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Ultrasonic Preamplifiers


preamplifier 5682

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.

Equipment: There are seven models of Panametrics-NDT preamplifier, which offer bandwidth and gain choices as listed below. All of these models are compatible with a wide variety of flaw detectors, thickness gages, and pulser/receivers. For full specifications, please refer to the Preamplifier Data Sheet.
 

Model

5660B

5660C

5662

5670

5676

5678

5682

Bandwidth

20 kHz to
2 MHz

500 Hz to
2 MHz

50 kHz to
5 MHz

50 kHz to
10 MHz

50 kHz to
20 MHz

200 kHz to
40 MHz

500 kHz to
25 MHz

Gain

40/60 dB

40/60 dB

34/54 dB

40 dB

40 dB

40 dB

30 dB

 

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.

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

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

When a preamplifier is used with a single element transducer in a pulse-echo system, the transducer cable should be run into a "T" connector. One branch of the T is connected to the transmitter transducer connector of the ultrasonic instrument. The other branch of the T goes to the preamplifier input. The preamplifier output should be connected to the receiver transducer connector of the ultrasonic instrument. If the ultrasonic instrument has a mode selection switch (single element/dual element or pulse/echo- through transmission), it should be set to the dual element or thru transmission position, even though a single element transducer is being used.

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


Cable connections for through transmission system.
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.

Olympus IMS

Products used for this application


5660B

The 5660B ultrasonic preamplifier for use with ultrasonic flaw detection, thickness gaging, and acoustic emission instrumentation provides very low noise amplification of ultrasonic signals. Some features include: 20 kHz to 2 MHz bandwidth, 40 dB and 60 dB selectable voltage gain.

5660C

The 5660C ultrasonic preamplifier for use with ultrasonic flaw detection, thickness gaging, and acoustic emission instrumentation provides very low noise amplification of ultrasonic signals. Some features include: 500 Hz to 2 MHz bandwidth, 40 dB and 60 dB selectable voltage gain.

5662

The 5662 ultrasonic preamplifier for use with ultrasonic flaw detection, thickness gaging, and acoustic emission instrumentation provides very low noise amplification of ultrasonic signals. Some features include: 50 kHz to 2 MHz bandwidth, 34 dB and 54 dB selectable voltage gain.

5670

The 5670 ultrasonic preamplifier for use with ultrasonic flaw detection, thickness gaging, and acoustic emission instrumentation provides very low noise amplification of ultrasonic signals. Some features include: 50 kHz to 10 MHz bandwidth, 40 dB voltage gain.

5676

The 5676 ultrasonic preamplifier for use with ultrasonic flaw detection, thickness gaging, and acoustic emission instrumentation provides very low noise amplification of ultrasonic signals. Some features include: 50 kHz to 20 MHz bandwidth, 40 dB voltage gain.

5678

The 5678 ultrasonic preamplifier for use with ultrasonic flaw detection, thickness gaging, and acoustic emission instrumentation provides very low noise amplification of ultrasonic signals. Some features include: 200 kHz to 40 MHz bandwidth, 40 dB voltage gain.

5682

The 5682 ultrasonic preamplifier provides low noise 30 dB amplification of ultrasonic signals ranging from 500 KHz to 25 MHz. The preamplifier, housed in a rugged splash-proof enclosure, is very small and lightweight making it ideally suited for remote applications.

EPOCH 1000 Series

The EPOCH 1000 is an advanced conventional ultrasonic flaw detector that can be upgraded with phased array imaging at an authorized Olympus service center. Key features include: EN12668-1 compliant, 37 digital receiver filter selections, and 6 kHz pulse repetition rate for high speed scanning.

EPOCH 650

The EPOCH 650 is a conventional ultrasonic flaw detector with excellent inspection performance and usability for a wide variety of applications. This intuitive, rugged instrument is a continuation of the popular EPOCH 600 flaw detector with additional capabilities.
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