Vortex Sound Research

Hi! Im trying to learn FM with FM8. Im doing quite ok and understand the basics. There is one thing i cant grasp how to do, or if you can do it.

OP_F is carrier. OP_E is modulating OP_F.
Now i would like to have OP_Es level start on 0 and slowly go up to 100% then back to 0 and repeat. Like a LFO modulation. So i try to modulate OP_E with OP_D but this mostly adds sounds. I cant find a way to only control how much OP_E modulates OP_F. I know it can be done with the envelop. But id like to experiment with being able to do it like this as well, for example if i sue the envelope for something else.