Novel balanced RF duplexer using bulk acoustic wave resonator with low TCF structure

A novel RF duplexer with balanced receiving output ports and low temperature coefficient of frequency (TCF) is proposed. The topology of balanced receiving RF filter is designed with both lattice and ladder type. RF resonators in this filter are designed based on a bulk acoustic wave (BAW) structure in which each layer has alternative negative and positive TCF values. The RF circuit block performing as a balance-to-unbalance transformer as well as a phase shifter is embedded in the low temperature co-fired ceramics (LTCC) substrate. The RF duplexer is fabricated for 3GPP2 Band Class 14 G-block, whose receiving (Rx) and transmitting (Tx) bands are respectively 1930-1995 and 1850-1915 MHz. The 15 MHz band gap between Rx and Tx bands corresponding to about 0.8% of fractional band gap is such a narrow cellular standards that it is difficult to achieve. By embedding all capacitors inside the BAW filter, the size of the duplexer is as small as 2.5 mm × 2.0 mm. The maximum insertion losses of the fabricated duplexer are measured to be 2.4 dB and 3.7 dB in the pass-band for transmitting and receiving band, respectively. The amplitude and the phase balance are within ±1 dB and 180 ± 10° in the receiving band. The isolation characteristics are 52 dB and 55 dB for transmitting and receiving band, respectively.

I. Introduction

There have been great demands for the balanced BAW devices as RF filters and duplexers in mobile communication systems for its high Q- factor and low noise characteristics. As wireless frequency resources are limited, band gap between uplink and downlink frequency is getting narrower, while band width is coming wider. Balanced RF structure has been widely demanded in the low noise amplifier (LNA), for its advantages of common-mode noise rejection, which leads to requirement of the post-LNA filter and balanced duplexer. Currently, most balanced duplexers and post-LNA filters are using surface acoustic wave (SAW) technology for its simple and easy fabrication process [1], [2]. However, SAW devices show a low Q- factor around a few hundreds and weak temperature stability, which makes it difficult to achieve narrow band gap duplexers such as Band Class 14 G-block (Transmit:1850–1915 MHz, Receive:1930–1995 MHz) that shows a fractional band gap lower than 1 %.