Try replacing the DD with a reference to the DropDown Object.
Let me see if I can find an example.
#Pull a Reference to the DropDown Object you want to
YourOwnDD = event.source.parent.getComponent('dd_Filter')
#Then branch from there
#If you call from an event generator on the dropdown
event.source.addPopupMenuListener(myPopUpMenuListener(YourOwnDD))
#If you call from anywhere else.
YourOwnDD.addPopupMenuListener(myPopUpMenuListener(YourOwnDD))
#If you call from a a custom method.
self.addPopupMenuListener(myPopUpMenuListener(YourOwnDD))