Lamb Wave Filter Layout With Floating Electrodes for Band-Edge Rejection
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Solution Overview
Problem
Modern Lamb wave filters exhibit poor out of band rejection at the edges of their passband, limiting the number of signals that can be transmitted within a frequency range and requiring multiple filters for multiple channels.
Innovation Solution
A Lamb wave filter design incorporating a substrate with a center region and edge regions, featuring a first and second set of electrodes as interdigital transducers and a third set of floating electrodes on the center region, which interferes with signal propagation to create a band gap and improve out of band rejection.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If a conventional Lamb wave filter design is used, then the filter structure is simple, but the out of band rejection is poor at the edges of the passband
Solution Approach 1:
The filter is segmented into three distinct electrode sets: input interdigital transducer electrodes, output interdigital transducer electrodes, and floating electrodes positioned in the center region. This segmentation allows each electrode set to perform a specific function, with the floating electrodes specifically dedicated to creating band gaps and improving out of band rejection without interfering with signal propagation between the input and output electrodes.
Solution Approach 2:
The floating electrodes act as an intermediary element between the input and output electrodes. These electrodes interfere with signal propagation to create band gaps at specific frequencies, thereby improving out of band rejection. The floating electrodes are electrically isolated (not connected to ground or power), allowing them to modulate the acoustic wave without creating a direct electrical path, thus maintaining signal integrity while enhancing rejection characteristics.
2Reliability
If the band gap is made wider to improve out of band rejection, then the rejection of out of band signals improves, but the signal propagation delay is affected
Solution Approach 1:
The floating electrodes are strategically positioned in the center region of the substrate between the input and output electrodes. This local positioning allows the band gap effect to be created only in the central region where it is most effective for filtering, while the signal propagation path near the input and output electrodes remains relatively unaffected. The localized interference pattern creates sharp rejection at band edges without significantly impacting the overall propagation delay.
Solution Approach 2:
The pitch of the floating electrodes can be adjusted to control the frequency at which band gaps occur. By optimizing the pitch parameter of the floating electrodes relative to the wavelength of the signal, the filter achieves improved out of band rejection while minimizing the impact on signal propagation delay. The electrical isolation of the floating electrodes also allows for parameter optimization without the constraints of electrical loading effects.
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
The improved design achieves high Q rejection of out of band signals without interfering with signal propagation delay, allowing for multiple channels to be filtered using a single filter, especially at frequencies over 10 GHz.
Implementation Method 1
a first set of electrodes arranged as a first interdigital transducer and configured to receive an input signal and transform the input signal to a Lamb wave
Implementation Method 2
a third set of floating electrodes disposed on the center region of the substrate... which interferes with signal propagation to create a band gap and improve out of band rejection
Data Source
AI summary
A Lamb wave filter is provided which has improved out of band rejection, the filter including a substrate having a center region and two edge regions, a first set of electrodes arranged as a first interdigital transducer displaced on one of the two edge regions and configured to receive an input signal, a second set of electrodes arranged as a second interdigital transducer displaced on the other of the two edge regions and configured to provide a filtered output signal, and a third set of electrodes disposed parallel to the first and second sets of electrodes on the center region of the substrate.