This article will focus on the transducers, but briefly look at the link arrangement itself as well.
The Van Atta Cell
An RF Van Atta structure consists of an array of antennas interconnected in symmetrical pairs. This concept was the starting point for the multi-element array, but with major differences due to the nature of the piezo transducer and interconnection. They began with a switching technique known as pass/absorb, where the switches are simultaneously controlled to alternate between these two states to achieve retroreflective backscatter.
However, applying the same idea in underwater backscatter isn’t that simple. Although it’s in some ways similar to an RF phased array and associated beamforming, there are significant differences. First, unlike antennas that typically have one main external feeding point (meaning they are single-ended), piezoelectric transducers are differential. Therefore, they needed a new switch architecture to alternate between different reflective/absorptive states and optimize the backscatter SNR.
Second, in addition to the need for a novel switching architecture, the piezoelectric transducers must be matched to maximize the retroreflective SNR. Unlike antennas that can be easily designed or purchased to have 50- or 75-Ω impedance, it’s very difficult to design a piezoelectric transducer to have a real and consistent impedance. A piezoelectric device is capacitive by nature and even with tight mechanical tolerances, the fabrication of a sealed transducer results in significant process variations in the measured impedance.
Instead, they used a second switching technique via an arrangement which maintains that the connection between the two nodes be “ON” at all times, but toggles it between in-phase and counter-phase polarities (Fig. 2).