Application: Use of preamplifiers in ultrasonic flaw detection, thickness gaging, and other ultrasonic test and measurement applications.

Background: Certain ultrasonic applications require the addition of a separate preamplifier to standard flaw detectors, thickness gages, or other instruments in order to provide sufficient gain or broadband signal-to-noise enhancement for optimum echo acquisition. This sometimes happens in industrial applications that involve testing thick sections of material that exhibit high ultrasonic attenuation or scattering, such as nodular cast iron, austenitic steel, brass, bronze, rubber, fiberglass, or certain composites. A preamplifier is also an important component of acoustic emission test systems that require amplification of low amplitude signals. Preamps can also be used to compensate for amplitude losses caused by very long cables between the transducer and the test instrument, to improve signal-to-noise ratio in scanning applications involving large test specimens, and to amplify small tip diffracted signals in time-of-flight diffraction (TOFD) tests. Panametrics-NDT preamplifiers have also been used in many diverse research applications such as elastic modulus measurements in rocks and minerals, analysis of acoustic scatterers such as bubbles or plankton in water and other liquids, and biological tissue analysis. In general, a preamplifier can be used in almost any application where additional gain is required for optimum performance.

Theory of Operation: The primary purpose of a preamplifier is to improve receiver signal to noise ratios in applications involving low level acoustic signals. By providing supplementary low noise amplification, a preamp allows use of lower gain in the primary test instrument, thus limiting baseline noise. A preamp may permit detection of small echoes that cannot be detected even at maximum gain in the primary test instrument. Additionally, the RF bandpass filtering in preamplifiers further helps limit high frequency noise.

The waveforms below represents a thickness measurement of a 5.6 in. (140 mm) rubber tire tread over fabric reinforcements in a heavy equipment tire, using a special 500 KHz dual element transducer. In the absence of a preamplifier in some challenging tests like this, instrument receiver gain must be set near maximum in order to obtain an echo, and there can be significant baseline noise:

But if a preamplifier is added to the receiver side of the system, then instrument gain can be lowered by more than 30 dB while still maintaining a strong backwall echo, greatly reducing baseline noise:



Connection Procedure:
There are two ways of connecting a preamplifier into a test system, as outlined below.
1. Pulse/echo systems with single element transducers


Figure 1 -- Cable connections for pulse/echo setup.

2. Through transmission, pitch/catch, or dual element transducer systems



Figure 2 -- Cable connections for through transmission system.

For systems using separate transmitter and receiver transducers or dual-element probes, the preamplifier is connected on the receiver side of the system between the receiver transducer and the receiver input of the test instrument. If the ultrasonic instrument has a mode selection switch (single element/dual element or pulse/echo-thru transmission), it should be set to the dual element or thru transmission position.

General notes: As noted above, whenever using preamplifiers the test instrument should normally be set to through transmission, dual element, or pitch/catch mode even when using single element transducers in a pulse/echo setup.

Preamplifiers are most effective in situations where signal loss is due to attenuation rather than scattering from grain boundaries, reinforcing fibers, or other discrete reflectors. This is because scatter noise will be amplified along with the desired echoes.

The amount of gain that can be effectively used in any given ultrasonic system is limited by the internal noise levels and amplifier characteristics of the instruments involved. Panametrics-NDT preamplifiers have very low internal noise (5µv peak to peak referred to the input for the 5660B), but generally system RF voltage gains above 100 dB are not practical. Panametrics-NDT flaw detectors and pulser/receivers should be used in the lowest practical gain setting when a preamplifier is used to avoid amplifier saturation.

In some cases, a high gain preamplifier output can overload input circuitry of certain ultrasonic instruments, resulting in baseline offset and non-linear amplitude presentations. Careful adjustment of RF filtering and instrument gain can help minimize this effect. In extreme cases an external attenuator can be inserted between the preamplifier output and the instrument receiver input.

In high gain systems, it is important to insure that the coaxial integrity of the transducer cables is maintained in order to prevent external RF noise pickup.

When operating in pulse/echo mode, the transducer excitation pulse must not exceed 500 volts for the Model 5682, or 300 volts for all other models, to prevent possible preamplifier damage. This restriction does not apply to through transmission or dual element setups.

When operating in pulse/echo mode, amplification of the excitation pulse or interface echoes can result in reduced near surface resolution. Careful adjustment of RF filtering and pulser damping can help minimize this effect.

When using a preamplifier to compensate for transmission losses in very long cables, the preamp must be located at the transducer end of the cable.