GND plane under USB Common mode choke

Question

Wanting to remove common mode noise (CMN) from my USB data line (12/1.5 Mbps speeds) I use common mode choke. My layout is shown below. FL4 is the common mode choke (DLW21HN181SQ2L), and IC3 is a USB isolator ADUM3160. The GND plane is split and my design works as I expect it to be.

I want to take it a step further, and I wonder if removing the GND plane under the USB traces makes sense, (similar idea/question here) since the GND plane will carry CMN, and I suppose CMN will affect the USB data lines after the common mode chock. So at the end I think the common mode chock will have no effect at all (or very little effect) if I have the GND plane underneath it.

So I am thinking removing the GND plane like that:

Since USB (data lines) need GND only for the handshake, removing the GND plane under the data lines wont affect the signal integrity (hopefully) and I suppose it will do a better job in keeping CMN away from my data lines.

(Side note: My design is in metal enclosure with AC input, so metal enclosure is tied to earth)

1. Has anyone tested such layout?
2. What I say makes sense?
3. What problems could that layout cause?

As an extra update to the board, I will try adding a capacitor (C77) between the GND and earth in case it also helps removing CMN. (Idea from here)

Answer 1

You can't remove the ground under the USB data tracks because then you would have an impedance jump where the ground plane starts or ends and in general an impedance mismatch over the whole length. Those lead to reflections and thus, lots of noise.

If you want to ensure that noise from other parts of the circuit doesn't bleed into that ground under the USB tracks, you have to use a wide ground track that is isolated from the ground plane at all but one point.