Thick Film Transducers for High Frequency Coded Ultrasonography

Recently a new technology of piezoelectric transducers based on PZT thick film
has been developed as a response to a call for devices working at higher frequen-
cies suitable for production in large numbers at low cost. Eight PZT thick film
based focused transducers with resonant frequency close to 40 MHz were fabricated
and experimentally investigated. The PZT thick films were deposited on acoustically
engineered ceramic substrates by pad printing. Considering high frequency and non-
linear propagation it has been decided to evaluate the axial pressure field emitted
(and reflected by thick metal plate) by each of concave transducer differing in radius
of curvature – 11 mm, 12 mm, 15 mm, 16 mm.
All transducers were activated using AVTEC AVG-3A-PS transmitter and Ritec
diplexer connected directly to Agilent 54641D oscilloscope. As anticipated, in all
cases the focal distance was up to 10% closer to the transducer face than the one
related to the curvature radius. Axial pressure distributions were also compared to
the calculated ones (with the experimentally determined boundary conditions) using
the angular spectrum method including nonlinear propagation in water. The com-
puted results are in a very good agreement with the experimental ones. The trans-
ducers were excited with Golay coded sequences at 35–40 MHz. Introducing the
coded excitation allowed replacing the short-burst transmission at 20 MHz withthe same peak amplitude pressure, but with almost double center frequency, result-
ing in considerably better axial resolution. The thick films exhibited at least 30%
bandwidth broadening comparing to the standard PZ 27 transducer, resulting in an
increase in matching filtering output by a factor of 1.4–1.5 and finally resulting in
a SNR gain of the same order.