# How Does the RMS of White Noise Change with Sampling Frequency?

There is an analog system which includes the continuous-time linear equalizer (CTLE). With some .noise analysis the power-spectral density (PSD) of the noise in that system is provided. So let's not care about the details how PSD is provided, the only thing that matters is that I have access to the noise PSD of the circuit. The noise PSD is not necessarily white and it can have an arbitrary shape, but it dies out after a certain frequency (let's say most energy is contained within 100 GHz). By having the noise PSD, I integrate it over a large enough frequency range (in this example from 0 to 100 GHz) to get the (almost) total power of the noise, let's name it $\sigma^2$.

Now, in my MATLAB simulation I have a clean sampled signal $s(n \,\,\Delta t)$ with sampling frequency $f_s$, where $f_s = 1 / \Delta t$ (To add more details, $s(t)$ is a continuous-time signal that has a bandwidth of 30GHz and I am oversampling $s(t)$ by taking 32 samples per symbol time. So the sampling frequency in this example would be $f_s$ = 32 x 30GHz = 960 GHz . Maybe these details don't matter).

I want to add discrete white Gaussian noise to $s(n \,\,\Delta t)$ such that this discrete Gaussian noise has the same power (i.e. variance) as the continuous (non-white) noise which was calculated to be $\sigma^2$. I know I can do something like $N = \sigma$ x randn( size(s) ) in MATLAB to generate noise with power $\sigma^2$, but my main question is that what is the dependency of the discrete noise in this example to the sampling rate $f_s$. If I change $f_s$ how the randn() function or the $\sigma$ should change.

Cause I see in some pages it is mentioned that:

"Given a continuous White Noise (Wide Sense) with variance $\sigma^2$ and you want sample it at rate of $f_s$ you should generate discrete noise samples with variance of $f_s \, \sigma^2$",

but I don't know what is the mathematical reasoning behind it and if the same applies in my case or not.

If you have any good read on this matter, that would be really appreciated.

• I would refer to this answer dsp.stackexchange.com/a/8632/26081. Ideal sampling is filter with finite bandwidth $f_s$, then you generate samples with variance $f_s \sigma^2$. – AlexTP May 31 '18 at 12:07

## 1 Answer

It changes according to the bandwidth of the LPF Filter in the sampling chain.
Namely if you use the same LPF yet with two different samplings rates the RMS of the noise is the same.

Now, the theoretical RMS of White Noise is given by its PSD which is a constant.
The RMS of the sampled noise is the constant value multiplied by the BW of the LPF in the sampling chain.

• Can you be more mathematically specific about your answer. Basically what you propose does not make sense energy-wise. Cause higher sampling frequency means more frequency separation and hence wider bandwidth for the white noise. So I don't get why the RMS should remain the same. I tried to explain the problem in other words in the following post. Please read it and let me know if you have any explanation on that. dsp.stackexchange.com/questions/49574/… – shampar Jul 19 '18 at 19:52
• It does makes sense. Imagine ideal LPF. Then the RMS of the Noise going through the analog chain (LPF + Sampling) is exactly the White Noise Level multiplied by Bandwidth. – Royi Apr 16 at 9:26