Broad-band, large delay range, high precision, and variable analog delay lines for a high-frequency phased array beamformer

A set of prototype RF delay lines have been developed in order to meet the required bandwidth, delay accuracy, and delay range requirements for a previously developed 64 channel endoscopic phased array prototype. Each delay line is based on a planar PMN-32%PT pulse-echo transducer situated in a water bath, reflecting off a polished brass reflector. The pulse-echo transducers are mounted on individual in-house fabricated piezoelectric bimorphs, which are `simply´ supported. This creates an electronic delay time proportional to the distance between the single-element transducer and the reflecting surface in the water bath. When the physical length between the transducer and the reflecting surface is changed, the delay time also changes. A variable high voltage signal causes bimorph actuation and, in turn, the variable delays. The experimentally measured centre frequency and bandwidth for the pulse echo transducers was 45 MHz and 60% respectively. The pulse echo insertion loss without considering water attenuation or loss into the brass reflector was measured to be -20.8 dB, therefore one single low-noise amplifier with 20 dB gain was required for each delay line to have zero insertion loss. The signal-to-noise ratio was only decreased by 1.1dB with this amplifier in place. The total delay range measured by adjusting the bimorph voltage by +/- 80V was 1.4 μs, and the accuracy/repeatability of the pulse delay was less than 3ns. To date 16 channels of individually controlled parallel delays have been fabricated and the fabrication/testing of a full 64 channel system is ongoing.