Square wave DC vs AC coupling

[vk6zgo]

Hi.

I am using a cheap ebay signal generator to display a 1Vpk 1Khz square wave to my oscilloscope.
When the scope is in DC coupling the signal goes from +500mV to -500mV from 10% to 90% duty
cycle and its vertically centered.

I then change the scope to AC coupling and when the duty cycle is less or greater than 50% the
signal is offset positive or negative on the display.  Shouldn't the signal remain vertically centered
despite the changing duty cycle?

Is this normal ?
Thanks.

Some generators produce square waves which are not symmetrical around zero volts,
The signal from these will approximate to zero volts on one half cycle,so you will see  one half cycle at (near as dammit) zero volts,& the other half cycle elevated to + or - whatever voltage is set by the generator controls..

Go to AC coupling,& the square wave will be displayed symmetrically about zero volts.
If you now change the duty cycle of the signal,(incidentally,it is then no longer a square wave,it is a "rectangular wave"),the signal distribution above & below zero volts will change------this is normal.

This case is fairly straight forward,but many generators produce a square wave symmetrical about zero volts.
In this case,the same thing happens with AC coupling as before,but it can be a bit more difficult to discern.

It is exactly the same process---the AC coupling circuit doesn't really care!