In electronics, excess noise ratio is a characteristic of 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 ...

such as a "noise diode", that is used to measure the noise performance of

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 ...

s. 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 ...

method is a common measurement technique for this purpose. By using a noise diode, the output noise of an amplifier is measured using two input noise levels, and by measuring the output noise factor (referred to as ''Y'') 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 the amplifier can be determined without having to measure the amplifier gain.

Background

Any amplifier generates noise. In a

radio receiver In radio communications, a radio receiver, also known as a receiver, a wireless, or simply a radio, is an electronic device that receives radio waves and converts the information carried by them to a usable form. It is used with an antenna. Th ...

the first stage dominates the overall noise of the receiver and in most cases thermal, or

Johnson noise Johnson is a surname of Anglo-Norman origin meaning "Son of John". It is the second most common in the United States and 154th most common in the world. As a common family name in Scotland, Johnson is occasionally a variation of ''Johnston'', a ...

, determines the overall noise performance of a receiver. As

radio signal Radio waves are a type of electromagnetic radiation with the longest wavelengths in the electromagnetic spectrum, typically with frequencies of 300 gigahertz (GHz) and below. At 300 GHz, the corresponding wavelength is 1 mm (shor ...

s decrease in size, the noise at the input of the receiver will determine a lower threshold of what can be received. The level of noise is determined by calculating the noise in a 50

ohm Ohm (symbol Ω) is a unit of electrical resistance named after Georg Ohm. Ohm or OHM may also refer to: People * Georg Ohm (1789–1854), German physicist and namesake of the term ''ohm'' * Germán Ohm (born 1936), Mexican boxer * Jörg Ohm (b ...

resistor A resistor is a passive two-terminal electrical component that implements electrical resistance as a circuit element. In electronic circuits, resistors are used to reduce current flow, adjust signal levels, to divide voltages, bias active el ...

at the input of the receiver as follows: : where: : =

Boltzmann's constant The Boltzmann constant ( or ) is the proportionality factor that relates the average relative kinetic energy of particles in a gas with the thermodynamic temperature of the gas. It occurs in the definitions of the kelvin and the gas constant, ...

= 1.38 × 10⁻²³ J/ K : = Temperature : =

Bandwidth Bandwidth commonly refers to: * Bandwidth (signal processing) or ''analog bandwidth'', ''frequency bandwidth'', or ''radio bandwidth'', a measure of the width of a frequency range * Bandwidth (computing), the rate of data transfer, bit rate or thr ...

Thus, receivers with a narrow bandwidth have a higher sensitivity than receivers with a large bandwidth and input noise can be decreased by cooling the receiver input stage. A noise diode is a device which has a defined excess noise ratio (ENR). When the diode is off (unpowered) the noise from it will be thermal noise defined by the above formula. The bandwidth to be used is the bandwidth of the receiver. When the diode is on (powered) the noise from it will be increased from the thermal noise by the diode's excess noise ratio. This figure could be 6 dB for testing an amplifier with 40 dB gain and could be 16 dB for an amplifier with less gain or higher noise. To determine the

of an amplifier one uses a noise diode at the input to the amplifier and determines the output noise ''Y'' with the diode switched on and off. Knowing both ''Y'' and the ENR, one can then determine the amount of noise contributed by the amplifier and hence can calculate the noise figure of the amplifier. Other techniques exist for making this measurement but either require accurate measurements of impedance or are inaccurate. The following formula relates Y-factor to ENR: :

\mathit=10\log_\frac

Measurements

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 ...

measurements can be made with a noise diode, a power supply for the noise diode, and a

spectrum analyser A spectrum analyzer measures the magnitude of an input signal versus frequency within the full frequency range of the instrument. The primary use is to measure the power of the spectrum of known and unknown signals. The input signal that most co ...

. They can also be made with a specialist

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 8970A8970A Noise Figure Meteris a Keysight product numbers that were formerly part of Agil ...

. The advantage of the noise figure meter is that it will automatically switch the noise diode on and off, giving a continuous reading of Y; it will also have the correct bandwidths in its receiver to average the received noise in an optimum fashion. However, accurate noise figure measurements are possible with the noise figure meter and a spectrum analyser.

References

{{Reflist Noise (electronics) Engineering ratios