I am routing LVDS pair on a flex board. The signal is 264MHz. I used Saturn PCB design to calculate width and spacing between conductors. The targeted impedance for diff pair is 100 ohms, which is dependent on the width/space between conductors and hight to the reference plane. The reference plane is a ground which I choose to be layer 3 (of 4) in my design. Since this is the flex board it is very thin 8 mills total thickness, those it is difficult to create required hight to the reference plane. The reference plane is not far enough to accommodate 100-ohm differential impedance.
I created coplanar waveguide on the same layer as the differential pair to serve as a coupling for the diff pair. Unfortunately, the geometry of the board does not allow me to have this waveguide run all the way to the source. Those now I have a coplanar waveguide with a 6 mills spacing to diff pair guiding LVDS signal half way to the source and after LVDS signal coupled to a reference plane which is 3 mills away (Zdiff = 69 ohms). I also should mention that layer 2 is a power layer, those I believe LVDS signal introduces noise on the power supply. I could move power on layer 3 and have ground on layer 2. However, Zdiff going to get even worse then. I am torn here. Any suggestions on how to route differential pair on a very thin board? Would it be better if I only couple to the ground plane and not use a waveguide?