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JuddJones

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I am attending ultrasonic training and I am finding myself with more unanswered questions than knowledge. When searching for leaks or mechanical anomalies, we set the instrument to a specific frequency (40khz for airborne, less for Mechanical). Is this a filter only allowing the 40khz frequency into the instrument? I asked the question of the trainer and got an interesting story about how it was explained to him that it is like layers in a cake and we are looking at only one layer. So I asked again if it was a band filter that only allowed the 40khz frequency in, and got more "this is how it was explained to me answers". I am leaning more to the filtered setting only being used to reproduce the heterodyned signal heard in the headphones, and the instrument is recording a waveform of the full range of the instrument? The second part of my confusion is that I can then record a WAV file of this filtered reading and produce a spectrum of the WAV file. The fmax on the spectrum is 4khz and it was produced with an instrument that measures between 20k-100kHZ??? Is the WAV file recorded after it is heterodyned and then converted to FFT? I hope I am making sense, I am a little frustrated at spending 4 days of my life in a certification class and not being able to get knowledgeable answers from the instructor.
TIA for any clarification, I will take whatever I can get at this point.
trapper

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Reply with quote  #2 
Big J,

From my reading, the actual piezoelectric crystal is designed for the 20k-100kHz range.

As best I can tell, there is a bandpass filter as the first thing the output goes through. Their example illustrates the instrument tuned to 40kHz with a 4kHz ( +2kHz/-2kHz ) bandpass. This works out to a 10% bandpass filter. It doesn't explain if this is a fixed kHz bandwidth or a fixed 10% of tuned frequency bandpass.

The instrument then heterodynes these higher frequencies to the audible range so you can hear them with the headphones. It doesn't explain whether this is the upper sideband or lower sideband but theoretically they should be identical. I then opened the spectral software and saw that the converted FFT has a range of just above 0 Hz to about 22 kHz.

I haven't dug much deeper into it since the software is really cumbersome and crude to use with little details on its technology or functionality. We mainly use ultrasonics here to airborne air leak/electrical testing so my testing was just a little "play time" with a known bad bearing here.

I've not tried feeding the headphone output to our 2140s yet since we don't have an adapter cable but I'd like to try that sometime.

If you're taking the UE Systems class, look on pages 26-27 of Rev 14 version of the Level 1 course book.

Appreciate any more details if you find out more.

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JuddJones

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Reply with quote  #3 
Thank you, that helps a great deal. I spent hours reading the manuals and searching the manufacturer website looking for that info.
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