Maybe you can use a square wave source and also use a DC offset that suits you so that the total value goes up and down and you can see the result, you can set it at a low frequency.
Also i think you can set the square wave to swing between the normal value and a lower value that you want for testing (not 0) so that you don't have to use the DC offset
You can also do the same trick using a triangle or sine wave.
There are probably custom source types that can be programed (custom signals) but i can''t help you on that.
Alex