Mutual Magnetic Coupling Between the Common Modes of Bifilar Windings in Equal-Delay Transformers

A comprehensive model for equal-delay transmission-line transformers (TLTs) is given which includes the coupling of common-mode (CM) magnetic flux between bifilar windings. It is shown that the CM magnetic flux can be thought of as being due to a CM voltage drop or gradient along the bifilar winding. The effect of the coupling of CM magnetic flux on the TLT performance is shown to depend on the topology of the TLT as well as that of the source and the load. A shunt-series, equal-delay TLT which employs two bifilar windings and can operate as a balun/hybrid as well a transformer is analyzed as an example. The coupling of the CM magnetic flux of the two bifilar windings is beneficial in a transformer application for a particular load configuration, but detrimental to balun operation. The new model is compared with lumped circuit analysis, in particular the coupled inductor feature in SPICE, and is thereby verified over the lower portion of the operating frequency range. It is believed that this model has not previously been published.