you are right, actually i missed that force is an current source, consider as voltage source, but the main idea is the same...
In this case, as you said, voltage across Rx is always is fix, voltage across the current source increases depending on RC network impedance which is dtermined by frequency.
A similar method is used to define earth/soil electrical charecteristic, hence there is no real current source, in that applicaiton, force source is an AC voltage source...