You write "It does not show up on
any of the system performance data but I know it
shouldn't be there." What do you mean by this? It
is not affecting your results or the UUT? The power
supply data doesn't spec it? Other?
60 Hz is a sine wave then it is probably facility
power leaking thru. This is probably do to poor
filtering, shielding or grounding. If it is not
a sine wave (ramp, sawtooth, etc) then it is originating
in the power supply (I assume supply is a switcher?).
I'm sure you have checked the obvious, that the
supply has adequate filtering on the output and
the facility power is also adequately grounded and
If there is a poor ground or shield
open then the 60 Hz can be radiating so that why
it is on the 50 ohm lines. I have experienced this
in the past. The external scope is also plugged
into the same facility power as the system so the
facility power can be coupling into the scope through
the power cord (especially the safety ground pin).
Try to isolate this scope by using another power
source, if possible, or a filter in the power input
line. The level is pretty high so you may be able
to use a scope probe to "sniff" around to locate
the source of the 60 Hz.
Some equipment have
internal 60Hz nad 400Hz rejection capability. The
DMM and Dig-Oscope I use here do. Check if your
You can always try another scope to
convince yourself that it is not the problem.
I know my response states the obvious but I
can't see any other reason why you would have 60
Hz feed thru. This kind of problem is always hard
to isolate and will require a lot of trail &
error to locate and eliminate.