Using an earthed function generator with an earthed oscilloscope and ground loops
To measure high-voltage signals up to 40KV, we used 2 channels of the scope and 40KV probes connected to the device being tested. The scope inputs were grounded to earth gnd. In this case the signal ground was NOT used, as the voltages would have arced inside the oscilloscope and destroyed it. The 40KV probes divided the signal by 1,000 so it was within the limits the scope could read. The 2 channels were combined with software to yield a differential result, seeing only the signal across the 2 probes.The same setup can be used to measure lower voltage waveforms without any ground-loop issues. Most scopes have a 50 volt signal limit but the probes that come with the scope normally have a x10 switch built in, allowing a maximum signal level of 500 volts without using special HV probes.If the device being tested has a floating power supply, or you use a isolation transformer, then issues with ground loops go away.