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EMFi based Ultrasound Transceivers
Silviu Epure, Radu Belea, and Dorel Aiordachioaie
Electronics and Telecommunication Department
“Dunarea de Jos” Galati University, Romania
Abstract—The paper presents some transceivers for ultrasonic
transducers based on Emfit Ferro-Electret Film (EMFi). The
proposed and discussed transceivers use the wide bandwidth of
EMFi material – from audio up to hundreds of kHz, for in-air
biomimetic sonar applications, allowing thus to develop new
algorithms for object detection, recognition and navigation based
on frequency modulation algorithms. Technological process of
building new ultrasonic transducers, starting from blank EMFi
foil, is presented as well as electronic circuits used for transmitter
and receiver. Three transmitter circuits were designed and
presented, all of them being capable to drive a capacitive load of
about 30pF with 300Vpp sinusoidal signal, up to 300 kHz.
Keywords- ultrasounds, transducer, transceiver, receiver, EMFi
I.
INTRODUCTION
The work is part of the research grant “adbiosonar”, [1].
Among others research directions based on sonar image
processing, the main objective of the grant is to develop new
bio-mimetic sonar heads and new methodologies for sonar
based applications having nature as source of inspiration. The
new sonar heads must have wide frequency bandwidth, e.g.
from 50 kHz up to 300 kHz, in order to be able to develop new
algorithms for object detection, recognition and navigation.
The absorption of sound in air is significant, at least
compared with liquid media, e.g. water. Also, we consider only
the behavior in far end field. A model of the computation of
sound pressure is presented in [2]. Following [3], transmission
losses should be considered as two terms: one generated by the
geometrical spreading and the second one by the absorption.
The first component does not depend on frequency and in not
considered here. The basic classical mechanism of absorption
can be expressed as:
p(r )  p0  e( f )  r
( f )  0.022  f  0.6
(2)
with f–frequency [kHz] and  ( f ) [dB/ft]. Absorption grows
more important as frequency get larger. When using
transmitted signals with large bandwidths, the strong frequency
dependence of absorption distorts the pulse shape considerably,
explaining why it is necessary to include this block in the
model. At large distances the low pass characteristic of air
absorption might even restrict the bandwidth of the
transmission channel beyond that of the transducer itself, [7].
Starting from EMFi film without electrodes, we have
created transducers and all the necessary circuitry. The paper
presents them, as well as the obtained results. Section 2
describes the ultrasonic materials used for transducers, section
3 investigates the receiver. The emitter structures are presented
in section 4. Finally, section 5 presents conclusions of the
work. Experimental results are presented at the end of each
section.
II.
THE ULTRASONIC TRANSDUCERS
We are interested in a wide band frequency transducers,
capacitive or piezoelectric, i.e. which could be used in the
frequency range [50 kHz–300 kHz]. Based on results reported
in literature of ultrasonic sensors, e.g. [8], [9], [10], [11], [12],
[13] the EMFi material, [14], was considered. This material has
strong electro-mechanical response and can be acquired for
research purposes to create custom transducers. It is delivered
in sheets of 70-80 µm thick flexible film. By considering a
square surface for sensor of 1 cm x 1 cm, the following
parameters are valid [14]:
(1) sensitivity of the EMFi sensor: Sq [pC/N] = 25-250;
(2) the specific capacitance C0 [pF/cm2] = 40.
(1)
where p0 denotes the pressure without taking absorption into
account, r is the propagation distance, and  ( f ) is the
absorption coefficient measured in attenuation per unit length
[dB / m], and frequency depended.
Approximate analytical expression for the absorption
coefficient  ( f ) derived in [4], [5], could be used for objects
at different distances. Since an ultrasonic sensor usually is
required to operate at all possible humidity, target range
calculations should use the largest value of attenuation, [6]. A
good estimate for the maximum attenuation in air at room
temperature over all humidity for frequencies between 50 kHz
and 250 kHz is presented in [6].
Working with no significant load for ultrasonic sensor, e.g.
a resistive load greater then 1 MΩ, for a pressure variation of
P [N/cm2] the output voltage will be:
V  (1/ C0 )  Sq  P
(3)
Experimental results require a minimum voltage at the
input of the receiver, thus the minimum necessary pressure at
the reception point is
P  V  C0 / Sq
(4)
The simplest type of source is the pulsating sphere.
Assuming that the radius a of the sphere is small compared to
the wavelength λ of the emitted sound wave, the pressure field
can be expressed as
a
j  t  kr 
p(r , t )  j 0  c U 0   k  a  e 
r
(5)
where: U0 is the speed amplitude of the vibration; a average
radius of the sphere; ρ0 mass density of an undistributed fluid
element.
electronics circuits presented in following sections, we have
determined the bandwidth, and the directivity of these
transceivers, Fig. 2. Continuous sinusoidal signal was used for
measurements.
A source of particular interest is the rigid piston of radius a
mounted flush with the surface of an infinite baffle, and
vibrating with a time harmonic motion. The final expression
for the pressure of a single source at the point placed at a point
of distance r and θ angle is, [2]:
  c  U 0  ka 2 j   t  kr   2 J1 (ka sin ) 
p (r , , t )  j  0
e

 (6)
2r
 ka sin  
where J1 represent the Bessel-function of the first kind of
order 1.
Experiments with EMFi sensors show that the resonance
and also the directivity are strongly dependent of the
construction of the transducer. The way in which the glue
substance is used to connect electrodes to both sides of the
sensors has a great impact. This is a draw back of this material
because all results concerning the frequency behavior must be
associated with the used technology in building of the
ultrasonic sensor. A transducer that can be used in emitter as
well as in receiver has been built based on following steps:
 using low density cyanoacrylate adhesive, the EMFi foil
is glued to a double sided PCB, Fig. 1a); one side of the PCB
will be the active electrode of the sensor; the other will be used
as electro-magnetic shield;
 the second electrode is created by covering the EMFi foil,
Fig. 1b) with a thin graphite layer connected to the ground
plane, Fig. 1c). Using this solution, capacitive coupling
between active electrode in receiver and the transceiver is
minimized. Furthermore, the user cannot touch the high voltage
active electrode on the transceiver stage.
Figure 2. a) Measured frequency response of the channel transducer-airtransducer; b) Polar diagram for EMFi transducer, at 280 kHz
Fig. 2a) shows that this transducer have a wide frequency
response, with resonant frequency around 250 kHz, unlike the
39-40 kHz transducers that have a bandwidth of a 2..5 kHz.
This characteristic must be taken in consideration when the
transmitter-air-receiver system will be modeled.
The directivity graph has been obtained using ultrasounds
with 280 kHz frequency. As can be seen on Fig. 3, at these
frequencies, the transducer produces a highly directional beam
of ultrasounds, with no measurable lateral lobes.
These two properties make the custom transducer well
suited for our purpose.
III.
THE RECEIVER PREAMPLIFIER
At the receiver stage, the transducer can be modeled as a
10..20 pF capacitor in series with a wide band signal generator.
A preamplifier circuit is needed for signal conditioning. This
circuit must have the following properties:
 high input impedance (higher the better);
 high sensitivity (50 µV); gain greater than 50dB;
 bandwidth of [50..500] kHz;
 single 5V power supply;
 low distortion;
 high immunity to capacitive coupling and e.m. noise.
Figure 1. Custom transducer: a) PCB; b) PCB and EMFi; c) end product
This technology has two important advantages over some
classical approach [15], where the EMFi foil is fixed using
double-adhesive tape:
 the adhesive layer, dielectric material in both cases, will
act as a parasitic series capacitor, thus tinier layer means higher
sensor efficiency;
[12] reported a 100 V sensibility receiver using two
blocks: a preamplifier with high fixed gain followed by a
variable gain stage.
In our paper, the receiver in Fig. 3 is based on circuit
OPA2301 – a low noise, high-speed, CMOS inputs, operational
amplifier, with 150MHz bandwidth.
 the cyanoacrylate layer is stiffer than adhesive tape, and
provide a firmer support for EMFi foil and better sensor
efficiency.
EMFi transducers have been built in two versions: one with
an active area of 4 cm2 used on the transmitter and the other
with 1.5 cm2 active area, used on the receiver. Using the
Figure 3. a) Schematics, b) Layout of the preamplifier circuit
We have used half of the circuit to collect and amplify the
signal from transducer terminals. R1 give the input impedance
of the preamplifier, R2 and R3 set the gain and capacitor C1 is
used to avoid high frequency auto-oscillations. The second
operational amplifier is used as low-pass Butterworth filter, to
limit the bandwidth at 500 kHz. Fig. 4 presents the measured
bandwidth of the preamplifier circuit.
Figure 4. Gain of the preamplifier circuit – experimental values
The circuit was built on a dual-layer PCB, Fig. 3b): one
layer for components and the other used as ground plane. The
board size is the same with the transducer size. This way,
placing back-to-back the transducer and the preamplifier,
shielding the input signal is automatically solved and the length
of the wires between transducer and preamplifier is minimized.
One of the first tests was to find the sensitivity of the
transceiver/receiver pair: transmitter and receiver transducers
were placed face-to-face at a distance of 50 cm; a sinusoidal
signal close to the resonant frequency and minimum amplitude
was emitted, so that at the receiver’s output signal to be greater
than noise. Fig. 5a) prove that sensibility is better than 50 µV.
IV.
THE EMITTER AMPLIFIER
The EMFi-based transducer does not require a bias DC
voltage but the response of an EMFi-based emitter is
proportional to the applied AC voltage, so it is necessary to
drive it with high voltages to achieve high sound pressure
levels (SPL). For example in [12] signals of about 300Vpp were
used. In these respect, three schematic circuits have been built
and tested: a hybrid power amplifier with transformer, a
bootstrap high voltage amplifier and an active load A-class H
bridge amplifier. This chapter presents the most important
characteristics of the three circuits, from designing stage to the
experimental results.
A. Emitter based on transformers
A hybrid amplifier is a circuit that includes an operational
amplifier and other active components connected in such way
that the resulting circuit will exceed one or more op-amp IC
limits. Largely, the operational amplifier entity refers to a
general-purpose dc voltage amplifier that may be configured by
a passive network. This acceptation the hybrid amplifiers are
also op-amps.
The hybrid op-amp schematic in Fig. 6 consists in IC, T1,
T2, R4, R5, R6 and C3. The configuration network is: feedback
resistor R2, ground resistor R1 and in loop compensation
condenser C2. The circuit amplifier can drive a 2.2 nF
piezo-ceramic ultrasonic transducer or a 150 pF EMFI
transducer using a 1:30 rising voltage transformer. The hybrid
power amplifier was build with a 3 MHz bandwidth op-amp
(TL071) and two power transistors BD439 and BD440. The
supply voltages are V+ = 15 V and V- = -15 V to allow a
maximum output voltage span of 25 Vpp. The amplifier output
voltage Vo is applied to the load capacity CL via a 1:30 rising
voltage transformer. In this case we can use up 750 Vpp driving
voltage.
0°
5°
10°
15°
Figure 5. red – voltage at transmitter output; blue – receiver output,
a)sensitivity test, b) directivity test, different θ
To determine the directivity graph of the transducers,
presented on Fig. 2b) we have placed the transmitter in the
center of a 50 cm virtual circle and the receiver on the
circumference of the circle. Fig. 5b) presents the signal at the
transmitter output –red, and signal at the receiver output –blue,
for various positions on the circle.
The parameter θ represents the angle between transceiver’s
normal axis and receiver’s normal axis. Given the 280 kHz
frequency used, a slight variation of distance D will lead to
considerable phase shift between signals at transmitter and
receiver.
Fig. 5a) it is a screen capture of INSTEK GDS2062 digital
oscilloscope and Fig. 5b) it is a superposition of four captures.
Figure 6. The hybrid power amplifier with transformer
The unity gain bandwidth of the operational amplifier is 3
MHz and the close-loop amplification is 10. Consequently, the
close loop bandwidth is:
BW10 
BW1 3 MHz

 300 kHz
ACL
10
(7)
The hybrid amplifier has no stability problems, but the
in-loop compensation capacity value C2 must be
experimentally set [16].
B. Bootstrap high voltage amplifier
The supply voltages of the op-amp of Fig. 7 changes
dynamically as a function of vo voltage. So the op-amp can
cover a peak-to-peak voltage swings greater than the total
voltage apply across its supply rails. In the scheme are two
Bootstrap feedback loops for supply voltages VC0 and VE0, used
for increasing the output voltage span.
The TL071 op-amp slew-rate parameter is referred to the
ground voltage for constant supply voltages: VC0 = 15V and VE0
= -15 V. The datasheet typical value of Slew-Rate (SR)
parameter is SRamp = 13 V/μs. The swing time of the op-amp
output voltage is
Neglecting the basis current of the two transistors the
op-amp supply voltages VCO and VEO results from equations:
(12)
VC0 
(VCC  0.6) R5  vo R6
R5  R6
(8)
VE0 
(VEE  0.6) R7  vo R8
R7  R8
(9)
 swing 
VSamp
SRamp

27 V
 2.08 μs
13 V/μs
If we assume that the swing time of the op-amp output
voltage, and the swing time bootstrap amplifier output voltage
are equals, the bootstrap amplifier the Slew-Rate is
SRboot 
VSboot
 swing

93 V
V
 44
2.08 μs
μs
(13)
The amplifier haves two feedback loops: the negative
feedback loop R3, R4 and a positive feedback loop R1, R2. The
close loop amplification and the stability condition are:
R4
v
R3
Au  o 
vi 1  R4 R1
R3 R2
1
Figure 7. The Bootstrap high voltage amplifier
We use the same op-amp as in the Fig. 6. If VCC = 50 V,
VEE = -50 V, R5 = R7 = 10 kΩ and R6 = R8 = 24 kΩ. Fig. 8
presents the signals vin (green), vo (blue), VC0 and VE0 (red).
If VCC = -VEE, R5 = R7 and R6 = R8, from equations (8) and
(9) results that
VC0  VE0  VCC  0.6 
2 R5
R5  R6
(10)
In previous enounced conditions the difference between
supply voltages VC0 and VE0 is 29 V. The bootstrap amplifier
output signal amplitude is

R 
Vpk  VCC  0.6  1  6  Vsat
R5 

(11)
where Vsat is the op-amp saturation voltage. If we assumes that
Vsat do not exceed 1 V, the op-amp output voltage span (VS) is
VSamp = 27 V and bootstrap amplifier the voltage span is VSboot
= 93 V.
and
R4 R1
1
R3 R2
(14)
For R1 = 80 kΩ, R2 = R3 = R4 = 100 kΩ resistance values
results the voltage amplification Au = 10. If we will use high
voltage transistors (ex. STN83003 [17],STP 93003 [18]), faster
op-amp (ex. LM7171 [19]) R5 = R7 = 10 kΩ and R6 = R8 = 100
kΩ, the supply voltages can be increase at ±180 V. In above
enounced conditions the output signal span will be 336 V.
In conclusion, the bootstrap amplifier has a very good
Slew-Rate parameter, but due to positive reaction loop, the
latch-up risk mast not be ignored.
C. Modified active-load A-class amplifier principle
In almost all sonar and echolocation applications, the
ultrasounds are emitted as a short burst. We speculate this fact
to design a high quality amplifier, based on active load A class
amplifier principle. The presented circuit has two operating
modes: “stand-by” and “burst emission”. Heat will be
dissipated (as in all A class amplifiers) only in the “burst
emission” mode; this way the electrical efficiency of the circuit
is greatly improved.
The circuit presented in Fig. 9a) has four functional blocks:
(1) voltage-current converter: the op-amp, T2, D2, the shunt
resistance R2; (2) active load: T4, R8, and the resistive load
R4; (3) voltage divisor: RB1, TB and RB2 that generate the
signal Vbias used to bias the active load; (4) electronic switch
K that connects the resistor R6, in parallel with R2 when
ultrasonic burst is emitted.
Because of burst shape of the emitted signal, we propose to
reduce the power dissipation between two successive bursts by
reducing the collector current of T2 transistor. So we propose
that the amplifier works in two operating modes:
(1) Burst-Emission: when the contact K is close;
Figure 8. Signals vIN (green), vO (blue), VC0 and VE0 (red)
(2) Stand-By: when the contact K is open.
The voltage-current converter transistor T2 is an A class
amplifier all the time, but the constant current generator
transistor T4 works in switching mode. As a numeric example
for VCC = 200 V, R2 = 1.2 kΩ and R4 = 12 kΩ, in accord to
equation (20) we choose IC4 = 15 mA and results: IC4 = 6.6
mA, R6 = 0.36 kΩ and Vref = 5.8 V (adjustable).
The next schematic, presented Fig. 10, use H bridge
topology with the modified A-class amplifier on each side.
Figure 9. a) The modified A class amplifier principle; b) the output
characteristics of T2 transistor
The open loop active load amplifier has not a stable bias.
The resistor R4 is necessary to stabilize the T2 transistor
Quiescent-point (Q-point) position. Fig. 9b) presents
MATLAB model output characteristics of T2 transistor and the
DC Load Lines for the active load R4, T4 and R8, and the
Q-point positions for the two positions of the contact K. Due to
the active load the A class amplifier has two DC Load Lines.
When K = off, the transistor T4 is off, IC4 = 0 and consequently
results the equation
V
V
IC 2  CC  CE 2
R4
R4
(15)
When K = on, T4 is a current constant generator and we get
the equation
V
V
IC 2  CC  CE 2  IC 4
R4
R4
(16)
Figure 10. The A-class amplifier, H bridge topology
The current generators bridge arms are connected to the
ground and the active loads bridge arms are connected to the
supply voltage. As a rule, the inverting branch indexes are odd
figures and the non inverting branch indexes are even ones.
The capacitive load CL is connected on the other diagonal of
the bridge. Fig. 11 presents the output signals vC2, -vC1, and the
capacitive load differential signal vC2-vC1.
In Fig. 9b) the lower line is equation (15) and the upper line
is equation (16). The Q-point position is the intersection point
between the output characteristic equation and one of the two
previous DC Load Lines. In Burst-Emission mode the Q-point
position is set by adjusting Vref dc signal. In Stand-By mode the
Q-point position depends on R2 and R4 resistor values.
If the output signal is a sine wave, the maximum speed of
the output is
dvL
d

 max  Vpk sin t   Vpk
dt
 dt

(17)
If the output signal peak voltage is 150 V, at 300 kHz, the
maximum speed of the output signal is
dvL
V
 2  f Vpk  282
dt
μs
(18)
For the capacitive load class A amplifier the slew-rate is
SR 
I
CL
V
50 pF  14.13 mA
μs
The power dissipation was computed in the hypothesis that
the H bridge is symmetric and the ratio between burst duration
and burst period is 1/40. Table 1 shows the power dissipation
calculated for all the elements of H bridge elements. All the
components in the H bridge haves the values calculate in
previous section.
(19)
TABLE I.
The slew rate of the amplifier must exceed maximum speed
of the output signal SR > Voω. If CL = 50 pF, using equation
(19) we can calculate the minimum current value for the
constant current generator:
I min  SR  CL  282
Figure 11. Signals vc2 (yellow), -vc1 (green) and vc2-vc1 (red)
(20)
unit
T1, T2
T3, T4
R3, R4
POWER DISSIPATION OF H BRIDGE ELEMENTS
Burst-Emission
dissipate power
mW
1500
485
159
Stand-By
Medium
dissipate power dissipate power
mW
mW
556
579
0
12
419
352
V.
CONCLUSIONS
The objective of the work was to presents some results
concerning ultrasonic transceivers, both emitters and receivers,
tested on a new ultrasonic transducer based on EMFi material.
The investigation is completed with experimental results
obtained on the working field.
To obtain these results, custom transducers were built
starting from blank EMFi film. Building details are presented,
as well as two main characteristics of the transducers –
bandwidth and directivity. Using a latest generation operational
amplifier, with superior characteristics, we have built a receiver
preamplifier as interface between EMFi transducer and signal
processing unit. Gain over 50dB, bandwidth from 0 to 500
kHz, input impedance of 10 MΩ, output swing voltage of 4.8
V, sensibility greater than 50 µV and high immunity to noise
makes the receiver “perfect fit” for its purpose.
Concerning the transmitter circuit, three topologies were
presented, each with its own advantages and disadvantages:
 amplifier with transformer at the output, which will
produce relatively easy high voltage output, but with the
narrow bandwidth of the transformer;
ACKNOWLEDGMENT
The work was supported by the CNMP grant no 12079 /
2008 ADBIOSONAR under PNCDI-II.
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[10]
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