Research on miniaturized balun in RF front end

The miniaturized balun is studied in this paper. Theoretically, the balun, based on the transformer principle, is analyzed in a new way by combining the numerical methods of electromagnetic field with the microwave network theory and associating the standard scattering parameters with mixed mode scattering parameters. Practically, thorough electromagnetic simulation and optimization, a compact transformer balun with cross coupled planar spiral structure is designed and fabricated by using the integrated passive device (IPD) technology based on silicon substrate with high resistance. The size of the balun is only 1.85 mm×1.6 mm. The bandwidth with 10dB return loss at the single-ended terminal is from 2.07 GHz to 2.80 GHz. Within the frequency range, the insertion loss is less than 0.98dB, the amplitude imbalance is less than 0.43 dB and the phase imbalance is less than 3.8 degrees. The simulation results are validated by the measurement.