I am using MSOX4024A to measure a small envelop variation of a large CW signal. As it is large CW signal, I can't have the whole signal peak-to-peak seen in the screen at the small vertical scale. I have to shift 0V axis out of the screen. When I make the vertical scale to 500mV/div, I see 1V ac variation of the envelop. When I make the scale to 1V/div, the variation reduces to 400mV. I increase the scale further to 2V/div, the variation becomes very small. The variation changes unproportionately to vertical scale. If the variation is 1V at 500mV/div, I should still see half of a division in 2V/div but it is much less than it. Indeed, I also see the variation timing change from the vertical scale adjustment.
Is there limitation to decease the vertical scale in large input signal? I am not sure if it is really to have the 1V envelop variation seen at 500mV/div.