Is this an online UPS? True sine wave output on the UPS?
Most on line UPSs take line power, convert it to a DC buss, and then into an AC component. This may be a true sine wave or a nasty stepped square wave...yuk! Look at it with a scope. I'd guess you are trying to measure the AC component of a stepped square wave.
For what it's worth. I use off the shelf office type UPS in an industrial settings. These UPSs are fed from control transformers. I've noticed they have problems returning back to line once on battery. I'm thinking the spike to the autotransformer causes a buck/boost event which the UPS sees as a sag and then returns it to battery; alternating back and fourth. There's a sensitivity switch on most mid range APC models that allow them not be so fussy.
Can you bypass the UPS just for testing purposes?
RSLater,
RSmike
