risposta da un post su un forum in lingua inglese
"Old oscilloscopes used an AC coupling capacitor in series with the input which was switched in and out of circuit using a relay. More modern designs tend to use a dual path input amplifier with the feedback implementing the AC coupling function because this is much easier to switch electronically, however this requires the time constants of the DC and AC paths to be matched for a flat frequency response above the transition region.
To verify this, look to see how the low frequency transient response changes between AC and DC coupling using a low frequency square wave. When AC coupling is used, a single pole (exponential) high frequency response should be present. A clean response for comparison purposes can be made by placing an approximately 0.19 microfarad capacitor in series with the x1 or x10 probe tip with DC coupling; 0.18 or 0.22 will work fine for testing purposes. In your examples, you can see the problem where the power line signal has a different shape when AC coupling is used.
Another way to test this is to compare the phase and amplitude of low frequency sine waves with one channel set to DC coupling and the other channel set to AC coupling. The difference should be a clean single pole response, but it won't be.
I consider any oscilloscope with this behavior to be defective by design; there is no excuse for it. I would return it for a refund if possible."
stessa nostra conclusione!
@Pier: aggiungo la entry nel CV, quindi!
