The design of a compact LTCC balanced filter for Bluetooth applications

This paper reports on a design of a balanced filter based the miniaturized multilayer LTCC technology (Low Temperature Co-Fired Ceramics) which is widely used for Bluetooth and Wireless LAN. It is consist of a folded SIR (stepped impendence resonator) bandpass filter with two transmission zero and a improved Marchand balun. The two elements are cascaded via a through hole in vertical direction for further reduction of the whole size. This balanced filter transforms a 50Ω unbalanced input to 50Ω balanced output, Its working frequency ranging from 2.4GHz to 2.5GHz, in the frequency band its return loss is better than 15dB, the amplitude and phase imbalanced between the two output ports are within 0.5dB and 5°, the finally size of the balanced filter is 2.5× 3.5× 1.6mm³.