Transmitting circuit

The transmitting circuit includes a pulse generator which drives the ultrasonic transducer. The electrical impedance of the piezoelectric transducer is like that reported in Fig. 3.5, where the thickness mode vibration corresponds to the fourth resonance (approximatively at 2.56 MHz), and the maximum of transduction efficiency is obtained if the electrical stimulus is at the series resonance. Piezoceramic materials presents multiple resonances corresponding to different vibration modes: only one is the fundamental thickness mode which has to be excited for obtaining a longitudinal pressure wave in the liquid.

Since in TOF measurements a pulsed stimulus has to be used instead of a pure sine-wave, a preliminary analysis on the effect of the pulse shape was done.

1. a single sine pulse, with duration tS = 390 ns given by the period of a sinewave at ∼2.56 MHz;

2. a rectangular pulse with duration tRand rising and falling times shorter than 30 ns;

3. a Gaussian pulse with a full width at half maximum (FWHM) tG. For all pulses a peak-to-peak amplitude of 2 V was imposed.

The peak-ot-peak output signal at the receiver was approximatively 190 mV for the single-period sine pulse, increasing for a sine pulse train up to ∼1.3 V when 10 sine-wave periods were used. However, a pulse train generator is more complex than Gaussian or rectangular pulse generators.

Gaussian and rectangular pulses should present a sufficient power spec-tral density in a narrow bandwidth close to the fourth resonance of Fig. 3.5.

For the Gaussian pulse, the output signal reaches its maximum peak-to-peak value of ∼180 mV for tG = 165 ns, which corresponds to a −3 dB bandwidth f−3dB ∼1.9 MHz, on a good agreement with what was expected.

On the contrary, the behaviour of the rectangular pulse was little strange.

The peak-to-peak output value varies, but not monotonically, as the pulse width shortens, as shown in Fig. 3.6(a). An explanation was found in the fact that both the rising and the falling edges of the rectangular pulse excite the mechanical vibration of the transducers and generate two waves which, reach-ing the receiver, can interfere, increasreach-ing or not the output signal dependreach-ing on their relative phase. Such phase shift is a function of the rectangular pulse width. The system response to rising and falling, well-spaced, edges were mea-sured for obtaining the two different waves, and so numerically estimating the interference effect by varying the phase shift, Fig. 3.6(b). The peak-to-peak values of that resulting waveform were compared with the measures obtained for different pulse widths showing a good agreement.

(a) Simulation (line) and measurement re-sults (markers) with rectangular pulse.

(b) Simulated interference.

Figure 3.6: (a) Variation of the peak-to-peak output signal as a function of the width of the rectangular pulse on the transmitter. Markers refer to experimental results, line to simulations. (b) Single responses to the rising and falling edges of the transmitted pulse, and their combined effect.

Another relevant parameter is the edges’ slope of the excitation signal.

Faster fronts have wider bandwidth ensuring greater excitation of the piezoce-ramic transducer resonance. Some measurement results obtained with different edge times are reported in Fig. 3.7. It is evident that the wave edges have to be shorter than 50 ns for the effectiveness of the stimulus.

Therefore, a simple step signal can be used as excitation stimulus for ob-taining a clear response on the receiving transducer. Moreover simpler and more reliable buffers and pulse generator circuits can be designed.

The transmission buffer has been reduced to a simple CMOS driver, the only constrains were on the rise, and the fall, time of the output and on the maxi-mum driving amplitude. In Fig. 3.8 a simple representation of the transmission system is shown.

The electrical model of a piezoelectric transducer can be described as a capacitor in parallel to few RCL series circuits, one for each resonance. It is possible to extract each component of such equivalent model through an interpolation of experimental data, obtaining for the used transducer a parallel capacitor Cp= 726 pF and a series resistance for the interesting series branch

10¹ 10² 10³ 0.1

0.2 0.3 0.4 0.5 0.6

tfall [ns]

Output amplitude normalized

Figure 3.7: Measured peak-to-peak amplitude of the received signal for dif-ferent slopes of the step signal used as stimulus.

TX

~ nV

Half-Bridge Driver

TTL

Figure 3.8: General outline of the transmitter.

Square wave

generator CMOS

Buffer Voltage Multiplier

V_DC

~ VDC ~ 2VDC ~ 3VDC ~ 4VDC

Figure 3.9:Block diagram of the voltage multiplier used for the power supply of the half bridge in the transmitter.

R_s= 13 Ω: the buffer must be able to drive these loads.

To this aim, the transducer has been connected to the central terminal of a half-bridge of dual n-MOS, able to drive up to 600 V of VDS with about 15 ns of rise and fall time. This circuit, the STP6NK60Z, is normally used for high-current and high-speed switching designs, so it could be quite oversized for the application but adapted at this phase of the study because it is still not known the exact power needed in the transmission for obtaining the desired accuracy on the TOF measurement.

An additional driver circuit is necessary for achieving the correct driving potentials since the half-bridge is made of two n-MOS. The IRS21834 driver, with TTL inputs has been selected.

In addition, a voltage multiplier has been designed for obtaining, from a standard VDC = 24 V supply, higher voltage values for supplying the half bridge. A Villard cascade voltage multiplier circuit was designed and four different outputs (∼ VDC, ∼ 2VDC, ∼ 3VDC and ∼ 4VDC) were obtained using a four stages architecture; the voltage drop on the diodes of the circuit limits the effective multiplied voltage to some volts below the expected value.

The multiplier should be powered with an alternate potential that has been obtained using a simple CMOS buffer/driver, the TC4432, able to drive loads up to 30 V, driven by a simple square wave generator. An overview of the whole multiplier circuit is reported in Fig. 3.9.

The half-bridge driver has a static absorbed current limited to the

leak-Cm = Qsw

0.05Vtot = 150 nC

0.05 · 96 V '31 nF . (3.12) The value of these capacitors, together with the frequency of the chopped supply voltage, ensure that the outputs of the multiplier will be maintained on the correct values also in loaded condition. Experimentally, using 47 nF capacitors, a frequency of ∼40 kHz and a DC supply of 24 V, the output of the last stage remains at about 91 V with the piezoceramic connected as load.

For comparison, 93 V were measured when no loads were connected to the multiplier outputs.