these rectifiers can exert negative non-linear effects
which have to be taken into account, because they could
seriously distort some positive half-cycles of echo signals
exceeding the cumulated conduction thresholds of all
the diodes included in RI and RII . This situation is
CC
clearly shown in the waveforms of Fig. 5, calculated
for the circuital node Vecho with an alternative, simpler
equivalent circuit depicted in Fig. 6. This circuit repre-
sents, in Spice format, a specific electrical transceiver
with a rectifier branch RI formed by only two diodes,
C
and an untuned PZT5 transducer, 12 mm in diameter
(air-backed and nominal frequency of 2.2 MHz). Ele-
inments C3 and R4 correspond to Rin and Cos in the ZRecos
block (5). The diodes included in the reception stage and
C4 represent respectively, the residual influence during
reception of the pulser components RÃ and Ceq (4) inC
Fig. 3.
In this Fig. 6, the ultrasonic pulse-echo probe is rep-
resented by the three-port blocks (US PROBE) incor-
porating a well-known Spice model of the piezoelectric
transducer [11]. The terminals e, b and f denote respec-
tively the electrical and mechanical (back and front)
ports of the probe. The quadripole E1 symbolises a de-
pendent voltage source which, from the electrical point
of view, is used for decoupling both ultrasonic stages.
The gain of block E1 has been determined, considering
the behaviour of the emitted force FM and the wave
amplitude decay due to its interactions with the plain
steel reflector, taking into account expression (6).
At this lower frequency range and in the absence of
tuning, it has not been necessary to introduce all the
non-conventional elements included in the general model
proposed in Fig. 3.