When designing a broadband transformer, there are several parameters
which can be adjusted for an optimized frequency response:
Increased coefficient of coupling, decreased leakage inductance, can be
achieved by placing the primary and secondary windings closer together.
While keeping winding to winding capacitance at a low or optimal value.
Ferrite core loss is reduced by increasing the primary winding
inductance. While keeping secondary inductance and wire length in range
for acceptable leakage inductance and bandwidth.
A set of compromises are demonstrated here:
Core size, stack of two FT114-43, and primary turn count, four turns, is
selected for a Ferrite core loss of about 0.3 dB in the HF band.
Two secondary ratios are shown. The secondary turn counts are selected
to maintain minimum leakage inductance.
The primary windings are interlaced within the secondary windings to
increase flux coupling between the primary and secondary. Increasing
coefficient of coupling.
The location of the primary winding with respect to the secondary
winding is selected for optimal winding capacitance. The capacitance
will aid in canceling the leakage inductance.
The frequency response with the above adjustments is very flat through
the 3.5 to 28 MHz HF band. The device enters resonance conditions above
about 40 MHz.
John KN5L
