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Question asked by sidney.higuchi on Jan 11, 2012
I'm measuring the 60Hz potential across a 0.1 ohm shunt from a precision AC current source that has a switching power supply.  The 3458A is in ACV SYNC, LF filter ON, RES .002, ACBAND 40, 80.  I'm trying to verify a +/- 0.25% accuracy specification.  The problem seems to be the triggering of the 3458A.  The sample light is not stable, so I assume that the DMM isn't triggering at the same point.  This is the way I could looked at the potential output of the shunt, with a 10x passive probe/HP 35670A analyzer.  The 60Hz harmonics are >35dB down and the switching supply noise is also >35dB down.  I've changed the shunt with a better spec'd one.  The harmonics/noise dropped >46dB down and the 3458A performed better, but it was not stable enough.  Putting a 10kHz low pass filter on the DMM input made the 3458A readings useable.  Had similar results using the Fluke 5790A.
      I have two questions.   Is the harmonics/noise causing the 3458A TARM (?)  to be unstable or is it something else?  Is there a way for me to raise the trigger threshold, so I can trigger on only the 60Hz?