Integration Design of Compact Balun Filters and Duplexing Balun Filter With Arbitrary Frequency-Selective Response

In this paper, a multifunctional microwave balun filter and balun diplexer design method is introduced. Through the proposed novel coupling structure and the use of miniaturized grid resonators, not only can the overall circuit area be reduced, but also features such as frequency selection, out-of-phase power splitting, and frequency division can be realized. This study actually designed and fabricated two balun filters and one balun diplexer. The simulated predictions of their frequency responses are in good agreement with the measurement results, validating the feasibility of the proposed technique. The results show that the circuit area of the developed balun filter and balun diplexer is approximately
$0.08{\lambda }_{g}^{2}$
and
$0.136{\lambda }_{g}^{2}$
, respectively. They also exhibit satisfactory performance, with in-band insertion loss
$ < 0.7 \mathrm {dB}$
, in-band return loss
$>15 \mathrm {dB}$
, port-to-port isolation
$>30 \mathrm {dB}$
, maximum in-band magnitude imbalance of 0.9 dB, and in-band phase imbalance of 180°±8°.