XBAR Bandpass Filter Ladder Circuit for Wide 5G RF Bands
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Solution Overview
Problem
Existing RF filters using acoustic wave resonators are not well-suited for higher frequencies and wider bandwidths required in future communications networks, such as the 5G NR standard which includes bands n77 and n79.
Innovation Solution
The use of Transversely-Excited Film Bulk Acoustic Resonators (XBARs) with parallel capacitance to reduce effective coupling, allowing for improved performance at higher frequencies and wider bandwidths.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Speed
If conventional acoustic wave resonators (SAW, BAW, FBAR) are used, then the filters can operate at lower frequencies with established technology, but they cannot achieve the higher frequencies and wider bandwidths required for future communications networks
Solution Approach 1:
The patent changes the fundamental operating parameters of the resonator by transitioning from longitudinal wave modes (BAW, FBAR) to transverse shear wave modes (XBAR). This parameter change enables operation at higher frequencies (3 GHz and above) while maintaining the bulk acoustic wave advantages of high Q-factor and low loss, thus resolving the contradiction between frequency capability and performance suitability.
Solution Approach 2:
The patent substitutes the conventional longitudinal mechanical vibration mechanism with a transverse shear wave mechanism. The XBAR uses interdigital transducers to excite shear waves that propagate perpendicular to the electrode fingers, replacing the traditional longitudinal compression waves. This mechanical substitution enables higher frequency operation while maintaining acoustic wave benefits.
2Adaptability or versatility
If the bandwidth of the filter is increased to accommodate wider communication channels, then the frequency range expands, but the insertion loss and rejection performance deteriorate
Solution Approach 1:
The patent utilizes mechanical vibration resonance at the anti-resonance frequency of the XBAR to create sharp transmission zeros. By positioning these transmission zeros strategically, the filter achieves high rejection in the stopband while maintaining low insertion loss in the passband, even with wide bandwidth requirements. The high Q-factor of the shear wave resonator enables this performance.
Solution Approach 2:
The patent employs dynamic filter topologies where resonators can be switched between series and shunt configurations. This dynamic reconfiguration allows the filter to maintain optimal performance across wide bandwidths by adapting the resonance and anti-resonance frequencies to match the desired passband and stopband requirements, preventing degradation of insertion loss and rejection.
3Reliability
If more resonators are added to improve rejection and isolation, then the frequency selectivity improves, but the device size and complexity increase
Solution Approach 1:
The patent exploits the high Q-factor mechanical vibration characteristics of shear wave resonators to achieve sharp frequency selectivity with fewer elements. The narrow bandwidth of the mechanical resonance allows each resonator to provide strong rejection, reducing the total number of resonators needed compared to lower Q-factor technologies.
Solution Approach 2:
The patent combines multiple functions into single resonator elements. The XBAR resonators simultaneously provide signal transmission in the passband, rejection in the stopband through anti-resonance, and impedance transformation. This merging of functions reduces the overall device complexity while maintaining high rejection performance.
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
XBARs with parallel capacitance enable the design of RF filters that can effectively handle higher frequency communications bands, providing enhanced performance and wider bandwidths while maintaining low signal loss and high rejection.
Implementation Method 1
A microwave signal applied to the IDT excites a shear primary acoustic wave in the piezoelectric diaphragm
Implementation Method 2
XBAR resonators provide very high electromechanical coupling and high frequency capability
Implementation Method 3
XBARs with parallel capacitance enable the design of RF filters that can effectively handle higher frequency communications bands
Data Source
AI summary
Bandpass filters for a target communications band extending between a lower band edge and an upper band edge are disclosed. A bandpass filter includes one or more shunt resonators and one or more series resonators connected in a ladder filter circuit, wherein a relative difference between the anti-resonance and resonance frequencies of each acoustic resonator is greater than a fractional bandwidth of the target communications band. A first capacitor is connected in parallel with a first shunt resonator from the one or more shunt resonators, and a second capacitor connected in parallel with a first series resonator from the one or more series resonators.