Noise figure meter
Encyclopedia
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.
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.
Thermal noise in a resistor. Resistor in liquid nitrogen. Resistor in boiling water.
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.
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.
describes using a noise diode to measure noise factor.
s and avalanche diode
s. The two mechanisms have different noise behaviors.
The Zener effect (or internal field emission effect) dominates below 7 volts. 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.
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
.
Noise figure
Noise figure is a measure of degradation of the signal-to-noise ratio , caused by components in a radio frequency signal chain. The noise figure is defined as the ratio of the output noise power of a device to the portion thereof attributable to thermal noise in the input termination at standard...
of an amplifier
Amplifier
Generally, an amplifier or simply amp, is a device for increasing the power of a signal.In popular use, the term usually describes an electronic amplifier, in which the input "signal" is usually a voltage or a current. In audio applications, amplifiers drive the loudspeakers used in PA systems to...
, mixer
Frequency mixer
In electronics a mixer or frequency mixer is a nonlinear electrical circuit that creates new frequencies from two signals applied to it. In its most common application, two signals at frequencies f1 and f2 are applied to a mixer, and it produces new signals at the sum f1 + f2 and difference f1 -...
, 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
Avalanche diode
In electronics, an avalanche diode is a diode that is designed to go through avalanche breakdown at a specified reverse bias voltage. The junction of an avalanche diode is designed to prevent current concentration at hot spots, so that the diode is undamaged by the breakdown...
) 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 noiseThermal noise in a resistor. Resistor in liquid nitrogen. Resistor in boiling water.
Shot noise
Noise (electronics)#Shot noiseElectrons 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 emittedThermionic emission
Thermionic emission is the heat-induced flow of charge carriers from a surface or over a potential-energy barrier. This occurs because the thermal energy given to the carrier overcomes the binding potential, also known as work function of the metal. The charge carriers can be electrons or ions, and...
from the hot cathode. The operating conditions are set to avoid a space charge
Space charge
Space charge is a concept in which excess electric charge is treated as a continuum of charge distributed over a region of space rather than distinct point-like charges...
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 diodeZener diode
A Zener diode is a special kind of diode which allows current to flow in the forward direction in the same manner as an ideal diode, but will also permit it to flow in the reverse direction when the voltage is above a certain value known as the breakdown voltage, "Zener knee voltage" or "Zener...
s and avalanche diode
Avalanche diode
In electronics, an avalanche diode is a diode that is designed to go through avalanche breakdown at a specified reverse bias voltage. The junction of an avalanche diode is designed to prevent current concentration at hot spots, so that the diode is undamaged by the breakdown...
s. The two mechanisms have different noise behaviors.
The Zener effect (or internal field emission effect) dominates below 7 volts. 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
Avalanche breakdown 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 insulators. It is a type of electron avalanche...
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.
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
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 reflections. A transmission line that is properly terminated, that is, terminated with the same impedance as that of the characteristic...
.