Arbitrary waveform generators permit the user to design and generate
virtually any waveform in addition to the standard function generator
signals (i.e. sine wave, square wave, etc.). Waveforms are generated
digitally from a computer's memory, and most instruments allow
the downloading of digital waveform files from computers.
Ultrasonic generation pulses must be varied to accommodate different
types of ultrasonic transducers. General-purpose highly damped
contact transducers are usually excited by a wideband, spike-like
pulse provided by many common pulser/receiver units. The lightly
damped transducers used in high power generation, for example,
require a narrowband tone-burst excitation from a separate generator
unit. Sometimes the same transducer will be excited differently, such as in the study of the dispersion of a material's ultrasonic
attenuation or to characterize ultrasonic transducers.
Section of biphase modulated spread spectrum ultrasonic
In spread spectrum ultrasonics (see spread spectrum page), encoded sound is generated by an arbitrary waveform
generator continuously transmitting coded sound into the part
or structure being tested. Instead of receiving echoes, spread
spectrum ultrasonics generates an acoustic correlation signature
having a one-to-one correspondence with the acoustic state of
the part or structure (in its environment) at the instant of measurement.
In its simplest embodiment, the acoustic correlation signature
is generated by cross correlating an encoding sequence (with suitable
cross and auto correlation properties) transmitted into a part
(structure) with received signals returning from the part (structure).