A noise-figure meter is an instrument for measuring the

noise figure Noise figure (NF) and noise factor (''F'') are figures of merit that indicate degradation of the signal-to-noise ratio (SNR) that is caused by components in a signal chain. These figures of merit are used to evaluate the performance of an amplifier ...

of an

amplifier An amplifier, electronic amplifier or (informally) amp is an electronic device that can increase the magnitude of a signal (a time-varying voltage or current). It may increase the power significantly, or its main effect may be to boost the v ...

, mixer, or similar device. An example instrument is the 1983-era

Agilent Agilent Technologies, Inc. is an American life sciences company that provides instruments, software, services, and consumables for the entire laboratory workflow. Its global headquarters is located in Santa Clara, California. Agilent was establi ...

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

Measurement methods

One way to perform the measurement is described on the

Y-factor The Y-factor method is a widely used technique for measuring the gain and noise temperature of an amplifier. It is based on the Johnson–Nyquist noise of a resistor at two different, known temperatures. Consider a microwave amplifier with a 5 ...

page. A noise-figure meter could automate that procedure as follows: A gated broadband noise source (such as an

avalanche diode In electronics, an avalanche diode is a diode (made from silicon or other semiconductor) that is designed to experience avalanche breakdown at a specified reverse bias voltage. The junction of an avalanche diode is designed to prevent current con ...

) 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—a

noise generator 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 ...

. 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 In electrical engineering, a transmission line is a specialized cable or other structure designed to conduct electromagnetic waves in a contained manner. The term applies when the conductors are long enough that the wave nature of the transmis ...

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. 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. 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 Space charge is an interpretation of a collection of electric charges in which excess electric charge is treated as a continuum of charge distributed over a region of space (either a volume or an area) rather than distinct point-like charges. This ...

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. describes using a noise diode to measure noise factor.

Zener and avalanche diodes

Voltage breakdown diodes are often used as noise generators. There are two breakdown mechanisms: Zener and avalanche. Diodes with the corresponding effects are known as

Zener diode A Zener diode is a special type of diode designed to reliably allow current to flow "backwards" (inverted polarity) when a certain set reverse voltage, known as the ''Zener voltage'', is reached. Zener diodes are manufactured with a great varie ...

s and

s. The two mechanisms have different noise behaviors. The Zener effect (or internal field emission effect) dominates below 7 volts. 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. 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 Avalanche breakdown (or avalanche effect) is a phenomenon that can occur in both insulating and semiconducting materials. It is a form of electric current multiplication that can allow very large currents within materials which are otherwise good ...

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 Avalanche breakdown (or avalanche effect) is a phenomenon that can occur in both insulating and semiconducting materials. It is a form of electric current multiplication that can allow very large currents within materials which are otherwise good ...

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. 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 Mismatch loss in transmission line theory is the amount of power expressed in decibels that will not be available on the output due to impedance mismatches and signal reflections. A transmission line that is properly terminated, that is, terminate ...

Notes

References

* * (29MB download) * * *{{Citation , first= Howard L. , last= Swain , first2= Rick M. , last2= Cox , title= Noise Figure Meter Sets Records for Accuracy, Repeatability, and Convenience , journal= HP Journal , date= April 1983 , volume= 34 , issue= 4 , pages= 23–34 , url= http://www.hpl.hp.com/hpjournal/pdfs/IssuePDFs/1983-04.pdf Also *HP Application notes *Newer generation manuals *Ailtech fixed IF Electronic test equipment