I´m working on an ethernet switch, the basic setup goes like this: PHY - differnetial mircostrip - transformer - diff. microstrip - connector (see attached image)

I have a ground plane underneath the lines going from PHY to transformer, but I am struggling with what should the plane on the other side of the transformer be connected to. Would it be the point between R1, R2 and C9?
Does the plane even need to be connected to something, isn´t the presence of a metal plane enough to create the differential impedance between the two data lines?

 

I don't think it has to be connected to anything. Just the coupling between the lines and lines to plane is what counts.

 

But wouldn´t the plane, if not connected to anything, be very susceptible to noise?

 

Yea, I think your right. Everything I read says power or ground.

 

In theory you don't need to connect the plane to anything. Because any noise induced on the plane will be capacitively coupled to both lines of the balanced transmission line, and the voltage between the two lines would remain zero.

If the lines are connected to an active device like an amplifier, then there would be a problem if the noise is so great that it exceeds the limits of the amplifier's inputs. Also, I wonder what might happen if the frequency of the noise is outside the bandwidth of the active device.

I can't imagine having a problem with a small plane, though, as long as there are no external wires connected to it which otherwise would act as antennas.

One thing you might consider is splitting the termination resistor into two resistors (in series) and connecting their junction to the plane. (I don't see a termination resistor in your circuit. You really need one, with its resistance equal to the characteristic impedance of the transmission line. Otherwise you'll get hammered with reflections.)