RF Electronics Chapter4: Transmission Line Transformers and Hybrids Page 87
2022, C. J. Kikkert, James Cook University, ISBN 978-0-6486803-9-0.
Figure 4.37. Double loop Branchline coupler amplitude response.
It is possible to increase the bandwidth further by using three loops for the coupler. The
outer track impedances are then 187 . For this RO4003 substrate, this corresponds to a
0.027 mm wide track, which is not possible to realise. Doubling the substrate thickness
doubles the track width for the same impedance. Using thicker substrates may make some
of these configurations realisable.
Figure 4.38 and 4.39 show a comparison between the single, double and triple loop
Branchline coupler. The red curves are for the single line coupler, the blue curves are for
the double line coupler and the green curves are for the triple line coupler. In figure 4.38,
the most significant difference is the isolation (between port 1 and 4). The bandwidth
over which a good isolation is obtained is increased significantly by using multiple loops.
However, figure 4.37 shows that for a Microstrip realisation, increasing the number of
loops results in a reduction of the depth of the isolation at the centre frequency of the
hybrid.
Figure 4.38. Ideal Multi-loop Quadrature hybrid amplitude response.
RF Electronics: Design and Simulation
87 www.cadence.com/go/awr
