How is Noise Floor expressed in units?

Hello.

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.

Example:

http://www.pimmlabs.com/web/noise_floor_0db_1k_diff.jpg

Thanks

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?