I have never seen a tranformer used this way. Can someone explain? Looks like isolation.

 

That is called a balun. Differential signals enter on 1 and 2 and exit on 3 and 4. The coupled inductors act as a common mode choke, exhibiting high inductance (and impedance) to common mode signals while have relatively little effect on the differential signals. This is a common technique to filter radiated noise from RF and other high frequency signals.

ak

 

Thanks!

 

That is called a balun. Differential signals enter on 1 and 2 and exit on 3 and 4. The coupled inductors act as a common mode choke, exhibiting high inductance (and impedance) to common mode signals while have relatively little effect on the differential signals. This is a common technique to filter radiated noise from RF and other high frequency signals.

ak

The explanation of working is fine, but the term "Balun" is not. It is a Series Filter.
"Balun" is a short form of the "Balanced" to "Unbalanced" transformer used to interface Balanced Transmission Lines to Unbalanced. Like a 75 Ohm Coax feeder to a 300 Ohm Antenna.

 

its called a common mode filter, used in filtering noise off power lines. very common in switching power supplies.

 

The explanation of working is fine, but the term "Balun" is not. It is a Series Filter.
"Balun" is a short form of the "Balanced" to "Unbalanced" transformer used to interface Balanced Transmission Lines to Unbalanced. Like a 75 Ohm Coax feeder to a 300 Ohm Antenna.

Yup. My bad. A while ago I combined the two in three straight projects, and they got compounded in my little brain.

ak