Techniques for selectivity enhancement of lumped-distributed baluns are proposed. Transmission zeros are introduced by adding circuit components to the balun. First, novel balun circuits that possess a transmission zero in differential-mode operation are proposed. A shunt capacitor is then added to the balun to provide a mechanism for tuning the position of the zero. To further improve the frequency selectivity, one more transmission zero that appears in both three-port and differential-mode operations is introduced by adding one transmission line and two capacitors to the balun. The proposed circuits are realised by using a low-temperature co-fired ceramic (LTCC) process and validated by measurements.
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