Looking for a cheap or free, frequency-dependent, tremolo plug-in for a work project. I have a tone that I'm raising and lowering by a tritone or so every few seconds. I want to apply a tremolo that would get audibly faster or slower as the pitch rises or falls (respectively). I would be doing this in Reaper. It's sort of a one-time deal, which is part of why I'm not looking to spend a lot on it. Thoughts?