In this article, we explore how acoustic wave filters starting with shunt resonator require particular reflection phase conditions to ensure that the synthesized filter is feasible. The position of transmission zeros (TZs) along with the phase of the objective filter function might lead to nonfeasible solutions where the first and last resonators require elements with positive reactance slope in the static branch, or equivalently, a nonphysical negative static capacitor. Since the reflection phase of a duplexer-oriented filter is fixed to reduce loading effects, the feasibility problem is solved by bringing the resonance frequency of the first resonator beyond the central frequency of the counter band. However, this entails surpassing the limits of the electromechanical coupling coefficient. We demonstrate how two reactive elements at the input overcome this resonance position limitation and provide a simple rule to decide the right topology. The position of the first TZ of duplexers will play an important role. Moreover, we provide rules on the reflection phase values that ensure that all resonators have a capacitive static branch.

The usage of a commercial 24 GHz frequency-modulated continuous-wave (FMCW) radar sensor to retrieve sound signals is investigated in this article. Thanks to the great phase measurement capabilities of current radar sensors, low frequency audio signals can be recovered by measuring the vibrations they induce in objects due to their pressure wave nature. To prove the concept, a sound emitted by a speaker is recorded by analysing the small-scale displacement of a reflective surface using a radar sensor originally devised for automotive and UAV applications at the 24 GHz ISM band.

This work proposes the first analytical method for the direct synthesis of multiplexers based on acoustic wave (AW) ladder filters connected in a star junction topology. It describes the computation of the polynomial representation of a multiport ( ${K}$ $+$ 1 ports) network with fully canonical (FC) channels and derives a set of ${K}$ distorted channel polynomials from which each of the AW ladder filters is synthesized. The channel polynomials are distorted in a way such that when connected together at the star junction, the desired multiplexer response is obtained avoiding degradation due to channel loading. The method allows to freely set the fractional bandwidth (FBW), return loss (RL), and transmission zeros (TZs) of each channel enabling the search for multiplexers that fulfill the technological requirements of AW technology in terms of electromechanical coupling coefficient and area occupancy among others. Moreover, since the multiport network polynomials incorporate the complete effects of the loading between channels as, for example, additional TZs, the synthesized multiplexers depict an improvement of isolation when compared with a design based on only controlling the reflection phase of each channel filter. The proposed method is rigorously demonstrated by means of a synthesized multiplexer example providing the Butterworth–Van Dyke (BVD) elements of each channel filter. This approach avoids intensive optimization procedures and represents a useful technique for the design of carrier aggregation (CA) filter modules in a direct and controlled manner.

Extracted pole filters alternating transmission zeros (TZs) above and below the passband are intrinsically related to the use of nonresonant nodes (NRNs) of different nature (inductive and capacitive). This work demonstrates that it is possible to synthesize an extracted pole filter showing TZs on both sides of the passband but implementing all NRNs of the same reactive nature, overcoming the limitations imposed by some technologies. The proposed feature is achieved by special consideration of the input reflection phase of the generalized Chebyshev filter function, affecting input and output resonators and placing at most two zeros at the opposite band. A third-order all-capacitive prototype filter has been synthesized and fabricated to validate the proposed statement. The concept allows us to achieve all-capacitive or all-inductive filters.

This work assesses three different approaches to the design of dual-band filters with single-ended input and output based on acoustic wave (AW) technology together with the filter synthesis methodology behind each structure. First, the straightforward option, already on the market, is to connect two standalone ladder filters in parallel. This work provides a synthesis view to this option and also describes its strong limitations regarding achievable isolation. Nonetheless, departing from the classical ladder, an innovative topology is proposed in which series and shunt resonators of different sub-bands are concatenated forming a double-ladder topology. Although this latter structure is able to achieve better isolation levels, this work describes the particular constraints arising from this innovative solution in terms of response and technology requirements. Finally, the conventional ladder structure with a single signal path is proposed as a strong candidate to accommodate dual-band filtering responses for the first time. All three structures are discussed from a filter synthesis perspective, describing the particular constraints behind each topology. Also, each structure is reviewed and compared in technological terms to highlight its main advantages and disadvantages.