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Technical Discussions
Harshad
NDT Inspector, Sr. NDT Engineer
Kineco Kaman Composites Pvt Ltd, India, Joined Aug 2015, 5

Harshad

NDT Inspector, Sr. NDT Engineer
Kineco Kaman Composites Pvt Ltd,
India,
Joined Aug 2015
5
12:59 Feb-09-2018
Receiver Bandwidth

Dear experts,

Receiver Bandwidth of NDT UT equipment is mentioned has 0.3 MHz to 20 MHz at -6 dB.

What does this -6dB means?

Please clarify.

Your help is highly appreciated.

1
 
 Reply 
 
Rolf Diederichs
Director,
NDT.net, Germany, Joined Nov 1998, 609

Rolf Diederichs

Director,
NDT.net,
Germany,
Joined Nov 1998
609
14:16 Feb-09-2018
Re: Receiver Bandwidth
In Reply to Harshad at 12:59 Feb-09-2018 (Opening).

I am wondering why this question comes from an ASNT UT level 2. It seems that ASNT certifications can be purchased like PCN.

-6 db means 50% amplitude drop.

Rolf

2
 
 Reply 
 
Ed Ginzel
R & D, -
Materials Research Institute, Canada, Joined Nov 1998, 1286

Ed Ginzel

R & D, -
Materials Research Institute,
Canada,
Joined Nov 1998
1286
14:50 Feb-09-2018
Re: Receiver Bandwidth
In Reply to Rolf Diederichs at 14:16 Feb-09-2018 .

Good observation Rolf. I too must wonder why some questions are posted to this forum when a simple Google search provides excellent descriptions complete with illustrations! I looked up "Receiver Bandwidth definition" and found hundreds of useful links (with illustrations). I suppose some people think that it is easier for others to do the work for them.

1
 
 Reply 
 
Joe Buckley
Consultant, ASNT L-III, Honorary Secretary of BINDT
Level X NDT, BINDT, United Kingdom, Joined Oct 1999, 526

Joe Buckley

Consultant, ASNT L-III, Honorary Secretary of BINDT
Level X NDT, BINDT,
United Kingdom,
Joined Oct 1999
526
11:52 Feb-10-2018
Re: Receiver Bandwidth
In Reply to Ed Ginzel at 14:50 Feb-09-2018 .

Good point Ed..
If I was going to expose my ignorance I would try not to do it in a forum where my future colleagues and employers were watching.
Always google first.

And Rolf, as you know all the 'purchased' PCN certificates that we are aware of have been cancelled. Not sure how many of those affected re-took there exams.

Rest assured that any other schools doing similar tricks will also be investigated.

The vast majority of 'ASNT' certifications around are in fact SNT-TC-1A certifications, that is only a guideline, so its really up to employers to decide what level of competence they are wiling to pay for. I have never heard of any issues with an ASNT administered exam.

 
 Reply 
 
Harshad
NDT Inspector, Sr. NDT Engineer
Kineco Kaman Composites Pvt Ltd, India, Joined Aug 2015, 5

Harshad

NDT Inspector, Sr. NDT Engineer
Kineco Kaman Composites Pvt Ltd,
India,
Joined Aug 2015
5
07:37 Feb-12-2018
Re: Receiver Bandwidth
In Reply to Rolf Diederichs at 14:16 Feb-09-2018 .

Hi,

I am aware that -6db means 50% amplitude drop, that we can determine from-
dB = 20log10(h1/h2)

Some customer specification require bandwidth of an instrument at -3db whereas sometimes at -6db.

I am wondering why only -3db and -6db points are considered to express the bandwidth of an instrument.

I have goggled it but answer I got are not clear.

 
 Reply 
 
Daniel Braun
Daniel Braun
10:09 Feb-12-2018
Re: Receiver Bandwidth
In Reply to Harshad at 07:37 Feb-12-2018 .

The bandwidth is the parameter characterizing the frequency response - simply Google on the "frequency response" and you'll find plenty of data available, for example here https://en.wikipedia.org/wiki/Frequency_response

To measure the frequency response of the receiver of ultrasonic flaw detector follow the procedure according to EN 12668-1 - this will allow you to determine the bandwidth at any desired level

 
 Reply 
 
Randy Fong
USA, Joined Aug 2012, 6

Randy Fong

USA,
Joined Aug 2012
6
20:52 Feb-12-2018
Re: Receiver Bandwidth
In Reply to Harshad at 12:59 Feb-09-2018 (Opening).

Dear Harshad,

Receiver Bandwidth of .3 to 20 MHz, essentially means the instrument will accurately replicate signals comprised of sine waves between .3 and 20 Mhz (think Fourier Transform, all waveforms are a summation of a series of sine waves). In your case, your instrument is all you have. You cannot alter it's bandwidth. However, you can check its bandwidth width with a function generator and an oscilloscope capable of handling a 20 MHz sine wave. In your case, if you input a pure sine wave of say 21 MHz at a certain voltage, your instrument will show a sine wave of the same frequency at less than half the original voltage (-6dB).

Randy

 
 Reply 
 
seraniz
seraniz
18:38 Jan-20-2019
Re: Receiver Bandwidth
In Reply to Randy Fong at 20:52 Feb-12-2018 .

The bandwidth is the parameter characterizing the frequency response - simply Google on the "frequency response" and you'll find plenty of data available, for example here https://en.wikipedia.org/wiki/Frequency_response

 
 Reply 
 
Marc-Antoine Blanchet
Director,
KOROK Technologies, Canada, Joined Nov 2018, 2

Marc-Antoine Blanchet

Director,
KOROK Technologies,
Canada,
Joined Nov 2018
2
15:29 Jan-21-2019
Re: Receiver Bandwidth
In Reply to Harshad at 12:59 Feb-09-2018 (Opening).

Harshad,

It is indeed a good question as not all NDT equipment manufacturers will report the bandwidth specification in the same fashion. When you read 0.3 MHz to 20 MHz at -6dB, it means the lower and upper boundaries, here 0.3 MHz and 20MHz values are measured when the signal crosses the -6dB line. This is how the width of the band is defined if you will. A broader bandwidth offers the benefit of covering a wide range of application with the same unit.

You can refer to the example from the link below showing a measure done at -3dB. Therefore, the receiver bandwidth can be shown as being "better/wider" if measured at -6dB versus -3dB. At least, when it is stated by the manufacturer, you know exactly what the unit is up to. It also helps to compare apple-to-apple!

https://en.wikipedia.org/wiki/Bandwidth_(signal_processing)#/media/File:Bandwidth_2.svg

https://www.ndt.net/ndtaz/content.php?id=53

 
 Reply 
 

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