Incorrect Cursor readings on Rigol DS1054Z when scale is changed? - Page 1
Forum section: electronics / beginners

author: nietaki
author_rank: Newbie
timestamp: July 24, 2022, 12:59:21 pm
content: For context: I'm a programmer with some experience with digital electronics and I've recently been learning the basics of analog electronics. I recently got a used oscilloscope in great condition to do experiments on things I'm learning from the handbooks.I wanted to start out with something like a Voltage/Current characteristic of some diodes I had lying around, so I came up with this:...with the voltage on the resistor being a proxy for the current going through the diode. I connected it all up (with ground clips connected to ground, DC coupling, 10x probes) and initially got the result I was expecting:The forward voltage seems to be around 600mV and I could calculate the currents if I wanted to. I got similar results from the X-Y view.however...When I change the scale of the input voltage channel to 500mV/div or less, the voltage values read with the cursor don't make sense anymore - it looks as if the voltage opening up the diode was negative!This is a consistent result, not just a freak "single run". It's not just that the values read by the cursor are incorrect - you can see the trigger level on Channel 0 (which was at 0V), which is above the voltage opening up the diode. I first noticed it in X-Y mode, which had results consistent with this problem.I thought maybe there was something broken with Channel 1, so I moved the probe to Channel 3 and got a similar problem:Any idea what might be happening here? I went through all the menus and didn't find any suspicious setting. The scope is callibrated and so are the probes.I can't find any other explanation other than "my scope is broken for sub-1V scale", but I really hope it's not that - I would really appreciate some help debugging the situation. The reason I got the Rigol was that it was supposed to be a reliable machine for beginners like myself...edit: (please bear with me while I try to figure out how to show the attachments inline)

author: themadhippy
author_rank: Super Contributor
timestamp: July 24, 2022, 01:36:26 pm
content: Looks more like the 0 point is moving,just twiddle the vertical position until its back in the middle

author: nietaki
author_rank: Newbie
timestamp: July 24, 2022, 02:08:01 pm
content: It's not an analog scope, it takes into account the position of the 0 point with the measurementsHere's correct measurements in X-Y mode with different offsets with 1V/div:And incorrect with 500mV/div.

author: ozcar
author_rank: Frequent Contributor
timestamp: July 25, 2022, 06:44:55 am
content: Quote from: nietaki on July 24, 2022, 12:59:21 pmOf course, as soon as the diode starts conducting, the CH1 voltage is no longer the voltage across the diode - not that that would cause what you are seeing.Is the trigger coupling perhaps set to something other than DC? It might be a good idea to restore everything to default and start over (Storage->Default), in case you have something else unexpected set.Incidentally, how are you generating that test waveform? From your first image with CH1 scale set at 1V it appears decidedly non-linear.

author: eugene
author_rank: Frequent Contributor
timestamp: July 25, 2022, 01:07:24 pm
content: I can't explain the weird behavior. I second ozcar's recommendation to reset the scope to defaults.It doesn't address what you're seeing, per se, but you can see the voltage across the diode by setting the math channel to display ch1-ch2. It might be necessary to have both channels on the same scale: either 1V/div or 500mV/div.

author: hexreader
author_rank: Frequent Contributor
timestamp: July 25, 2022, 01:13:31 pm
content: The signal appears to be too large to fit on screen.Your scope gets confused with signals that go off screen. (just a fact of Rigol life)Try again with the Y range set so that the signal remains within top and bottom screen boundaries.