When I try to repeat the same test using the same settings, even when testing only a single channel (leaving the probe disconnected as Fungus apparently did), I get 60Hz noise that makes it impossible to detect the true noise floor. Noise remained unchanged with or without the ground cable attached to wall socket Earth Ground.
But your probe is not disconnected! Your photo shows that it is connected to the scope, and its tip is left "floating", not connected to any defined potential. It is normal that the probe essentially acts as an antenna then, picking up all kinds of noise from the environment.
I don't know how Fungus had configured the inputs for his earlier measurements, but it am pretty sure it was not the configuration you show:
- When people refer to "open inputs", they mean that nothing at all is connected to the scope's BNC jack.
- To really measure the internal noise of the scope, the inputs should be shorted directly at the BNC jack, or a 50 Ohm termination installed there.
- If you want to measure the background you can expext in a "real world" scenario, you can connect the probe to the scope, and then connect the probe's little ground strap to the tip. Even that little loop can pick up magnetic fields from the environment; we had reports from one user who could receive nearby FM radio stations that way.
Thank you for explaining. I choose to be more clear in my own personal wording in that if I mean for somebody not to connect any probe at all, I will tell them "no probe attached." That's 100% clear, with zero guesswork for veterans or newbies alike. Words like "disconnected" can mean multiple different things. Even "open input" isn't necessarily clear if you've not learned the meaning. "No probe attached" is just that.
I don't have a 50Ω terminator, and I am basically trying to replicate what Fungus did because that is established data we can go by. No, his work is not a gold standard, but it is a nice frame of reference that other people can try to see if we get the same thing.
Currently, my scope's "Probe" settings for each channel are set to
10x, and that's because I normally use my Probes mechanically switched to the 10x setting. But we're doing this particular test with "no probe attached" so...
- When displaying only CH1 and with CH1 set to Probe: 10X: The "Current" column shows about 600uV (Fungus was about 200uV)
- When displaying only CH1 and with CH1 set to Probe: 1X: The "Current" column shows about 60uV (Fungus was about 200uV)
I assume Fungus must have used 1x or 10x, so in either of those cases, my numbers are not close to his.
When I attach my probe to CH1 and mechanically set its switch to 10x, then set the scope's CH1 setting to Probe: 10x too, and then connect the probe's GND to the probe's tip (as per your bullet point 3): The "Current" column shows about
476uV (Fungus was about 200uV) But you will likely explain that as being a matter of the ground strap picking up noise and radio stations.
I then tried your bullet point #2, shorting the inner gold pin of the CH1 BNC to its ground using the shortest wire possible. I get a "Current" column value of
256uV, which is higher than the values Fungus showed, but we don't know exactly how he measured either. Even so, it least this test was in the ballpark of his. So I am largely satisfied with that result.
Thank you for your kind assistance.