While trying to filter the ripple of my switching PSU, I've come to a phenomenon that
is probably well known to professionals, but seems absurd to me.
Admittedly, my scope is not of the highest quality, but it can sample at 100MHz and gives usually coherent results.
Furthermore, it is only connected to a floating portable computer working on batteries; so, no parasitic current can flow through the ground or through another connection.
The phenomenon is the following: I took an ATX power supply like the one in your desktop computer, and connected the TWO probes of my scope to the ground wire (the black one) of the PS, AT ITS EXTREMITY A (that is, point A is connected to nothing, except the two probes). In my scope (set in AC mode), I could observe a small ripple.
As this seems absurd, I first thought this is an artifact, so I connected the first pin of a toroidal inductor (5mH) to A, the ground probe of the scope to A, and the other probe to the other pin of the inductor (connected to nothing except the probe); surprise ! my scope now show a ripple of up to 40mV, ranging from 10KHz to 20MHz (centered at 4MHz). I was trying to explain this to myself by invoking wave propagation, when the next experience mystified me completely: I connected the ground probe in the other side of the inductor, and the other probe to point A. Then an incredible ripple of up to 0.5V was shown by my scope !
These experiences contradict everything I know on currents, and in particular, why is there an asymmetry regarding the connection of the probes with respect of the two sides of the inductor (the ripple is AC no?)
I would greatly appreciate any insight.
If I shut down the PSU, while maintaining everything connected as is, the scope shows nothing.
If I don't shut down the PSU, but I disconnect the two probes from the ground wire, while maintaining them connected one to the other, the scope shows nothing as well.
Here is what I believe to be a beginning of explanation: the electrical wave is exploring the ground wire, and most of it is reflected at the first pin of the inductor. So, between the PSU and the inductor appears a standing wave. At the other pin, few of the AC components of the wave has passed the inductor, hence the ddp between the two pins of the inductor. If there is no inductor, a similar effect appears (but much less strong) because the two probes have a somewhat different inductance. As convincing as this may seem, this does not explain the asymmetry of the strength of the ripple with respect to the pins of the inductor.