known as Townsend discharge. Gases used include mercury vapor, xenon, neon, and hydrogen. Unlike a vacuum tube (valve), a thyratron cannot be used to amplify signals linearly.

Control grid

The control grid is an electrode used in amplifying thermionic valves such as the triode, tetrode and pentode, used to control the flow of electrons from the cathode to the anode (plate) electrode. The control grid usually consists of a cylindrical screen or helix of fine wire surrounding the cathode, and is surrounded in turn by the anode. The control grid was invented by Lee De Forest, who in 1906 added a grid to the Fleming valve to create the first amplifying vacuum tube, the Audion (triode).

Gas-filled tube Assembly of electrodes at either end of an insulated tube filled with gas

A gas-filled tube, also commonly known as a discharge tube or formerly as a Plücker tube, is an arrangement of electrodes in a gas within an insulating, temperature-resistant envelope. Gas-filled tubes exploit phenomena related to electric discharge in gases, and operate by ionizing the gas with an applied voltage sufficient to cause electrical conduction by the underlying phenomena of the Townsend discharge. A gas-discharge lamp is an electric light using a gas-filled tube; these include fluorescent lamps, metal-halide lamps, sodium-vapor lamps, and neon lights. Specialized gas-filled tubes such as krytrons, thyratrons, and ignitrons are used as switching devices in electric devices.

Voltage regulator

A voltage regulator is a system designed to automatically maintain a constant voltage. A voltage regulator may use a simple feed-forward design or may include negative feedback. It may use an electromechanical mechanism, or electronic components. Depending on the design, it may be used to regulate one or more AC or DC voltages.

Gunn diode

A Gunn diode, also known as a transferred electron device (TED), is a form of diode, a two-terminal semiconductor electronic component, with negative resistance, used in high-frequency electronics. It is based on the "Gunn effect" discovered in 1962 by physicist J. B. Gunn. Its largest use is in electronic oscillators to generate microwaves, in applications such as radar speed guns, microwave relay data link transmitters, and automatic door openers.

Voltage-regulator tube

A voltage-regulator tube is an electronic component used as a shunt regulator to hold a voltage constant at a pre-determined level.

Electronic component Discrete device in an electronic system

An electronic component is any basic discrete device or physical entity in an electronic system used to affect electrons or their associated fields. Electronic components are mostly industrial products, available in a singular form and are not to be confused with electrical elements, which are conceptual abstractions representing idealized electronic components and elements.

Diode logic Constructs Boolean logic gates from diodes

Diode logic (DL), or diode-resistor logic (DRL), is the construction of Boolean logic gates from diodes. Diode logic was used extensively in the construction of early computers, where semiconductor diodes could replace bulky and costly active vacuum tube elements. The most common use for diode logic is in diode–transistor logic (DTL) integrated circuits that, in addition to diodes, include inverter logic to provide a NOT function and signal restoration.

Valve RF amplifier Device for electrically amplifying the power of an electrical radio frequency signal

A valve RF amplifier or tube amplifier (U.S.), is a device for electrically amplifying the power of an electrical radio frequency signal.

Technical specifications and detailed information on the valve audio amplifier, including its development history.

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. 8970A Noise Figure Meter is a Keysight product numbers that were formerly part of Agilent.

References

  1. "World Power Systems, PDF, Sylvania Type 6D4" (PDF). Retrieved 6 November 2010.
  2. Motchenbacher & Fitchen 1973 , p. 289
  3. Google Calculator result for 1 kΩ room temperature 10 kHz bandwidth
  4. Ott 1976 , pp. 208,218
  5. Motchenbacher & Fitchen 1973 , pp. 289291
  6. "Philips: Standard noise sources K81A, K50A, K51A" (PDF). Retrieved 14 June 2013.
  7. Hewlett-Packard 1981 Catalog, page 437, "The 347A waveguide sources are argon gas discharge tubes carefully mounted in waveguide sections for frequencies from 3.95 to 18 GHz. Model 349A also uses an argon tube in a coaxial configuration for frequencies from 400 to 4000 MHz."
  8. "Sylvania: 6D4 Miniature triode thyratron data sheet" (PDF). Retrieved 25 May 2013.
  9. Motchenbacher & Fitchen 1973 , p. 180
  10. 1 2 3 Motchenbacher & Fitchen 1973 , p. 181
  11. http://www.home.agilent.com/agilent/product.jspx?nid=-536902744.536880071.00&lc=eng&cc=US, 346C Noise Source, 10 MHz to 26.5 GHz, nominal ENR 15 dB