Application Note

Using Spectrum Digitizers in Ultrasonic Applications
The use of Ultrasonic products is increasing as new techniques and improvements in instrument
performance constantly expand the range of applications. Spectrum digitizers are ideal tools formaking ultrasonic measurements and can play a key role required in the development, testing andoperation of these products. Spectrum digitizers and arbitrary waveform generators offer a wide
range of bandwidths, sampling rates, and dynamic range to match the broad spectrum of
ultrasonic measurement needs.

Figure 1: Spectrums M4i.4451-x8 – a PCIe 4 channel 14 bit 500 MS/s digitizer in acquisition ofultrasonic signals

Ultrasonic Applications
Ultrasound is an acoustic sound pressure (acoustic) wave with a frequency greater than the upper
limit of the human hearing range. Ultrasound devices operate with frequencies from 20 kHz up toseveral gigahertz.
Table 1 summarizes the characteristics of a range of some the more common ultrasonicapplications.

© Spectrum GmbH, Germany

1/4

Application Note
Application

Typical Frequency
Range

Simplex or Duplex

Dynamic Range Recommended
Spectrum DigitizersNondestructive
Testing (NDT)

0.1-100 MHz

Duplex (Transmit/Receive) Moderate

M2i.20xx, M2i.49xx,
M4i.22xx

Medical Imaging

1-18 MHz (High-Res up toseveral GHz)

DuplexHigh

M2i.49xx, M4i.44xx,
M4i.22xx

Ultrasonic cleaning 40 kHz

Simplex (Transmit)

Low

M2i.20xx, M2i.47xx

Ultrasonic Welding 20-100kHz

SimplexLow

M2i.20xx, M2.47xx

Range Finding

40 kHzDuplexModerate

M2i.47xx

Parking Assistance 40 kHz

DuplexModerate

M2i.30xx, M2i.47xx

Scanning Acoustic 2 MHz – 1 GHz
Microscopy (SAM) 2- 230 MHz typicalDuplex

High

M4i.22xx, M4i.44xx

Flowmeters

640 kHz – 1 MHz

DuplexModerate

M2i.49xx, M4i.22xx,
M4i.44xx

Time of Flight
Diffraction

1 – 15 MHz

DuplexModerate

M2i.49xx, M4i.22xx,
M4i.44xx

Table 1: Characteristics of common ultrasonic applications along with the recommended Spectrum
digitizers

The range of frequencies used in each application reflect an engineering trade-off. Increasing theoperating frequency allows detection of smaller artifacts by increasing resolution but the higher
frequency signals don’t penetrate as far. The common problem with ultrasound applications issignal attenuation, which is inversely proportional to the signal frequency. As a result, very highfrequencies tend to be used in surface study applications while lower frequencies are more
dominant whenever greater penetration and power is needed. Of course, increasing the dynamic
range of your digitizer also allows you to detect smaller signals.

Sample Rate
The basis for the products selection is related primarily to the frequencies used in the application.
The sampling rate of the digitizer, in general, needs to be 5 to 10 times the application’s frequency.
Except when the application uses Doppler shifts, even though the frequencies may not be thathigh, the timing resolution needs to be higher as frequency shifts, which are often fractions of asignal period need to be measured. In Doppler applications the sample rate of the digitizer may
need to be much more than 10 times the used frequency.

Bandwidth
The digitizer bandwidth should exceed the highest frequency used in the application by at least afactor of two. Working with a lower bandwidth will lead to attenuation of higher frequency signals
and may limit measurement resolution and accuracy.

Dynamic Range
Increasing the digitizer’s dynamic range (number of bits) allows the detection of smaller signals.
Higher resolution ADC’s generally deliver better signal-to-noise ratio making it possible to detectlarge and small signals in the same acquisition. This is why leading edge systems often use higher
resolution ADC’s or signal processing (like averaging and filtering) to improve their overallmeasurement sensitivity.

© Spectrum GmbH, Germany

2/4Application Note

Other Considerations
The digitizer’s input circuits must be a good match to the ultrasonic sensor’s output impedanceand coupling requirements. Many of the Spectrum digitizers offer a choice of input paths,
configurations and termination impedances allowing for the best possible match.
Depending on the nature of the ultrasonic signal the digitizer’s acquisition modes may also beimportant. Applications using multiple signal bursts, or pulsed ultrasound, benefit from thedigitizer’s ability to accept and process multiple acquisitions with minimal dead-time between theburst events. Multiple (segmented), gated and streaming acquisition modes can all play a part inmaking sure each event can be accurately captured and analyzed.
Additionally, Spectrum digitizers offer signal processing such as averaging, peak detection, filteringand Fast Fourier transform (FFT). Of these, averaging and peak detection are available as FPGA
based internal processing functions. The other signal processing features are available inSpectrum’s SBench 6 or third party software.

A Typical Ultrasonic Application
The following measurement of an ultrasonic range finder illustrates some of the features available
in Spectrum digitizers. The device transmits a burst of five, 40 kHz, acoustic pulses. Themeasurement sensor for this test was a 100 kHz bandwidth instrumentation microphone. The
microphone required a one megaohm input termination, DC coupled. Figure 2 shows theSpectrum SBench 6 software’s display of this measurement.

Figure 2: The use of Spectrum’s SBench 6 software to acquire and process a 40 kHz ultrasonicsignal. The five segments of a multiple acquisition are processed using a band pass filter,averaging, and FFT

The digitizer was setup using multiple acquisition mode. It acquired the five ultrasonic pulses as asingle measurement. The preview pane at the top of the display shows these burst pulses. Each of© Spectrum GmbH, Germany

3/4

Application Notethese events was time stamped and the timestamp table in the lower left corner of the screen
shows the event time both absolutely and relative to the other events. Time stamping provides asimple way to measure the pulse repetition period while the multiple acquisition mode makes it
easy to compare individual events and make other measurements such as pulse duration, duty
factor, spatial pulse length, peak amplitude and time.
A zoom display of the first of the acquired bursts, including the attenuated reflection from thetarget, is shown in the upper left displayed trace. Note that the trailing edge is not flat. A view of theFFT shows the spectrum of the acquired signal in the lower right quadrant. In addition to the 40
kHz primary frequency there is a second harmonic at 80 kHz and significant low frequency
spurious components. The baseline rise in the acquired signal is due to the low frequency spurious
pickup. Based on this spectral view a band pass filter with cutoff frequencies of 20 and 50 kHz isapplied to the signal (upper right grid). Filtering resulted in a flattening of the trailing edge of thesignal. The average of the five acquired bursts is displayed in the lower left grid. The vertical axisof each of these views is scaled by the microphone’s sensitivity and reads in units of soundpressure (Pascal). These views provide significant quantified information about the acquired
signal.
In addition, measurements of the signal frequency and maximum and minimum signal amplitude
are shown in the box marked ‘Info’. This is a small sample of the measurements available.
The digitizer and its accompanying software provide a multitude of measurement and analysis
tools to aid in the development of ultrasonic applications.

© Spectrum GmbH, Germany

4/4