I am trying to generate an equation driven line that is basically a Spiral with a small sinewave superimposed over the spiral line.  I have tried one method using the swept surface with a twist going around a sprial but the frequency changes when I use the twist along path option like in the picture below. I've got the spiral equation in parametric form but for the life of me can't remember the math to impose the sinewave onto an equation. Anyone able to help with ideas?

Thanks!