Title: Non-reciprocal Acoustic Devices for RF Communications
Description: The purpose of this research is to develop, demonstrate, and characterize a novel architecture based on surface acoustic wave (SAW) devices capable of non-reciprocal propagation of forward and reverse signals. To begin, a novel topology is introduced based on asymmetrical delay lines and a current source representing a copy of the input signal. An analysis of this structure demonstrates it is capable of functioning as an isolator with the added capability of tuning the frequency response by controlling the phase relationships be- tween input signal and its copy. The structure is dependent on creating large phase shifts of 360◦ and 180◦ which is implemented in the acoustic domain. The current source functioning as a copy of the input signal is implemented by a parametric circuit. When a non-linear capacitor is pumped by a large signal at exactly twice the input signal frequency, an amplified copy of the input signal is reflected with a phase shift according to the pump signal phase due to the presence of negative resistance.

This is precisely the behavior required for the topology to function. This type of parametric amplification is known as phase-coherent degenerate parametric amplifier. To investigate further, models are developed for the surface acoustic wave transducers, the non-linear capacitor, and the overall structure in Keysight’s Advanced Design Systems (ADS). Harmonic balance simulations in ADS verify the theory and demonstrate the same tunable behavior. These simulations are then used to design the SAW device and the peripheral circuitry required to match and isolate the pump signal from the input signal. The SAW device is fabricated on bulk LiN bO3 and bonded to a PCB board containing the pump circuitry. The first implementation is based on a bi-directional center transducer design and demonstrates overall functionality with limited bandwidth due to the single resonant parametric circuit design. A secondary device with a uni-directional SAW center transducer, which better matches the bandwidth of the input and output transducers, and a 2nd order resonant network result in improved performance demonstrating gain and isolation throughout the bandwidth of the SAW filter.

Furthermore, the tunable aspect is also demonstrated by controlling the phase relationship between input and pump signals. This device, however, requires phase-coherence between signals and the relationship fP=2fS to be maintained. This is difficult to implement in practical systems and requires additional complicated circuitry. For this reason, a phase-incoherent version was also investigated. The addition of the uni-directional center transducer which couples the negative resistance to the acoustic waves, generates another form of asymmetry which also results in non-reciprocal propagation of forward and reverse signals. Due to its general applicability, the focus of the work is shifted to this prototype. The device is capable of functioning under different pump frequencies each with its advantages and disadvantages. For this reason, the performance both phase-coherent and incoherent modes of operation in terms of gain, isolation, noise, and linearity are characterized and understood under the lens of para- metric amplification.

When compared to the state-of-the-art, the device exhibits superior performance in terms of isolation and insertion loss.