X-cut LiNbO3 Coupled Resonators for Narrow-band Filtering Applications

This paper reports the first implementation of a narrow-band filter exploiting two electrically coupled high performance X-cut Lithium Niobate (LiNbO
₃
or LN) Laterally Vibrating Resonators (LVRs) operating around 108 MHz. By selecting the proper coupling capacitance at the common node, the series of identical resonators attains a bandpass response with insertion loss of 2.8 dB and an average return loss of 10 dB in 300 kHz with 50 Ω terminations. The group delay is approximately 1.8 μs and it varies less than 7% inside a 0.09% fractional bandwidth (BW%). The out-of-band rejection is > 60 dB and the 20dB-shape factor is 2.33. This filter topology is a perfect candidate for spectral sensing and interferes removal in new Radio Frequency (RF) modules, as the channel selective filter exhibits low losses and narrow-band response combined with constant group delay and high rejection, within a simple 2-mask process fabrication and small form factor. The overall performances can be further improved through a monolithic integration of the filter, drastically reducing the impact of parasitics coming from the test board. Simulated and measured results are shown along with an even-odd mathematical analysis.