TABLE OF CONTENTS There are two basic methods for constructing broadband, impedance-matching transformers. One employs the conventional transformer that transmits the energy to the output circuit by flux linkages; the other uses the transmission line transformer to transmit the energy by a transverse transmission line mode. With techniques exploiting high magnetic efficiency, conventional transformers have been constructed to perform over wide bandwidths. Losses on the order of one decibel can exist over a range from a few kilohertz to over 200 MHz. Throughout a considerable portion of this band, the losses are only 0.2 dB.
On the other hand, transmission line transformers exhibit far wider bandwidths and much greater efficiencies. The stray inductances and interwinding capacitances are generally absorbed into the characteristic impedance of the transmission line. As such, they form no resonances that could seriously limit the high-frequency response. Here the response is limited by the deviation of the characteristic impedance from the optimum value; the parasitics not absorbed into the characteristic impedance of the transmission line; and, in some transformer configurations, the length of the transmission line.
With transmission lines, the flux is effectively canceled out in the core and extremely high efficiencies are possible over large portions of the passband losses of only 0.02 to 0.04 dB with certain core materials.
Therefore, the power ratings of transmission line transformers are determined more by the ability of the transmission lines to handle the voltages and currents than by the size and conventional properties of the core.
The earliest presentation...
