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Question about Polarity on Oscilloscope Vertical Position / Offset

Question asked by glowitz on Jun 22, 2014
Latest reply on Jun 23, 2014 by glowitz
I am seeing an interesting discrepancy between various Agilent scope products relating to the polarity reading of the Vertical Position/Offset.

On my Agilent U1602A handheld scope, if I adjust the waveform upwards on the scope (positive offset relative to zero baseline), indeed the numerical display shows a positive offset reading (e.g. Offset = +1.0 V)

If I do the same thing on my U1620A (a larger version of Agilent handheld scope), moving the waveform up causes a negative offset reading, and moving it down causes a positive offset reading. 

This puzzles me, so I looked on another Agilent scope I have (4000 series) and it also shows a negative number for offset when moving the waveform in the positive direction.

I hadn't noticed this before, but typically I'm accustomed to expecting a positive offset resulting from a move upwards on the Y axis and find this counter-intuitive.

Can someone explain this behavior and the rationale? But whatever rationale exists, two handheld Agilent scopes work differently, which makes it even more puzzling. 

I suppose it depends on your context for what is the "reference"  --- but if I have, for example, a 0-5V square wave and the baseline (0 volts) is sitting on the center X-axis of the screen -- and then I adjust the waveform "upward" by, say 1 volt (such that now the waveform is between 1 and 6V, the display shows the offset as -1.0V. Really? Isn't the offset 1V? Or is it saying I need to "add" -1V to get the true reading? Of course, this is the same as subtracting 1V -- and it seems much more intuitive to think in terms of upward = positive offset, and downward = negative offset.

I'm interested in what others see on on their scopes and why different models of Agilent scopes would treat vertical offset/position differently.