Exact 125MHz indicates it may be the interleave spur of the ADC. The DS1000Z, if I remember correct, use HMCAD1511, which use 8 small ADCs internally interleaved to act as single 1GSPS, dual 500MSPS, or quad 250MSPS ADCs.
Maybe try different vertical scale to see if the noise is scale dependent.
When I set Coupling – GND in the channel menu, the beam becomes thin and does not depend on the position on the screen. So I assumed the problem is in the input amplifier. But may be you're right, not me. I don't have a schematic diagram so I don't know where the input closes to GND. May be at the ADC input.
Thanks again to everyone who is trying to help me figure out the problem. It is very important to set exactly my measurement conditions when you simulating my case on your devices. It is important that:
- Only one channel is involved.
- Acquire menu: Mode – Normal; MemDepth – Auto or 24.0M pts; Anti-aliasing - OFF.
- Channel menu: Scale – 500mV/Div; BW Limit – OFF; Probe – 10X.
- Slider on the probe must necessarily be set to 10X.
Some of you have attached screenshots in which the probe was set to 1X. But noise is lower in such case and test meander and ramp signals looks more symmetrical on my Rigol (see my screenshots). Please see how test meander will look if the probe is switched to 10X.
before, i was using a BNC cable straight from the generator into the scope. Now i'm using a 200MHz probe set to 10x straight into the generator's BNC. Using the settings you describe, auto memory (and 24M points, altho i didn't get a screenshot for 24M) I get the same result, the waveforms appear symmetric no matter what i do.
Since you're getting different results with the different probe setting, the probe being the problem might be the case? have you tried different probes or a BNC cable?
But yeah, if the probe isn't the problem, i would ask for a replacement