Hi @amp-haus & @skipburrows, being USB powered, the QA403 doesn’t have a solid ground reference. A USB “ground” on a desktop PC might provide a ground in name, but when comparing to instruments with AC plugs, the AC-powered equipment are required to have grounds that are able to handle 33A (per IEC 61010) in the case of a fault, which is something a USB ground could never do.
So, when making measurements you need to keep in mind that a setup without an AC ground reference present will see the AC as a common mode signal and this can be conducted or radiated into the QA403. It’s not really possible to shield power line given the frequency. For H-fields, you can to some extent with exotic metals such as mu-metal. But for E-fields, it’s very difficult when hi-z inputs are involved.
Fortunately, it’s usually just a matter of giving the QA403 a solid ground reference and ensuring it’s a reasonable distance from magnetics associated with power line.
Here’s an experiment to try. Get a grounding plug with an alligator clip. These are commonly available on amazon for ESD control in labs:
https://www.amazon.com/Static-Care-Grounding-Alligator-Garments/dp/B08V5D9L7L
Now, make a measurement on a fully floating setup. In this case, I’ll use a QA403 and QA461 (QA462 is powered by a 12V supply, but that output is floating too. This is common for all ACDC bricks with two wire outputs, including laptop supplies).
Note the power line is ~74 dB below the signal.
Now, take your grounded alligator clip and touch it to any of the BNC on the QA403 or QA462. This gives the floating setup a “ground” reference, and with that reference, the CMRR of the input stages can reject the common signals (power line).
For what it’s worth, I just a got a Rigol MHO98 1 GHz scope to replace my Siglent 300 MHz scope. The MHO98 is a killer scope for $1400 (almost 1/3 the price of the Siglent when it was new). But in cutting costs, Rigol went with a USB-C power brick instead of a mains supply. Normally, a scope’s BNC inputs are tied to ground–they use the IEC PE (protective earth) to ensure the BNC inputs are tied tightly to ground. But with the USB-C brick, that ground reference is no more and there are measurement scenarios that benefit there, too, from a ground reference.
The upshot is that measurement equipment powered from a brick can benefit from a ground reference.
Skip, for your Fig 1, touch a ground to the loopback measurement (any of the BNC shells) and see if that makes it better. Chances are you picking up nearby power line E or H fields. You can verify this by rotating the QA403 in the air (while in loopback) and see if you can impact the height of the power line. If you can, it’s being radiated into the QA403 from nearby equipment and a ground reference will help. If you can’t, then it’s likely being conducted into the QA403 interior via the USB cable.
But in general, if you are seeing power line, it’s because everything is floating and it just needs a ground reference.
