You can use a waveform with a 100% offset inside, as MM_ModSquare, and add an offset of 5% to Out1. With this setting, the amplitude will be always positive, and never negative.
I attach a screenshot with the parameters you have to set.
1.- You have to set a 5% offset.
2.- You have to select the waveform.
3.- You have to select Inverse+sync.
For more details, please check the link: