Question Regarding XY Mode

n4buq
 

I have, among other models, a 2465B and I have what I assume to is a rather basic question regarding XY mode.

Placing the scope in XY mode and with an AC signal on CH1, with no other channels set to display, I get a display pattern that appears to have both X and Y components (e.g. a 45-degree moving beam). Without a signal on a vertical input, I would have assumed that CH1's voltage would affect only the X and without any vertical input, the beam would move only horizontally but apparently that's not the case.

XY mode does work as I'd expect with signals on, say, CH1 and CH2, but with only one signal on CH1, I didn't expect to see a vertical component. Is there something about that setup that causes the scope to assume a vertical value that's, maybe, equivalent to the horizontal component when no vertical signal is supplied?

If this is outlined somewhere in the manuals, then I apologize in advance and maybe someone can point me to that. From what I've found, though, XY mode seems to be covered pretty lightly in the manuals I have.

Thanks,
Barry - N4BUQ

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