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Synthetic Impulse™

The SIA-7 is capable of operating as three different types of ultrasonics systems.  In effect, the SIA-7 is three boxes in one.  The SIA-7 switches between modes at the click of a mouse.

Ultrasonics systems employ methods including pulser receivers, continuous tone, tone burst systems and now Synthetic Impulse™.  Each has certain properties that can be exploited to measure physical characteristics of a material.

·        The waveform must be finite in length (even continuous tone systems must be started and stopped at some point).

·        The waveform must have a finite frequency range (also known as its bandwidth).

So what's the difference between a 'pulse', a tone burst, a continuous wave and a Synthetic Impulse™?

Pulser Receiver: 

Pulser receivers are not very sophisticated but they are familiar and easy to understand.  Analyzing pulser data is not nearly as simple as it appears.

Figure 1.   Pulser Receiver - This image shows a simulated pulser receiver response.  The top plot shows the time domain representation of the signal.  The pulse shows a small number of oscillations covering approximately 20 microseconds.  The second plot shows the envelope of the signal.  Note the side lobes on either side of the main peak.  The third plot shows the Fourier Transform of the signal.

A pulser receiver is a simple device.  The basic design often uses some type of avalanche discharge into a capacitive transducer.  The discharge acts like an impulse to create a short-lived sound wave.  The waveform produced is limited by the bandwidth of the transducer and by the electrical characteristics of the cables and matching components used to connect the transducer to the pulser receiver.

A meaningful analysis of the short duration pulses requires a detailed understanding of the physics.  For example, the time of flight is sometimes evaluated using simple zero crossing techniques.  This technique does not allow for the fact that the phase of the signal changes as it passes through each boundary.  A pulse that enters a plate from one side will be ‘upside down’ when it leaves the far side!  Failure to compensate for such phase changes causes large absolute errors in the estimated speed of sound.  The SIA-7 uses analysis tools designed to eliminate this type of error.

Short excitation - duration and shape is determined primarily by the frequency response on the transducer and the attenuation as a function of frequency.  The short excitation generally requires a very large number of samples per second to get an 'accurate' measure of the waveform.

Limited total energy - The amplitude of the input pulse and the sensitivity of the transducer limit the total energy.  The voltages applied to the transducer must stay below the transducers breakdown limits.  The signal to noise ratio of such systems is inherently limited.  This often requires large signal averaging to compensate for the limited dynamic range.  Even though pulser receiver units can operate at very high pulse repetition rates, the signal averaging necessary to ‘clean up’ the signals effectively eliminates this speed advantage.

Tone Burst: 

In this mode the unit produces short tones at a specified frequency.  This is often useful when performing certain high precision frequency dependent measurements.  Just detecting the signals is never enough.  The SIA-7 features specialized analysis tools to capture both the phase and magnitude of the waveforms.  This is critical especially when trying to learn about a material or sample.

Figure 2.   Tone Burst - This image shows a simulated Tone Burst response.  The top plot shows the time domain representation of the signal.  The tone burst shows dozens of oscillations covering approximately 200 microseconds. The length of the tone burst and the envelope of the signal are arbitrary.  The second plot shows the envelope of the signal.  The third plot shows the Fourier Transform of the signal.

A Tone Burst system is generally more complicated than a pulser receiver.  The system must include an arbitrary function generator that is programmed with different duration tone bursts and each tone burst may have a different phase and frequency.  A Tone Burst system operates in a relatively narrow frequency range.  To be effective, the tone burst frequency must fall within the bandwidth of the transducer.  An isolated tone burst is simple enough to visualize.  The signal is much more complicated in the presence of multiple interfering signals.

Long excitation - duration and shape is determined primarily by the settings on the Tone Burst system.   The long tone bursts can be accurately measured using relatively modest sampling rates.  Analysis of tone burst signals generally requires that the signals be well isolated so that interfering reflections do not distort amplitude and time measurements.  Determining the time of flight of a single tone burst is also a complicated operation.  The phase behaviour of the material also can affect a simple tone burst signal.

Unlimited total energy - A long excitation generally allows a large amount of ultrasonic energy to be applied without exceeding the breakdown threshold of the transducers.  The tone burst duration and the sensitivity of the receiver will determine the signal to noise ratio of such systems.  Tone Burst generators are often used with Lock In amplifiers to achieve very high signal to noise ratios at the cost of reduced speed of operation.

Synthetic Impulse™: 

Synthetic Impulse is VN Instruments patented signal processing that greatly boosts the dynamic range and accuracy of any transducer.  This mode offers the apparent simplicity of a pulser receiver but it is vastly more capable.  The effective dynamic range of Synthetic Impulse™ makes short work of almost all materials and applications.  The SIA-7 has a powerful suite of analysis tools that complement the Synthetic Impulse™ module.

Figure 3.   Chirp - This image shows a simulated Chirp response.  The top plot shows the time domain representation of the signal.  The Chirp shows dozens of oscillations covering approximately 200 microseconds. The length of the Chirp and the envelope of the signal are arbitrary.  The second plot shows the envelope of the signal.  Note the similarity between the envelope of the Chirp signal and the envelope of the Tone Burst signal.  The third plot shows the Fourier Transform of the signal.

The SIA-7 processes the chirp based signal into a Synthetic Impulse™ image.  The resulting image data has the very useful property that the pulse width is independent of the chirp length.  Increasing the chirp length increases the signal to noise ratio without affecting the apparent pulse width!  This is how the SIA-7 overcomes very large acoustic losses and to obtain very accurate image data in the process.

Long excitation - duration and shape is determined primarily by the settings on the Chirp system.  The long Chirps can be accurately represented using relatively modest sampling rates.  Using Synthetic Impulse™, the resulting image data has all the advantages of a tone burst system with the time resolution of a pulser receiver.  The SIA-7 fully implements the Synthetic Impulse™ module. It’s fast, simple and efficient.

Unlimited total energy - A long excitation generally allows a large amount of ultrasonic energy to be applied without exceeding the breakdown thresholds.  The Chirp duration and the sensitivity of the receiver determine the signal to noise ratio of such systems.  Each chirp can be used to create a Synthetic Impulse™ image.  This achieves good time resolution with each chirp. 

Large Available Bandwidth - The construction of the chirp creates a large frequency spread or bandwidth.  This makes a Synthetic Impulse™ system ideally suited for spectroscopy applications that require high bandwidth and high signal to noise ratio.

Figure 4.   Synthetic Impulse™ - The Synthetic Impulse™ has the ideal peak widths while maintaining very good coverage of the entire available frequency spectrum.

 

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Copyright © 2007 VN Instruments Ltd
Last modified: April 02, 2007