Need help on AD8036 chip application

Hello:

I am using an AD8036 clamping amplifier in this scheme

It is used to clamp a square wave with boundaries +3.8V and +1.2V at 9KHz. Output is connected to a UMJ-123-D14-G VCO.  This is the output I get:


Now I got very strange rising edge and falling edge.

Could someone please explain the reason and give me some advise to solve this problem?  I need high speed, high accuracy on timing (low jitters of rising and falling) and very stable voltage step.  Change another chip is also appreciable.

Sincerely,