Noise generator

(For a 1 kΩ resistor at room temperature and a 10 kHz bandwidth, the RMS noise voltage is 400 nV.

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

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.

The temperature of the cathode (filament) sets the anode (plate) current that determines the shot noise; see Richardson equation.

[5][6] If the plate voltage were too low, then there would be space charge near the filament that would affect the noise output.

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.

For lower frequency noise bands glow lamps filled with neon have been used.

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.

[clarification needed] Reverse-biased diodes in breakdown can also be used as shot noise sources.

[9] The Zener effect is primarily exhibited by reverse-biased diodes and bipolar transistor base-emitter junctions that breakdown below about 7 volts.

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