Anyone notice that after you run a calibration with a Rigol scope and then "test" the scope by feeding it some "known" voltage (from a decent function generator) that in addition to channel 1 and channel 2 on the scope reading somewhat differently from the same source that the measurements will also vary based on whether you have just the channel receiving the test signal on by itself, or whether you also have the second (idle) scope channel also turned on?
I've been using 50 ohm termination on both ends and looking at 6Vpp. I use the same cable to switch between the function gen and the scope inputs. Then I toggle on just the active scope input (with the second input off), and then I also toggle on the second idle scope input - and the voltage readings change.
I've found that I seem to get the best resolution when using the Vtop measurement and I've also tried HiRes mode in case that matters. Frequency seems to be consistently accurate within the displayed resolution of the scope counter (a 1 followed by all zeros at 1 MHz).
In some cases (after a fresh calibration) when measuring Vtop I can get the function generator and the scope to agree within about +/-5 millivolts, in other cases it's about +/- 10 millivolts, and occasionally it's about +/-40 millivolts. I've tried running the calibration routine several times (after having the scope turned on for an hour or more).
I realize that scopes are generally not ideal precision instruments for measuring voltage.
What I'm mostly curious about is to see if other Rigol users (or other brand users) find that whatever values you get when measuring with one channel, if you get a somewhat different measurement simply by turning on a second idle channel. Ballpark, I'd say that turning on the second idle channel can change the reading by +/-10 millivolts. In other words, even if I sometimes can get a calibration that shows channel 1 within +/-5 millivolts reading using Vtop (ie, somewhere between 2.995V and 3.005V) based on the expected 6Vpp from the function generator using 1 scope input, when I turn on the second channel (even though it's idle and not connected to anything), I either lose or add 10 millivolts relative to the reading on the active channel by itself. So, 2.995V to 3.005V could become anywhere from 2.985V to 3.015V. Turning off the second idle channel in this example would return the reading to 2.995V to 3.005V. It's not directionally consistent - sometimes turning on the second idle channel creates a plus, and sometimes a minus - but almost always (I think always) turning on a second idle channel changes the measurement reading of the active channel.
It's probably measurement minutiae but I'd like to understand if this is common and what causes it? (It's possible it's some combination of reasons including pilot error, I realize.
) Thx