Hi!
I have a post (
https://designhelp.fedevel.com/forum...lter#post19868) which is talking about RC filters, however the genesis of this was this schematic from @phils-lab's Mixed Signal course
specifically, for now, R100 + C102 forming an RC filter for "noise" - and this has lead me to a rabbit hole on noise. I have discovered that there seems to be a
lot of hand-waving when it comes to noise - its a blanket term that feels like it gets thrown around a lot "this component is noisy and so we add a filter" and that's it.
I am eager to dig into this in more detail, as there are, to my knowledge a lot of different types of noise - with reference to the above
- noise is anything that is not the signal we are interested in; so if we expect a 5V DC voltage, then anything that causes that voltage to not be 5V anymore is considered noise? i.e.: we might say, "the VBUS signal is noisy" if it's not a constant 5V; we don't say anything about what causes that noise
- the VBUS signal is coming from a USB port - we don't know for sure how nice the 5V from that port is, it might be nice it might not
- the USB cable might be long, it could be a good quality cable with some ferrite baluns or it might be cheap and nasty and we will get RF and EM radiation coupled into the cable
- L100, C100 and C101 for a Pi LC filter to clean this up?
(what sources of noise are we concerned about here?)- the 5V line after the Pi filter is then used to feed the LDO and the Buck converter
- if whatever is connected to the LDO draws current, we can expect that there might be higher than expected voltage drops on the 5V line and the voltage might droop
the above I am fairly confident in; but please correct me if i made a mistake, what follows I am less confident in
- in the lesson (lesson 5 @ 14:00) @phils-lab mentions the the "switching noise" will not only go to the output but will feed into the other power supplies -
what is this switching noise? is this due to the output switching and causing voltage drops at the switching frequency? is this due to high speed transients causing EM/RF coupling between components and traces or a combination? what does the voltage at "+5v" label look like? is it 5v + some ripple due to noise, and it's the noise component we are filtering using R100/C102?
Ps. I am currently watching this:
https://www.youtube.com/watch?v=49dfOQbmKvY - hopefully some insights; also please see this related post based on a series of videos from @robertferanec on noise (
https://designhelp.fedevel.com/forum...clarifications) and also this post on Ferrite beads (
https://designhelp.fedevel.com/forum...-ferrite-beads)
P.P.S looking at this circuit and this question in particular i dug deeper into RC filters and looking at current flows, which resulted in this post (
https://designhelp.fedevel.com/forum...ow-pass-filter) - leading to the question - is the reverse current that RC filters can generate in issue in this case?
thanks!