What is the difference between Noise Density & Noise Flo

[thotamail]

Hi All,

Can any one explain me what is the difference between Noise Density(ND) & Noise Floor.
According to definitions

1# "Noise density is defined as the average noise power in 1 Hz bin",
that means wide band noise when spread across the Oscilloscope Bandwidth, each 1Hz bin will have a noise of "ND". Isn't it?
Also explain me the difference between dBm to dBm/Hz conversion
lets say a signal of power 0.1uW = -40dBm, Can you convert it to dBm/Hz

2# "Noise floor is defined as the average noise power in “setup specific” bin"
Please can you detail me on this

Thanks & Regards
Vamsi Krishna Thota

 

[DarwinNE]

Re: What is the difference between Noise Density & Noise

I will try to define a little better the terms. With the term "noise" one means a wide range of signals with different origins. If you take for example the thermal noise, you can have a certain power (which depends on the temperature of your apparatus), which is *spread* on a very wide frequency band.
In this case, if you have an ideal Δf wide bandpass filter centered to the frequency F, you get a certain amount of power P (measured in watt). If you, let's say, double the bandpass of your filter to 2*Δf, you will get 2*P. This is what it is called a white noise spectrum.
From a practical point of view, the situation for other kind of noises (and not only noise) is not always the same. Often, some frequency bands can carry much more power than the average. Engineers thus define the spectral power density, which gives a local description: how much power is carried by an infinitely small frequency band around the frequency f? In other words, if we call γ(f) the spectral power density (measured in watt/Hz), the total power is given by the following integral:

\[
P=\int_B \gamma(f)\mathrm{d}f
\]

where the integral should be evaluated in the band B.
Let's come to your questions now

explain me the difference between dBm to dBm/Hz conversion
lets say a signal of power 0.1uW = -40dBm, Can you convert it to dBm/Hz

You need to describe your signal in terms of the power spectral density. For example, you can carry -40dBm with a very pure sinusoid at a frequency of let's say F=1 GHz. Its PSD will have a sharp peak at the frequency F and not very much elsewhere. But you can have the very same power from a noise generator whose spectrum spans from 50 kHz to 10 GHz.
There is not a general rule to convert the power in the PSD. The two signals carry the same power, but have a very different spectral signature.

"Noise floor is defined as the average noise power in “setup specific” bin"
Please can you detail me on this

It means that when you are using your measurement instrument, you are limited in the minimum value of the power spectral density you can detect. This depends btw on the quality and the prize of your instrument, even if there are some limits. This depends also on the configuration of your instrument, and this is why your definition speaks about the "setup specific" stuff.
In a spectrum analyzer, if you put an adapted charge in its input, what you will see is more or less the noise floor of the instrument in the configuration you are using.