I have some confusion about practical aspects of dealing with white noise.
Let's say you want to quantify or view the white noise component of a power supply output or a constant transducer output. How is the white noise component be extracted from a measured signal in practice? Such a measurement can be done by a scope or data acquisition sampling.
But white noise power spectral is constant over infinite frequency. But how can such a signal as white noise be sampled then? If it has infinite bandwidth what should be the minimum sampling frequency when sampling white noise? Or it doesn't matter?
I know the sampling theorem but this white noise is hard to grasp when it comes to practice. Again an example would help. Let's start we have a 9V power supply and we have access to its terminals. One can see the switching noise from the FFT of the logged output signal as long as sampled at least twice the switching noise or more ect. But suppose one wants to verify the white noise component, what could be done in practice?
Best Answer
When you come to measure the wideband noise of a system, you start with a definition of the bandwidth you're interested in.
How do you define that bandwidth?
It may be a bandwidth that's relevant to your use (say 20Hz to 20kHz if it's an audio application), or it may be that it's limited by your test gear (my 'scope only goes up to 50MHz), but somehow, somewhere, you have to choose a bandwidth. If you're building your own test gear, or you don't know enough yet about your system or application, then it may be an educated guess.
Once you've chosen your bandwidth, you put an anti-alias filter in front of your digitiser (or maybe it's there already, defining your bandwidth). The AA filter passes the whole band of interest, and protects the digitiser from aliasing.
If you don't realise that the measuring instrument has chosen a bandwidth for you, then you may run into calculation or interpretation problems later.
What do you do next?
If your measurements of noise correlate with the behaviour of your system, so it allows you to predict behaviour, design improvements, reject and investigate faults, then it's job done. If you find it doesn't, or suspect there's noise at a higher frequency than you're measuring, then you have to revise your guess. Build, buy or rent better test gear, and measure it to a higher frequency. Rinse and repeat.
This iterative method of seeing what bandwidth is relevant is useful, as it means you don't have to measure to infinite frequency for every job. Start with the simplest measurement that you hope will do the job, then increase cost and complexity if you find it's necessary.