When measuring noise over some bandwidth, how much time should you record? - Page 1
Forum section: electronics / beginners

author: comfortcube
author_rank: Contributor
timestamp: September 14, 2021, 01:17:03 pm
content: I feel 70% sure of the answer but I wanted to make sure. Let's say you are looking at noise of some circuit over a bandwidth of 0.1Hz to 10Hz, or 10Hz to 10kHz, etc. on a scope, and you've placed the appropriate pre-amplifier to look at the bandwidth of interest. You would set the scope to be able to measure just 1 period of the lower end of the frequency range, right? So, for 0.1Hz to 10Hz, that's 1s/div (on a 10 div horizontal), which corresponds to 1 period of 0.1Hz. And for 10Hz, that's 10ms/div, and so on. Is that correct? If you were to use a different time scale, then you are not exactly looking at the bandwidth of interest, yes?For example, what was done in this LT App Note:https://www.analog.com/media/en/technical-documentation/application-notes/an124f.pdfEDIT: I am referring to internal circuit noise - white and 1/f noise. Not external noise sources.

author: fordem
author_rank: Regular Contributor
timestamp: September 14, 2021, 01:45:53 pm
content: I may be misunderstanding what you're trying to achieve, but I don't see that the time you record is related to the bandwidth - I'm defining noise as "un-wanted signal" so the time you record is going to be determined by how long you need to capture the event causing the problem.You could have continuous noise, in which case a few seconds of recording would be sufficient, you could have intermittent noise, or noise that is only produced when a particular piece of equipment is switched on - I once had a problem where noise on a telco circuit caused me weeks of grief until I discovered quite by accident that the problem was only present during business hours, once I made that discovery I was able to track down the source of the noise the very next day - had I been able to record that noise for a twenty four hour period (or longer) I would have resolved the problem a lot sooner.

author: TimFox
author_rank: Super Contributor
timestamp: September 14, 2021, 01:49:14 pm
content: When measuring high-frequency noise, which was "white" (thermal) noise filtered through a band-pass filter, I found that the Marconi 2610 true-RMS meter with selectable averaging time (averaging the mean before the "root" in RMS) was very handy, since the fluctuation in the indicated level was reduced by the longer averaging time. Averaging the RMS value itself is not an accurate method.

author: comfortcube
author_rank: Contributor
timestamp: September 14, 2021, 03:03:28 pm
content: So I am referring to random white and/or possibly 1/f noise internally generated noise in a circuit, not external noise sources like you described in your example.The reason I am thinking you would want to consider time interval is it does seem like, from the examples I've looked at, that the time scale measured is related the bandwidth of noise they are looking at. I remember reading a while ago that the reason noise specifications usually don't go below 0.1Hz is because lower frequency noise would have require longer time spent sampling (I can't remember which LT app note mentioned this), and I think that makes sense; if you set the scope scale to 10ms/div (so the full screen displays at the low end a full period of 10Hz) and there is a relatively large 1Hz noise source, then you will find all that 10Hz+ noise riding up and down the screen on the wave of the 1Hz.Also, I mention scope screen because I would presume the oscilloscope will perform its computation of things like RMS and peak-to-peak voltage over what is displayed on screen.

author: Kleinstein
author_rank: Super Contributor
timestamp: September 14, 2021, 06:49:30 pm
content: The time to record the data and do the calculation sets a lower frequency limit. So with 10 s recording the effective lower cut off is at 0.1 Hz. So the amplifier for the noise should not have an additional roll off at 0.1 Hz. Two filters combines would give a different (higher) lower frequency limit.It depends on the scope which data are used to calculate RMS or Peak to peak values. Some use the screen only, some use the whole data.

author: David Hess
author_rank: Super Contributor
timestamp: September 14, 2021, 06:58:21 pm
content: Quote from: comfortcube on September 14, 2021, 01:17:03 pmThat is right; the lower frequency limit is the inverse of the measurement time.QuoteIt depends on the duration over which the measurement is made. As in your example, 10 seconds is required to measure down to 0.1 Hz. 100 seconds is required to measure down to 0.01 Hz.

author: comfortcube
author_rank: Contributor
timestamp: September 14, 2021, 07:18:29 pm
content: Awesome, thanks guys!