Download Coded Signals

Ultrasound Microscopy and
High Frequency Coded Signals
Antti Meriläinen, Edward Hæggström
www.helsinki.fi/yliopisto
Ultrasound Microscopy
What it is?
• Using high frequency acoustic
waves for mm-/µm-scale imaging
• Method is non-destructive
• It “Sees” inside the sample
• Ultrasound images differences of
acoustic impedances
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Ultrasound Imaging
Amplitude image
TOF image
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Ultrasound Microscopy
Basic techniques
Single transducer
pulse-echo
Phase Arrays
http://en.wikipedia.org/wiki/Ultrasonic_testing
http://www.nde.com/phased_array_technology.htm
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Focused Ultrasound Transducer
[Yu, Scanning acoustic microscopy and its
applications to material characterization, 1995]
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Tx/Rx for USM
• TX
• Pulser, delta spike excitation
• Gated sinus wave
‒ For high frequencies ~1 GHz
• RX
• Protection circuit & Pre-amplifier
• (Envelope detector / pulse shaper)
• Oscilloscope
Camacho, J., Fritsch, C.: ‘Protection circuits for ultrasound applications’ Ultrasonics, Ferroelectrics and
Frequency Control, IEEE Transactions, 2008, 55, (5), pp.1160-1164
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Challenges with current
techniques
• Delta spike excitation
• Stress for transducer and sample
• Energy/amplitude variation with high PRF
• Gated sinus
• Stress for transducer and sample
• Uncertainty of Time-of-Fly (TOF)
‒ Depth resolution
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Coded USM
•Coded signals
•Electronics
•Signal generation
•Switch and timing
•Preamplifier
•Signal Synthesis
•Ultrasound measurements
•RF-design
•Components
•PCB layout
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Coded Signals
• Tx signal is wave packed
• Frequency can be programmed
• Phase can be programmed
• Envelope (amplitude over time)
can be programmed
• Example linear frequency
modulation (LFM)/chirp
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Cross Correlation
dt descript depth resolution
dt depends on bandwidth
dt
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Coded Signal and SNR
SNR =1
SNR =10
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Signal generation
Numerical vector to Electric signal
• Arbitrary waveform generators
• Digital to Analog converter (DAC)
• Bandwidth up to 120 MHz (2 GS/s)
• If you have money: 5.6 GHz (24 GS/s)
• High frequency signal generators
• Output: continuous sine wave
• Frequency range up to 4+ GHz
• Narrow modulation bandwidth (less than 1
kHz)
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Modulation techniques
• Modulation = change carrier wave by signal
• Amplitude modulation (AM)
‒ Quadrature amplitude modulation (QAM)
• Frequency modulation (FM)
• Phase Modulation (PM)
• Many other ….
Modulation
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QAM / IQ-modulation
• AM:
• QAM:
•
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TRF370417 Modulator
• Arbitrary/modulation bandwidth
is 2*120 MHz
‒ dt = 4.2ns
• Center output frequency is set
by Local oscillator
• Output Bandwidth is NOT
maximum output frequency
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Modulator outputs
Q
Lo
I
RF Out
1 cm
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Carrier Feedthrough and
Sideband Suppression
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Preamplifier
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Preamplifier Design
• Amplification
• Cascade design
Modulator ->
Attenuator(-60 dB) ->
Preamplifier(+55 dB)
• Voltage range
• Max/Min signal input strenght
• Impedance maching
• Input impedance
• Output impedance
• DC-blocks
• Capacitors and inductos for high frequencies
• Same component can be tunet for different band
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Switch and Timing
• Receiving during
transmission is impossible
• Transducer delay line gives
time limit for coded signal
• Typically 0.3 – 5 µs
• Signal generator limits coded
length 8 µs
• Maximize signal time and
minimize switching time
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Switch Circuit
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Switch designing
• Power handling
• Bandwidth
• Attenuation
• Insertion loss (Smaller is better)
• Isolation (Higher is better)
• Switching time
• Glitch
• AC/DC coupling
• Control voltages
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Timing
• Circuit based on AVR
µController
• Programmable
• Predictable
• Timing resolution is
62.5 ns
• AVR trigs AWG and
oscilloscope and
controls the switches
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Timing Circuit
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Coded USM
•Coded signals
•Electronics
•Signal generation
•Switch and timing
•Preamplifier
•Signal Synthesis
•Ultrasound measurements
•RF-design
•Components
•PCB layout
www.helsinki.fi/yliopisto
Signal generation
How to generate I and Q
• I and Q are numerical signals that can be generated
by Matlab
LO
cos
LO
sin
RF
X
X
LO
cos
Q
AWG
X
LP
LP
Q
I
Matlab
LO
sin
I
X
+
I&Q
RF
Modulator
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Results with 100 – 300 MHz
Transmitted
signal
Received Aline
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B-scan
image
27/15
Results from 2010: 30 – 70 MHz Coded signal
• Signal-to-noise ratios (SNR) of surface echoes were estimated to compare
coded excitation and delta spike excitation
• Preliminary results showed that coded chirp signal excitation increased mean
SNR (16±3) dB for 75 MHz transducer
Pulse-echo measurement using a coded
5 Vpp chirp signal excitation at 30-70
MHz (left) and a 33 Vpp delta spike
excitation (right). The coded excitation
increased mean SNR (16±3) dB.
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Higher frequency and coded
signals
• Higher frequency gives resolution
• Modulator shift arbitrary band (Not increase bandwidth)
• Coded signals may improve SNR/CNR
• Cross correlation is sensitive for noise which has same
band than signal
• Bad modulator can generated ”noise” (Feedthrough)
• Effective bandwidth can be tuned by arbitrary code
• Transducer bandwidth
• Attenuation in immersion liquid
• Arbitrary codes able multitone transmission
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RF design
• Impedance matching
• Single-end vs. Differential signals
• Available IC components:
• Amplifiers
• Attenuators
• Switches
• Modulators / Demodulators
• Power detector
• Clock generator (PLL/VCO)
All components are SMD
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Single-End vs. Differential signals
• Differential signals:
•
•
•
•
Single supply
No ground loops
Longer signal path
Reduces common-mode noise
(noise from ground)
• Paired signal is required
• Single
There is amplifiers for conversion
• Simpler design
• (Dual supply)
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Available IC components
Amplifiers
• Low noise (Pre. Amp.)
• Noise figure <1dB
• Gain ~20dB
• Gain blocks
• 50 Ω line driver
• Power amplifier (Linear amplifier)
• Differential amplifier
• Variable gain amplifier (VGA)
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Available IC components
Modulators
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Available IC components
Modulators
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