Cap placement, oscilloscope visual

https://www.youtube.com/watch?v=9EaTdc2mr34

I liked this vid as it gives newbs (me) more reason to deploy the oscilloscope for learning.

I couldn't force myself to watch the entire video but one perhaps amusing anecdote is that scope probe capacitance can either cause sensitive circuits to oscillate, or perhaps stop oscillating when probed.

As an old legend goes at least one big government/military project shipped with a scope probe left attached to a circuit node, and hidden inside the chassis.  :

JR

stop oscillating when probed

Click to expand...

 

lol now that would be frustrating indeed!

This is just like the other video you posted. At high frequencies, return currents follow the path of least inductance which is the path that forms the smallest loop area with the send path which is the path directly under the send path (assuming there is a suitable ground plane). This explains why ceramic bypass caps need to be placed as close as possible to the chip they're bypassing such as with high-speed op amps since currents are returning the chip and not to the ground plane entry point even if the ground plane provides a more direct path. Good to know.