Measuring noise on DC rails with active probe
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Money no object? R&amp;S has a new probe specifically for this application, with separate AC and DC signal paths so that large offsets can be accomodated while measuring small AC signals. The cost of a new R&amp;S scope to use it with may be prohibitive, though. I'd be surprised if Tek, Keysight, LeCroy, etc., don't come up with similar probes in the near future.
Use a different probe that allows for ac-coupling.
Forget the fancy probe and just contact the power rail through a 50-ohm resistor with the center conductor of a chopped off piece of coax (and contact the o.c. to ground nearby), and ac-couple the other end to your 'scope.