Synthesis of wideband bulk acoustic wave coupled resonator filter combined with lumped components

Abstract

A novel wideband bulk acoustic wave (BAW) coupled resonator filter (CRF) combined with external lumped inductors and capacitors is proposed. The filter synthesis method based on the Butterworth-van Dyke (BVD) model of CRF (CRF-BVD) is also presented. To improve the synthesis accuracy, the CRF-BVD model is modified and the CRF configuration with a single rather than the traditional multiple coupling layer is adopted in this paper. By establishing equivalence relationships between the modified CRF-BVD model and the filter prototype synthesis network, exact CRF dimensions are solved through a set of equations without any further parameters tuning. The proposed method exhibits a maximum 37.4% (FBW) for a Chebyshev filter with 9.4-dB return loss, which overcomes the narrowband limitation of regular BAW filters with FBW below 5%. To demonstrate the synthesis procedure, an n77 band filter with 3.723-GHz center frequency and 24.2% FBW is designed and simulated. Good agreements are obtained among the simulation results of the prototype synthesis network, the modified CRF-BVD model, and the cosimulations of physical model based on the finite element method (FEM) and the CMOS 130-nm technology.bulk acoustic wave, coupled resonator filter, filter synthesis, wideband filter | INTRODUCTIONBulk acoustic wave (BAW) filters have been identified as one of the best filter options for front-end circuits in wireless mobile communication systems owing to their ultrasmall size, low insertion loss (IL), steep skirt selectivity, and high power handling capability. 1,2 However, the bandwidth of BAW filters is limited by the effective electromechanical coupling coefficient k 2 ef f of BAW resonators. 3,4 The k 2 ef f must be at least double the fractional bandwidth (FBW) of a radio band. Therefore, the commercialized aluminum nitride (AlN) based BAW filter with k 2 ef f less than 7% can only reach a fractional bandwidth (FBW) below 3.5%. 5 With mobile communication evolving into the 5G era, some new bands with large FBW such as n79 (12.8% FBW), n78 (14.8% FBW), and n77 (24.2% FBW) have been proposed. Such wide bandwidths are hard to achieve with regular BAW filters. 6,7 To improve the bandwidth of BAW filters, some approaches have been developed. One is adding auxiliary inductors in series/parallel with the shunt/series resonators. 8,9 However, the bandwidth cannot be extended too much due to the limited distance between the transmission zeros at the lower and upper sides of the passband. Unexpected passbands also appear in the stopband range and affect the broadband rejection performance. 10 One method is employing

Design and Fabrication of Temperature-Compensated Film Bulk Acoustic Resonator Filter Based on the Stress Compensation Effect

“…Furthermore, the measured frequency was lower than the simulated frequency because of the deviations between the actual and designed film thicknesses. Figure 13b shows the temperature characteristic test curve of the TC-FBAR, which has a TCF of 0.8 ppm/ • C at −55-125 • C. The test results were compared with different references, as shown in Table 1 [12][13][14][41][42][43][44]. From the comparison results, the lowest TCF and narrowest bandwidth were obtained in this work; thus, this resonator is suitable for fabricating a narrowband filter with a low TCF.…”