4) I tested AC vs DC with the square wave from the internal generator: problem NOT present
5) I then tested a square wave sweeping from 10Hz to 1KHz. For frequencies greater than 200Hz the problem seems no more present.
6) I also tested a sin wave sweep from 10Hz to 1KHz. Again, from 200Hz on, the problem seems no more present.
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.
Here is sine wave and square wave 100 Hz from a signal generator connected through a T-BNC and two coax cables (the same length) into CH1 and CH2. The channel CH2 is always DC coupling. The channel CH2 taken with AC and DC coupling.
Does it means that my unit is defective by design?
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.
Here is sine wave and square wave 100 Hz from a signal generator connected through a T-BNC and two coax cables (the same length) into CH1 and CH2. The channel CH2 is always DC coupling. The channel CH2 taken with AC and DC coupling.
Does it means that my unit is defective by design?
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.
OK, assuming everything was set up right, if the calculated RMS displayed on the scope was 10% low compared to the DMMs at 50Hz with DC coupling, then the scope is garbage.
Here are four screenshots that should be self-explanatory and show a Siglent SDS1104X-E
It looks that Siglent made some change in X-E model RF frontend. Do you hear relay switch when changing DC to AC coupling?
I think you've posted about your scope before and perhaps we've discussed it. It doesn't look good to me....
Here is sine wave and square wave 100 Hz from a signal generator connected through a T-BNC and two coax cables (the same length) into CH1 and CH2. The channel CH2 is always DC coupling. The channel CH2 taken with AC and DC coupling.
Does it means that my unit is defective by design?
That response looks bad but a lower test frequency is needed to see more of what is going on.
I get a difference of -0.5% for AC coupling vs. DC coupling on the scope. Compared to my DMM which measures 1.421V TRMS, AC coupling on the scope is -0.63% different and DC coupling on the scope is -0.13%.
Here are four screenshots that should be self-explanatory and show a Siglent SDS1104X-E with an approximately 1Hz cutoff with AC coupling, both with a direct 1Vrmsinput from a 50R impedance signal generator and with calibrated 10X probes.
Here is sine wave and square wave 100 Hz from a signal generator connected through a T-BNC and two coax cables (the same length) into CH1 and CH2. The channel CH2 is always DC coupling. The channel CH2 taken with AC and DC coupling.
Does it means that my unit is defective by design?
That response looks bad but a lower test frequency is needed to see more of what is going on. At higher frequencies, the square wave will look "tilted". Old AC current probes have the same response and this specification is actually given as "tilt".
Just found that you're doing it for 1 Hz. My previous screenshots was for 50 Hz. Can you please test your scope for sine with 50 Hz?
Your scope is weird. It appears to have a peak in the response at some very low frequency. Perhaps you could do a low frequency sine wave sweep from 0.1Hz to 10 Hz and see if there is a peak?