Electronic – New Rigol Scope is this Oscillation Normal

500kHz is a ways past the maximum specified frequency for square waves for this generator: 100kHz. It appears to have a single-pole RC of about 3.5µs, which would work great for a 0.35/(3.5µs)=100kHz square wave. The output may have a LPF for slew limiting. Also, it is a 50Ω source, so it should be terminated properly to avoid ringing. Try using the TTL and CMOS waveforms, too. B&K have put together this document: Function & Arbitrary Waveform Generator Guidebook .

The external CMOS inverter is not a 50Ω source -- it's source impedance is only a few ohms at most (for low currents) due to V_CC and ground impedances and FET R_ON equivalent resistances. Notice that the output duty cycle isn't 50%, and the edges are ringing.

Look at the case represented by your first schematics. Let's say there is some noise source that is coupling to the connection between the probe and the 24V supply. Just for example, the coupling is weak and is equivalent to many mega-ohms of impedance. Since the 24V is relatively low impedance, the noise coupling in would not register at all in the measurement.

Loot at the case represented by the second schematics. Let's say the same noise source is coupling to the connection between the probe and the 10Mohm resistor. The impedance of that point to the ground reference is around 5Mohm. So even weak coupling in many mega-ohms of impedance from the noise source will show up in the measurement.

My guess is that the situation can be improved by attaching the 10Mohm resistor to the probe as closely and directly as possible. Then extend the ground around the probe tip with metal foil to cover the connection and the resistor completely.

Look at the case represented by the third schematics. If there is ground loop current between the scope and the 24V supply, the addition of the 10Mohm would cause that to show up. Theoretically, 1uA of ground loop current would show up as 10V with the 10Mohm resistor.

It is asymmetrical to case 2 because any external current coupled to the whole 24V apparatus would show up across the 10Mohm resistor.

The fix could be to hunt down and eliminate any ground loop coupling. For example, it is not unusual for power supplies and equipments to have their functional ground (directly or indirectly) connected to chassis ground through a resistor of mega-ohms and/or a small capacitor in parallel.

But in this case, perhaps decide if this is indeed an issue, possibly aided by an estimate of the ground loop current (for example, take your scope measurement and divide that by 10Mohm). If decided as not a real issue, tolerate it when using the scope and do not use the set up as in schematic 3.