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5th
International Conference on NDT of HSNT- IC MINDT 2013
Athens Greece, Eugenides Foundation, May 20-22, 2013
Ultrasonics System for Arbitrary Pulse Width and Position Trains
Application for Imaging
L. Svilainis, V. Dumbrava, S. Kitov, A. Aleksandrovas, A. Chaziachmetovas,
V.Eidukynas. D. Kybartas, K. Lukoseviciute
Kaunas University of Technology, Department of Signal Processing,
Studentu Str. 50-340, LT-51368, Kaunas, Lithuania,
Abstract
Performance improvement of ultrasound application in measurement or imaging demands high
energy and wideband signals for excitation. Spread spectrum signals are used to increase the
signal energy and maintain the resolution. We suggest using novel spread spectrum signals
generation technique: trains of arbitrary pulse width and position (APWP) pulses. It is expected
that APWP should have properties of chirp (wideband and compressible) and of a single pulse
(low correlation sidelobes). In order to complete such study, ultrasonics signals acquisition
system has been developed, capable of APWP pulse trains generation and reception. System
structure and parameters investigation are presented. Acquisition system was developed to
satisfy the specifics of the APWP: ability to generate constant amplitude APWP pulse trains,
acceptable pulser performance, and pulser recovery time. Investigation of system performance
indicates that it can be used for 0.5MHz to 30MHz frequency range: both pulser and
preamplifier have balanced operation range. Excitation signal amplitude can reach 200V for
unipolar signals and 400V p-p for bipolar signals. Variation of pulser output impedance
measured according to EN 12668-1 standard is within 7
according to EN 12668-
Keywords: ultrasonic imaging, iterative deconvolution, composite imaging, ultrasonic
spectroscopy.
Introduction
Ultrasound application in measurement, imaging or navigation systems is mainly based on time
of flight estimate. Propagation time estimate accuracy depend on signal energy, input noise and
signal spectrum. Resolution of the imaging system depends on the envelope bandwidth [1].
Spread spectrum signals are used to increase the signal energy and maintain the resolution [2].
Such signals can turn useful when applied for structural noise reduction in composite materials
imaging [3]. We suggest using novel spread spectrum signals generation technique [4,5]: trains
of arbitrary pulse width and position (APWP) pulses. It is expected that APWP should have
properties of chirp (wideband and compressible) and of a single pulse (low correlation
sidelobes). In order to complete such study, ultrasonics signals acquisition system has been
developed, capable of APWP pulse trains generation and reception. System structure and
parameters investigation are presented.
ToF Estimation
5th
International Conference on NDT of HSNT- IC MINDT 2013
Athens Greece, Eugenides Foundation, May 20-22, 2013
The time of flight (ToF) of the ultrasonics signal is an essential parameter in temperature
measurement, chemistry, navigation or velocity measurement systems [6-9]. It’s estimate is
influenced by the quality of the signal. If estimation is done by locating the correlation
function’s peak [1] in the presence of noise with power spectral density N0, the of ToF variance
is defined by Cramer - Rao lower error bound:
0
22
1
N
EFe
ToF
, (1)
where E is the received signal energy, delivered to impedance Z:
0
*2 21dffSfS
Zdtts
ZE T , (2)
Fe is the effective bandwidth of the signal, constituted by center frequency f0 and envelope
bandwidth :
2
2
222
0
22
0
2
E
dffSf
E
dffSff
fFe
. (3)
The amplifier input noise power density can be simplified as [1]
Z
ZieeN nnVnT
222
0
, (4)
where enT is the thermal noise of the transducer impedance Re(ZT) at temperature T, 4kRe(ZT)T,
enV is the amplifier’s input voltage noise, in is the amplifier’s current noise. Here, SNR is treated
as the signal energy and the noise power spectral density ratio:
0
2
N
ESNR . (5)
The signal energy can be improved by increasing the excitation amplitude (usually limited by
the transducer design or electronics), or by increasing the signal duration.
APWP Signals
From equations (1)-(3) it can be seen that accuracy can be improved by increasing the signal
bandwidth or increasing the energy. Energy enhancement is possible by a probing signal
amplitude or duration increase. The excitation amplitude is limited by the transducer
construction and the electronics capabilities. Another approach would be to increase the duration
of the exciting signal. But if it is a simple CW burst then the envelope of the correlation function
is not sharp and resolution will be low. Spread spectrum (SS) signals allow to achieve
compression using matched filtering [2,6,10] thanks to wide bandwidth and long duration.
Wideband signals can also turn useful if applied for structural noise dispersion increase in
composites imaging [3]. Arbitrary waveform signals are not easy to generate if large voltages
have to be delivered to capacitive load [10,11]. If linear frequency modulation signal (Fig.1) is
used, such signal is significantly compressed during the matched filtering.
5th
International Conference on NDT of HSNT- IC MINDT 2013
Athens Greece, Eugenides Foundation, May 20-22, 2013
Fig.1. APWP pulse trains comparison against conventional excitation signals
But the correlation sidelobes of the chirp are higher than for simple rectangular pulse [10].
Nonlinear frequency modulation (NFM) signals are gaining popularity thanks to the ability to
control the shape of the spectrum and so the sidelobe level [11]. In turn rectangular pulses are
easy to generate and exhibit low correlation sidelobes, but do not possess the high energy and
orthogonality set of SS. Novel spectrum spread technique did not receive the proper attention in
ultrasound: trains of the arbitrary position and width pulses (APWP) [4, 5]. Technique is using a
chaotically placed train of square pulses with arbitrary position and width (Fig.1). We suggest
using this technique novel spread spectrum signals generation technique [10]: trains of arbitrary
pulse width and position (APWP) pulses.
Initial study carried out in [10] suggests that APWP should have wide, controllable bandwidth,
similar to chirp and same time should have slightly lower correlation sidelobes compared to
rectangular pulse (Fig.2).
Fig.2. Comparison of chirp, rectangular pulse and APWP signals’ correlation function
Analysis of correlation functions of the signals presented in Fig.2 indicates that sidelobes-
optimized APWP signal maintains the mainlobe width (similar to pulse). Meanwhile, APWP
signal energy is the same as chirp signal. But chirp has much higher correlation sidelobes if
rectangular slope is used.
Since APWP signal is constructed by the set of constant amplitude but arbitrary duration pulses
with arbitrary spacing, generation of such signals should be almost as simple as rectangular
pulses. But, new, better than pulse correlation properties can be obtained. We are aiming to
investigate such signals in ultrasonic imaging and ToF measurement applications. In order to
have the ability to generate such signals and carry out the research of APWP application,
dedicated acquisition system was developed.
System Structure
Ultrasonics signals acquisition system developed is capable of APWP signals generation and 3D
spatially correlated data acquisition. System structure is presented in Fig.3.
Pulse
CW burst
Chirp
APWP
-400.0n -200.0n 0.0 200.0n 400.0n
-1.0
-0.8
-0.6
-0.4
-0.2
0.0
0.2
0.4
0.6
0.8
1.0
Chirp
PulseAPWP
Ma
tch
ed
filt
er
ou
tpu
t, A
.U.
Time, s
5th
International Conference on NDT of HSNT- IC MINDT 2013
Athens Greece, Eugenides Foundation, May 20-22, 2013
Fig.3. Acquisition system’s structure and the 3D transducer positioning equipment
System is composed of the main acquisition module, analog front-end and the transducer
positioning equipment. Main module (Fig.4) is responsible for low power excitation signals
generation and analog signals digitization and transfer to PC using USB connection.
Fig.4. Main acquisition module structure and PCB design view
It is located at host PC end. It is using two 10 bit resolution analog-to-digit converters (ADC)
with programmable conversion frequency up to 100 MHz. Resolution and sampling rate of ADC
was chosen basing the analog electronics noise level and is balanced against expected ToF
estimation systematic errors [1].
Analog front-end (Fig.5) is located at scanner end and is used for low power excitation signal
conversion into high voltage excitation signal and received signals preamplification.
Fig.5. Analog front-end structure
Excitation signals amplitude is controlled by varying the high voltage DC/DC converter output.
It can be varied up to 200 V. High voltage is fed to pulser which basically chops the high
voltage and feeds it to RF transformer output, forming switched push-pull pulser [12].
Excitation signal’s rise/fall times of 10 ns ensure up to 30 MHz system bandwidth. Pulse train is
formed in main acquisition block at 100 MHz rate. Toggling step rate of APWP pulse train is
such way is 10 ns. Reception channel’s gain is controlled at analog front-end side (Fig.5) and at
the main acquisition block (Fig.4). It can be varied in 80 dB range.
Water tank
X
Scanner
controller
Main
acquisition
moduleHost PC
Analog
front-end
Transducer
Z
3D scanner
Y
ADCUSB
communication
and
controlADC
RS485
conn
conn
To front-end
USB,
to PC
APWP RAM Drivers
Pulser
Local
uControllerTo main
acquisition
module
conn
HV DCDC
RS485
5th
International Conference on NDT of HSNT- IC MINDT 2013
Athens Greece, Eugenides Foundation, May 20-22, 2013
Scanner (Fig. 3) is controlled by dedicated controller with USB or RS485 connectivity. Every
axis (X,Y,Z) has it’s own step motor driver. Minimum scanning step along X and Y axes is
10 Z axis.
Pulser Performance Measurement
Since essential parameters of APWP generation are defined by quality of the high voltage pulser
output, investigation of pulser output impedance and attainable bandwidth was carried out.
Standard EN_12668-1 defines the ultrasonic pulser output impedance Zo measurement
procedure by using two output voltages obtained at 50 V50) and 75 V75) pulser load:
07550
5075
5075
7550
NVV
VVZo
. (6)
We have slightly modified this procedure in order to investigate the output impedance at
particular frequency. Since system is capable of automated data collection using several
excitation signals, set of CW burst signals was formed and stored for automated generation and
acquisition. Voltages V50 and V75 were recorded using 1:100 voltage divider formed by 5
50 The amplitude and phase of the harmonic signal can be extracted by using the
Sine wave correlation (SWC) technique. SWC can be treated as correlation coefficient of the
signal’s sampled version sn with the sine and cosine signals:
1
1
0
1
1
0
2sin2
2cos2
WN
wsft
fS
WN
wsft
fC
N
n
nnn
N
n
nnn
, (7)
where, N is length of the observation window wn, W1 is a L1 norm of the window. It was
decided to use the excitation generator using the direct digital synthesizer (DDS). The CW
bursts used were 10 periods long and their frequency was in fixed proportion to system clock
frequency. Since data acquisition and excitation used common reference oscillator, frequency-
related amplitude estimation errors were minimized. Results for output impedance magnitude
variation with output amplitude are presented in Fig.6.
Fig.6. Pulser output impedance vs. frequency
500k 1M 10M 30M
3
6
9
12
15
18
21
24
27
30
Zo
ut,
Frequency, Hz
@ 50V
@ 100V
@ 150V
@ 200V
5th
International Conference on NDT of HSNT- IC MINDT 2013
Athens Greece, Eugenides Foundation, May 20-22, 2013
It can be seen that output impedance magnitude is stable with output voltage and is rising with
frequency. Variation is within 7 Obtained output impedance now can be used
for pulser performance together with ultrasonic transducer estimation. Measured transducer
impedance Zin can be used to calculate the complex power delivery:
*
2
inoino
ing
TZZZZ
ZeS
, (8)
where eg is the intrinsic generator voltage. Assuming that losses in matching circuit are
negligible then real part PT of the complex power delivered ST is equal to the power transmitted
by transducer (in case of the optimal transducer design). Then the power delivery to load
efficiency criteria can be established, which indicates the ratio of the real power conveyed to
load and the power available from the generator:
%100
4%100
Re4*2
inging
ing
g
Tg
ZRZR
ZR
e
SR
, (9)
Same equation (7) was used to obtain the pulser output voltage. Pulser main harmonic output
voltage plot against load type and frequency is presented in Fig.7.
Fig.7. Pulser output amplitude vs. frequency and load type
Pulser is using active switches for both positive and negative front’s generation. Such topology
should allow for quite efficient energy use: energy is consumed only during the pulse fronts.
Energy consumption during positive front and negative front is still present, since parasitic
capacitance of output MOSFET’s has to be charged. If capacitive load is used (piezoelectric
transducer will exhibit such load) additional losses occur. Efficiency of pulser was evaluated by
comparing the energy consumption per pulse against theoretically expected energy consumption
if only capacitive load was the cause of losses. Energy stored in capacitor C0 which is charged to
voltage V is
2
0
2 CVE HV , (8)
Then, power consumed by pulser generating burst of N pulses over 1 s at pulse repetition
frequency PRF is:
NPRFCVP HVMM 0
2
21 , (9)
500k 1M 10M 40M
0
50
100
150
200
250
Vo
ut, V
Frequency, Hz
50 load:
@ 50V
@ 100V
@ 150V
@ 200V
1000pF load:
@ 50V
@ 100V
@ 150V
@ 200V
5th
International Conference on NDT of HSNT- IC MINDT 2013
Athens Greece, Eugenides Foundation, May 20-22, 2013
Power consumption on HV power supply was monitored and equation (9) was used to obtain the
power per pulse consumption. Results for 200 V output with unloaded pulser (Fig.8 left) and
50 .
0.9 1 2 3 4 5 6 7 8 9 10 2020
10
20
30
40
50
60708090
100
200
300300 Theory SPD02N60C3
IRFRC20 SPD02N60S5
IRFRC20 without cancel winding
Ene
rgy p
er
pu
lse
(
J)
Frequency (MHz)
1 2 3 4 5 6 7 8 9 10 20 3030
2020
30
40
50
60708090
100
200
300
400
500
600700700 Theory SPD02N60C3
IRFRC20 SPD02N60S5
Ene
rgy p
er
pu
lse
(
J)
Frequency (MHz) Fig.8. Energy per pulse consumption at various pulser components combinations for unloaded case
(left) and 50
Unfortunately, pulser can not be used for frequencies below 1 MHz: transformer output is
beneficial at high frequencies, but at low frequencies magnetizing inductance of the output
transformer is tampering the output, core saturates and efficiency and output amplitude is
dropping.
Preamplifier Performance Measurement
Preampifier parameters define the reception performance of the system. AC response of the
preamplifier was investigated using SWC technique: input was fed by harmonic signal from
direct digital synthesizer, driven by the same clock source as the acquisition channel. Signal at
the input was registered by channel 1 (Fig.4) and the output signal was fed to channel 2.
Obtained frequency response is presented in Fig. 9.
Fig.9. Preamplifier AC response for the gain (left) and phase (right).
Fig.9 shows that at 45 dB gain passband frequencies are from 318 kHz to 33.2 MHz. With gain
AC response available, input noise measurement can be established. Usual approach for
amplifier noise estimation could have been using spectrum analyzer. But since preamplifier
output sampling by ADC was readily available, more simple noise estimation procedure was
100k 1M 10M 40M
38
39
40
41
42
43
44
45
33.2MHz
318kHz
Ga
in, d
B
Frequency, Hz
GdB
F1
100k 1M 10M 40M
-90
0
90
ph
ase
, d
eg
Frequency, Hz
phaseGmax
phaseGavg
phaseGmin
5th
International Conference on NDT of HSNT- IC MINDT 2013
Athens Greece, Eugenides Foundation, May 20-22, 2013
used: amplifier output noise was recorder using sampling ADC. Record length of 32 k samples
was chosen (equivalent to 3 kHz resolution bandwidth). FFT has been used for noise power
spectral density estimation. In addition multiple noise power density measurements were power-
wise averaged Cmax times to obtain the statistical noise estimate:
NCf
es
PSDs
C
c
N
n
nkN
i
n
max
1
21
0
2max
2
, (10)
where N is the record length, 32 k samples; sn is the recorded noise signal. Measured gain was
used for output noise density conversion into input noise voltage density (Fig. 10).
Fig.10. Preamplifier input voltage noise density AC response
Noise whiteness now can be estimated: it can be seen that 5 MHz to 7 MHz frequency range
frequency response for input noise is sufficiently flat and can be considered as for white noise.
Moreover, measurements confirm that input voltage noise is 2 nV/Hz which produces right
balance with 10 bit ADC conversion resolution [1].
Recovery time of the ultrasonic preamplifier is important in pulse-echo mode when operation in
close vicinity is required. If not essential for conventional pulse systems, APWP signal requires
special attention on preamplifier recovery time: APWP signals suppose to be much longer than a
single pulse. Recovery time was estimated using similar to EN 12668-1 procedure: preamplifier
input was fed with sinusoidal signal via high impedance circuitry and probing rectangular pulse
was fed into same preamplifier. Recorded signal’s envelope was taken and recovery time
estimated (Fig.11).
Fig.11. Preamplifier recovery time measurement, using output envelope
100k 1M 10M 40M
0.0
5.0n
10.0n
Volta
ge
no
ise
de
nsity, V
/sq
rt(H
z)
Frequency, Hz
Max gain
Nin150
Ninopen
NinShort
Mid gain
Nin150
Ninopen
NinShort
Min gain
Nin150
Ninopen
NinShort
-5.0µ 0.0 5.0µ 10.0µ 15.0µ0.0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1.0
1.1
8.8us
Norm
alis
ed
envelo
pe
, A
.U.
Time, s
5th
International Conference on NDT of HSNT- IC MINDT 2013
Athens Greece, Eugenides Foundation, May 20-22, 2013
Output pulse was deliberately delayed so envelope magnitude before the pulse can be noted.
According to the EN 12668-1, recovery time should be estimated at 0.5 level. Taking
measurements at this level, with pulse duration subtracted, recovery time is 2 If much
stricter requirement, 0.95 level is used, recovery time is 8.8 .
Conclusions
Design of dedicated system was needed in order to start the investigation of novel APWP class
of signals. Acquisition system was developed to satisfy the specifics of the APWP: ability to
generate constant amplitude APWP pulse trains, acceptable pulser performance, and pulser
recovery time. Investigation of system performance indicates that it can be used for 0.5MHz to
30MHz frequency range: both pulser and preamplifier have balanced operation range. Excitation
signal amplitude can reach 200V for unipolar signals and 400V p-p for bipolar signals.
Variation of pulser output impedance measured according to EN 12668-1 standard is within 7
to 21 -
Acknowledgements
This research was funded by a grant (No. MIP-058/2012) from the Research Council of
Lithuania.
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5th
International Conference on NDT of HSNT- IC MINDT 2013
Athens Greece, Eugenides Foundation, May 20-22, 2013
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