I'm studying switching converters and want a bench setup that will allow me to reliably observe the various voltage waveforms (output, inductor, switch) on my oscilloscope.
I discovered pretty quickly that just hooking up my 10x probe with witches hat and ground pigtail wasn't going to work very well, picking up large inductive spikes and hiding the rest of the action in noise:
This image is from the Analog Devices app note here on measuring switching regulator output ripple which is pretty much exactly the sort of thing I'm trying to do. The results I get are quite similar.
That app note advises a setup like this, which I'd like to build up:
The basic idea is a length of \$50\Omega\$ coax soldered directly to the output capacitor (SMD in this case), fed through a DC Blocking Capacitor (the tubular aluminum coupling on scope input 2), and using the built-in \$50\Omega\$ input impedance of the scope.
My scope (Rigol DS1054Z) doesn't have a \$50\Omega\$ option, but I have a feed-through terminator that accomplishes the same thing.
What I don't have is a DC Blocking Capacitor fitting.
So I was wondering. How hard could it be to put together at least a fairly usable one?
So my main question is how to size the capacitor and what type/dielectric would be suitable? I'm happy to order the right type and wait, but would also like to slap in the closest thing I have on hand that would work just to get a feel. The effective bandwidth would optimally be something like 10kHz to 100MHz.
I'm kind of supposing I would use an axial capacitor, solder it inline with the coax center conductor, shrink wrap that, then use some copper foil to wrap it and solder the coax braid to that, then shrink wrap the whole thing, maybe after wrapping some 2 liter Coke bottle plastic around it for mechanical strength.
If SMD caps were going to do the trick I could use a small bit of veroboard in roughly the same sort of way.
Anybody got any pointers on the capacitor selection or construction approach that might help me out?