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How is Noise Floor expressed in units?

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Junior Member level 1
Dec 10, 2011
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I'm wondering in what units Noise floor is expressed? Like Per hertz or something?

Can one figure out how much noise there will be over a certain bandwidth? Take for example that you have a constant noise of -70dB over a certain bandwidth, like 0-5kHz.



The most common way to express noise floor is dBm/Hz. Then you simply take 10*log(BW) and add it to the noise floor to get the noise power in that bandwidth.

For instance, if your noise floor is -174 dBm/Hz, then noise power in 1 kHz is -144 dBm.

If you are given a noise power in dBm. Then you also need the bandwidth of that measurement to calculate dBm/Hz, for a spectrum analyzer this will be the resolution bandwidth.

The concept of noise spectral density (unit V/√Hz respectively V²/Hz) should help to find the answer.

Usually the noise is quantified in terms of power-density with respect to the bandwidth, that is (using "dB" values) dBm/Hz. It can also be expressed in voltage as V/sqrt(Hz) or current as A/sqrt(Hz).
Then, if you have, for instance, a constant power density (that is power over 1Hz bandwith) of -70dBm/sqrt(Hz) over a BW=5kHz, then the total noise power over that bandwidth will be Pn=-70+10*Log(5000)=-33 dBm. If you have, instead voltage or current units, to convert to power you need to know the resistance on which it is applied and use P=V^/R or P=R*I^2 (and, if you want a log form, convert to dBm).

In you post you wrote -70 dB, but dB is not a measurement unit; it represent instead the ratio between two quantities expressed in log-form.
In the picture the title is power(dB) that is wrong: to express power in "dB" you have to use dBm (power referred to 1mW), dBW (power referred to 1W), an so on. The y-axis is in dBV that is a voltage, not a power.

Ok, thanks for the answers.

I'm using a measurement system with LabVIEW to measure total noise of a system with no signal applied. That is, by choosing 'Power Spectral Density', I assume I can calculate the total noise of a given bandwidth by using those formulas. Am I correct?

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