Hello.

I read some article telling thaf transformer not only reject common mode noise ans also high frequency differential mode noise.

For common mode noise, the rejection is obvious as identical signals on both wires connecting two end termnals of one side of transformer generates zero magnetic field so no coupling to other side.

But differential mode is just what transformer is supposed to receive. How high frequency in this mode is rejected from transformer? Is due to high impedance for high frequency as transformer is nothing but inductor when looking at only one side?

 

How would a transformer that rejects differential mode signals be useful for anything except as a doorstop?

Maybe the author was referring to the fact that some transformers attenuate high frequency differential signals. The reasons for attenuation could be eddy losses, leakage inductance, and large stray capacitance among other possible other reasons.

If the "transformer" being discussed is a Common Mode Choke, then yes, its main purpose is to stop differential signals. A Common Mode Choke is a special case of two or more coupled winding.