DIAdem FFT delta-f

Hallo,
ich hatte bereits eine Frage zur FFT unter DIAdem:
http://forums.ni.com/ni/board/message?board.id=60&message.id=6962&query.id=66517#M6962
Darauf aufbauend:
Als Vergleichskriterium für verschiedene FFT wurde mir nun "delta-f" präsentiert.
Dies soll sein 1 / (N*delta-t) mit N = Anzahl der Stützstellen (Intervalllänge) und delta-t = Abstand zweier Abtastpunkte.
Weiterhin soll ja die Intervalllänge immer möglichst groß gewählt werden.
Wir sind bei unserem Messsystem auf bestimmte Abtastraten festgelegt, z.B. 200Hz, 500Hz, 1000Hz.
Die Intervallbreite, also Anzahl der Stützstellen, bei der FFT muss aber immer 2^N sein, also 512, 1024, 2048 etc...
Wenn ich nun 20 Sekunden messe, habe ich bei 500Hz 10.000 Messpunkte.
Die Intervallbreite muss mindestens 2 x in die Messdatei passen, also kann ich mit einer Intervallbreite von 2048 oder sogar 4096 arbeiten.
Ist diese Annahme richtig?
Dabei ist ja nur relevant, dass durch die Abtastrate und Messdauer eine für die gewünschte Intervallänge ausreichend lange Messdatei entsteht. Die Abtastrate muss nicht ebenfalls ein Vielfaches von 2^N sein, oder?
Welche negativen Effekte können entstehen, wenn die Abtastrate kein Vielfaches von 2^N ist, aber die Messung stets mehrere Sekunden läuft?
Danke,
Heiko

Hallo Heiko,
Die ein- und zweikanalige FFT in DIAdem kann in den neueren Versionen mit jeder beliebigen Anzahl von Stützstellen rechnen. Damit hat sich das Problem mit den Messzeiten erledigt.
Delta-f ist in der Tat sehr wichtig, wenn man Ergebnisse vergleicht. Wenn ein Signal konstant über einen längeren Zeitraum vorliegt, kann man leicht verschieden lange Intervalle messen und FFTs oder Übertragungsfrequenzgänge berechnen. Wenn man solche Ergebnisse vergleicht, wird man feststellen, dass Sinusartige Peaks immer etwa gleich groß bleiben, aber mit größerer Stützstellenzahl schmaler werden. Rauschartige Anteile werden dagegen mit steigender Stützstellenzahl kleiner.
Wenn das Signal unverändert vorliegt, kann man kann also durch möglichst lange Messzeiten eine bessere Trennung von Sinuspeaks und Rauschen erreichen. Wenn man aber an den Rauschanteilen interessiert ist, muß man die Stützstellenzahl unbedingt beachten oder die Funktion PSD (Power Spektrum Density) benutzen. Bei PSD wird durch delta-f dividiert. Nun haben Rauschanteile immer die richtige Größe, Peaks sind dann aber nicht mehr zu vergleichen. PSD ist z.B. für Vergleiche von Filtern oder Verstärkern geeignet, da die Frequenzgänge meist keine Peaks haben. Hier interessiert man sich z.B. besonders für den Verlauf der Rauschwerte.
mit freundlichen Grüßen
Ulrich Bierwisch

Similar Messages

How can I use the Diadem FFT Analysis GUI to give me multiple outputs given multiple inputs?

I have a few hundred channels I would like to perform FFT analysis on. They are mostly of different lengths, but I know I will have groups of 7 that will have the same number of data points. The default GUI will accept those 7 sets since they are the same size, but it will only give me one output as a combination of the sets. Is there a way to get Diadem to give me an FFT output for each set without having to select each data set individually?

The way it worked with the default channels, I had to make sure I wasn't going crazy because the way you demonstrated worked very well for me. That being said, for some reason what I am doing does not work. I've recently found that it is in changing the details in the GUI that makes my output different than yours. The differences are as follows:
1) I do not use a time channel. That is always left blank.
2) in the 'Window Functions' tab, I change the window type to Hanning
3) In the 'FFT Functions' tab, only amplitude is checked with the properties 'Peak amplitude', No Third/Octave, and Average amplitude is changed to arithmatic.
4) In the 'Time Intervals' tab 'Length/Start/Overlap' is checked, Interval length is changed to 1024, and overlap (%) is changed to 50.
I've also found that changing any number of these details from the default will cause my proplem to crop up. I am fairly sure that most of those options are just as feasibly done to one channel as they are to multiple. Is this a bug, or am I misunderstanding a property of FFT that has gone over my head? Please try changing a few of those properties since it seems like it should work just as well for all the channels as it does for just one, but that doesn't seem to be the case.
Another point of confusion is that while the window previewing the inputs in the GUI will show all the inputs I have selected, the window previewing outputs only appears to preview a single unknown output. That is just a side note though.

Diadem FFT script

Hello
I’m trying to do an FFT and a PSD on some acceleration data that varies from amplitude in (g’s) to zero then to an amplitude.
I would like to remove the part when the data goes to zero then to amplitude before I run the FFT.
I currently do it manually using the band cursor in DIAdem. I’m new to the DIAdem and I was wondering if somebody can help.
Thank you,

Hi DSP_ott,
You can remove the offending values in your channel by using the CTNV function in the calculator:
Ch("YourChannelName") = Ch("YourChannelName") + CTNV(Ch("YourChannelName") = 0 Or Ch("YourChannelName") = AmplitudeValue)
This will replace all channel values equal to zero or AmplitudeValue with NoValue, which will be ignored when computing the FFT.
For more information about working with NoValue in the calculator, please refer to this KnowledgeBase:
Working with NoValues in the Calculator in DIAdem 10.1
Regards,
Message Edited by James M on 06-05-2007 10:57 AM

What are the fft based algorithms DIAdem is using?

Greetings! I am relatively new to DIAdem and am trying to find some basic information about how DIAdem is performing some calculations, specifically the FFT based functions. The DIAdem help does not cover in depth what I would like to know and I cannot find anything on NI's website or these boards (yet) that does. I did find one post that discussed the algorithms used in DIAdem Crash Analysis (originating from the NHTSA?) that gave a very good discussion, but this is specific to Crash. Is there a similar paper for the algorithms within DIAdem? Yes, I have run the same data through both DIAdem and an app we currently use and the results are very close, but this anecdotal evidence is not enough to satisfy the folks I work for. Can someone point me in the correct direction? Thanks!

Hi,
DIAdem is using several different FFT algorithms, depending on the version of DIAdem and the mathematical function. The basic FFT, witch is calculating the real/imaginary part out of the time signal, is well defined. Each algorithm gives almost the same result. There are only very small differences on the last bits of the float values. The basic FFT algorithm is usually not very large. It is easy to use different functions or libraries depending on the needs. The results have to be the same in all cases.
DIAdem 10.2 is using a new algorithm which is able to do FFT calculations with any number of values, not just the power of two. This algorithm is also used in other NI-products like LabVIEW. A large afford is going into the quality and speed of this algorithm because not only the simple FFT but also a number of additional functions like the sound and vibration toolkit for LabVIEW is depending on this.
The difficult part, when it comes to comparison between DIAdem and other FFT calculations, is always the large number of options. You need to know the number of values, sampling frequency, window function, window correction and the averaging. You can calculate peak, rms or power spectrum and so on. Depending on the options, you can get very different but still correct results.
Crash is much more a question of standards. Starting with sampling frequencies and ending with precise defined algorithms to get a result like the HIC-value witch has to be reproducible and comparable between cars from different vendors all over the world.
As soon as you start to do real world FFT calculations there are so many options and possible ways do get the results you need, that it is impossible to discuss all of them in a documentation of a Software like DIAdem. You could write a series of books about this.
If you have specific questions about the FFT results, you can give it a try and ask on this forum.
Ulrich Bierwisch

FFT and Overall Values Differ btwn VI and DIAdem

Hi all,
I have an application that displays live vibration readings including the spectrum and overall vibration level. All my report generation is done in DIAdem scripts, so it's critical that any values calculated and displayed on the LabVIEW side match the values calculated and reported in DIAdem. The attached VI and Script, when fed the input from the attached TDMS file, give me significantly different values for the magnitude of the FFT and overall RMS value. Can anyone spot what I'm doing wrong?
Thanks,
Scott
CLAD
Solved!
Go to Solution.
Attachments:
FFT and RMS Script.VBS ‏5 KB
FFT and RMS VI.vi ‏39 KB
VibrationSample.tdms ‏103 KB

Hello,
Here is a great reponse from R&D. I felt it would be better to place this on the web then keep it limited to your service request.
The difference has to do with the Window-correction. The results are identical if you select rectangluar (or no) window.
Usually a Window is taking a part of the singnal away, so in the first place, the result has a different shape and is smaller compared to result with rectangular window. This can be corrected with a specific factor for each window.
There are two possible corrections. Random and periodic.
- Periodic is used if you have pure sine-wave like signals and you want to measure the peak value. A typical use case is the flattop-window which is designed for this type of calculations. The periodic correction is too large if you want to add the values to calculate the total RMS of the signal or of a the summation of a certain frequency band.
- Random is correcting the signal back to the correct overall RMS value and should be used in all other cases.
The periodic correction for the Hanning window is a factor of two. The random correction is about 1.633. DIAdem will give the same result as LabVIEW if you use the following correction type:
FFTWndCorrectTyp = "periodic"
In the world of FFT-analysis, the different corrections are very often not obvious and hidden to the user. Unfortunately this is also the case in LabVIEW. You can find a good example of an explanation here:
http://blog.prosig.com/2009/09/01/amplitude-and-energy-correction-a-brief-summary/
There are different factors for the different window functions. If you go deeper into the FFT-VI you will find the "Scaled Window VI". This has an output "window constants". With this, it is possible to get the correction values for the window functions.
I also think, that periodic is wrong in most cases when Hanning is used, because Hanning is best for the summation of RMS values in frequency bands and the results are wrong without a random correction.
Hope that helps.
Jacob R. | Applications Engineer | National Instruments

FFT in diadem

Hello,
I found some information from the DIAdem help and DIAdem manual as follows.
"Note DIAdem calculates the FFT to powers with a base of two and therefore might not use all the measurement data. Example: If a time signal has 340 values, DIAdem only uses the first 256 (28) values for the FFT."
"DIAdem calculates a FFT for the entire length of the specified channels, even if the channel length is not a power of two."
I don't understand which one is right?

Hello xzhcong,
They are both right, but I agree this is a little misleading.
The original FFT calculation did require a power of two number of values, but it has since been replaced with the improved DFT calculation (and can be switched back to the original algorithm with a configuration setting in DIAdem).
The setting can be changed from the "Settings" menu in DIAdem, in the Settings > Options > General dialog under "Compatibility"
That setting Specifies whether DIAdem uses the improved FFT algorithm. If the variable has the value TRUE, DIAdem uses the new algorithm.
Description:
Note If the variable has the value FALSE, DIAdem uses the FFT algorithm from DIAdem 10.0 and earlier. DIAdem calculates the FFT of the older FFT algorithms to powers with a base of two and therefore might not use all the measurement data. Example: If a time signal has 340 values, DIAdem only uses the first 256 (28) values for the FFT.
Only use the older FFT algorithms if you want to compare your results with earlier data. The functions ChnFFT1, ChnFFT2, and ChnInverseFFT including the related dialog boxes FFT with one time signal, FFT with two time signals, and Inverse FFT are affected by these changes.
The default setting for this variable is True, so the new algorithm will be used unless you specify otherwise.
Please check the attached PDF for more details about the FFT calculation ...
Let me know if you have additional question,
Otmar
Specifies whether DIAdem uses the improved FFT algorithm. If the variable has the value TRUE, DIAdem uses the new algorithm.
Definition
UseNIFFT, Boolean variable
Note If the variable has the value FALSE, DIAdem uses the FFT algorithm from DIAdem 10.0 and earlier. DIAdem calculates the FFT of the older FFT algorithms to powers with a base of two and therefore might not use all the measurement data. Example: If a time signal has 340 values, DIAdem only uses the first 256 (28) values for the FFT.
Only use the older FFT algorithms if you want to compare your results with earlier data. The functions ChnFFT1, ChnFFT2, and ChnInverseFFT including the related dialog boxes FFT with one time signal, FFT with two time signals, and Inverse FFT are affected by these changes.
Otmar D. Foehner
Business Development Manager
DIAdem and Test Data Management
National Instruments
Austin, TX - USA
"For an optimist the glass is half full, for a pessimist it's half empty, and for an engineer is twice bigger than necessary."
Attachments:
FFT_Use_in_DIAdem.pdf ‏682 KB

Calculating FFT using the FFTAverageType variable in DIadem

Hi,
I have some doubts about how should I use the averagingType variable. I want to define this variable as "Arithmetic" and get the combined FFT of two trials of the same process, i.e I have two signals which represent the same process.
What I do is just calculate the FFT of each signal separately. Then I calculate the amplitude of each FFT signal and finally i combine these two signals by doing the mean for each amplitude. I.E if i have signal1FFT = (1 , 2 ......23 ) and signal2FFT = (2,4, ...43 ) the resulting signal should be ( 3/2 , 6/2 ...... 66/2 )
Am I correct about what this averaging type variable does? When i try to do the same in other mathematic programs I get different results but I don't know exactly what I am doing wrong,
Thank you so much for the help!

There are several ways to approach averaging in the FFT environment.
FFTAverageType is should be "Arithmetic" which means that all FFTs are weighted equal.
The Exponential averaging is kind of a running average for special calculations which are typically done during a running measurement.
Much more important is the parameter in "Calculate average from:" (Amplitude or Autospectrum)
Amplitude is giving a linear result and Autospectrum is giving the result based on squared values. The squared result is the correct result from an energetic point of view.
Example:
If you have a sine with a peak of 2 for one second and a peak of 4 for another second, you might want to get and averaged peak of 3.This might be wrong.
The RMS value of a sine with a peak of 3 would be 2.121.
The RMS which is representing the energy in a signal is the root of the average of the squared values. The RMS of a signal with half the time 2 and half the time 4 is 2.236. This means, that a sine with the peak of 3.162 for two seconds has the same RMS as the signal with a peak of 2 and 4 for one second each.
If you average FFTs in DIAdem you need to decide between the linear average (based on the peak Amplitude) and the energetic correct squared average (based on the Autospectrum). Right or wrong depends on your circumstances.
Usually, other mathematic programs are giving you the choice to decide or they might just use one or the other method.

Frequenzintervall bei FFT in Diadem einstellen

Ist es bei Diadem 9.1 möglich bei einer FFT das Frequenzintervall des Ergebnisses zu beeinflussen?

Hallo HermannGr,
ja, das ist möglich. Es ist abhängig von den Einstellungen der
FFT-Parameter. Ich habe dazu ein Beispiel erstellt. Dieses Beispiel
zeigt eine Variante die Parameter für eine Frequenzanalyse
einzustellen. Im Vordergrund steht hier die Vorgabe der Auflösung
im Frequenzbereich - z.B. möchte ich das FFT-Ergebnis in
1Hz-Schritten berechnen. Der in SUD erstellte Parameterdialog überprüft
auf Knopfdruck die Eingaben auf Plausibilität und berücksichtigt dabei
die Randbedingungen der FFT und des zugrunde liegenden
Datendatensatzes.
Vielleicht hilft das ja weiter.
Gruß
Walter
Attachments:
Sud-fft.zip ‏52 KB

Correct FFT settings Diadem

Hi,
I am a new user with Diadem (version 10), and use the program to view and analyze EEG data. In order to calculate a powerspectrum from parts (ca. 5 seconds long) of my EEG data (sampled with 5000 Hz) I tested the analysis FFT function in Diadem for one-time signals. Being unexperienced with the FFT details, I have trouble chosing the correct settings:
-type of window (Hanning ?)
-amplitude damping correction (none/periodic/random?)
-FFT functions: should be amplitude I guess. Averaging arithmetic? (I also tried exponential, which did not look too bad, but I guess this puts more stress on the latest calculated parts..). Then calculating the average from amplitude ?(other option is autospectrum)
-time intervals: this of course is linked to my sample size (e.g. 5 sec) and sampling rate (5000 Hz). With my 5 sec of 5000 Hz I believe I can get up to large FFT sizes like 1024 or 2048. But what about the overlap, what are the rules for this? I guess too much overlap will distort the frequency spectrum, but what should be chosen? Reject no values sounds good, but if this is the choice, why?
In the tests with the FFT I did, the 0 Hz peak is dominant. Can one choose the settings in a way that gets around this, or should one put a high-pass filter on the data to get rid of it?
Obviously ignorant, perhaps some expert can give me advice how to make the best powerspectra before I create non-sense. I checked the forum and examples in diadem on this, but I do not seem to find a clear explanation on the options that diadem provides.
Regards
Else

Hi Else,
there is no correct setting for FFT calculations. You can get a lot of
different results and most of them are making sense. On the first view, this
might look very complicated, but most of the time, the settings are only
important if you try to compare your results with the results of other FFT
Analysers or software. If you just need to find out more about your data, you
can try out different settings and look for the results you need.
Here are some answers to your questions.
Your signal is cut of at the begin and the end of the measurement which results
in an error. Most of the window functions are reducing this error. Hanning is
the most common window. You should compare Hanning and Rectangle (no window) to
see which one is giving you the better result. Forget all the other windows if
you don't know them.
Hanning is taking a part of your signal away. In the first place, all
FFT-values are smaller then the results without a window. It is possible to
multiply the results with a correction. If you are looking for a sinus wave
within your signal and you are interested in the amplitude of this sinus, you
should use the periodic correction. In all other cases you should use the
random correction. But the correction is just a factor. If you are not interested
in the absolute value of the amplitudes at all, it doesn't matter.
FFT functions:
It is always the same result, but sometimes squared or multiplied with a
factor.
Amplitude is OK, unless you compare your values with other results.
RMS = Amplitude/square root(2)
Autospectrum = Amplitude^2
Powerspectrum = RMS^2
Averaging is only important if you do more than one FFT calculation. In the
first place, I would do one FFT as large as possible. If your signal is a continuous
signal, you get better results if you take as much values as possible (4096,
8192, ...). Larger FFTs are giving you a much better frequency resolution. Most
of the averaging and overlapping is not useful anymore. We have so much memory
and speed in our computers, that we can do calculations with millions of
values. Today averaging is still very important for 2-channel calculations like
transfer functions on numbers of short measurements.
DIAdem can handle datasets with gaps where the equipment was not able to
deliver any value. "Reject no values" is not important for you,
because you don’t have "no values" in your measurement.
If 0 Hz is the dominant frequency, using a high pass filter is a good way to
get better results.
This are
just some short answers to your questions. Signal processing and FFT is a very
wide area with lots of things you might need to know. If you need more specific
help on analysing your data you can post a set of these data or send it by
mail. It also would help to know what you are trying to find out.regards,
Ulrich Bierwisch
[email protected]

FFT calculations in DIAdem

Hello DIAdem users,
R&D just made a great document available that explains how the FFT calculations work in DIAdem. In addition, it also contains a lot of very useful background information that I thought would be great to share.
Please download the attached PDF file. Please provide feedback by replying to this thread.
Otmar
Otmar D. Foehner
Business Development Manager
DIAdem and Test Data Management
National Instruments
Austin, TX - USA
"For an optimist the glass is half full, for a pessimist it's half empty, and for an engineer is twice bigger than necessary."
Attachments:
FFT_Use_in_DIAdem.pdf ‏682 KB

Hello Beejal,
There is a document that describes the functions available in the crash test analysis tool kit for DIAdem. It's located in the "..\National Instruments\DIAdem 11.1\Manuals" directory and called "CrashAnalysis.PDF". We are also adding more background on the mathematical function in DIAdem to the help system all the time, so that might be a good place to check out as well - I don't think any of these are up to the level of this document, but there is some very good information out there.
What functions specifically is your customer interested in? Maybe I can nudge someone in R&D to look into writing something more in depth if that makes sense ...
Otmar
Otmar D. Foehner
Business Development Manager
DIAdem and Test Data Management
National Instruments
Austin, TX - USA
"For an optimist the glass is half full, for a pessimist it's half empty, and for an engineer is twice bigger than necessary."

Difference between diadem and LabVIEW Report Generation Toolkit

I want to expand LabVIEW's reporting capabilities. Can someone tell me what the difference is between DIAdem and the LabVIEW Report Generation Toolkit for Microsoft Office.

Hello Mr Bean,
There are several major differences between the two solutions. I'll try and outline them as good as I can ...
The biggest different is that LabVIEW and its toolkits are programming tools and DIAdem is a ready to run application.
The LabVIEW Report Generation Toolkit basically provides an extensive library of VIs that allow you to connect LabVIEW to the MS Office library and create and edit reports in Word and Excel.
DIAdem is a stand alone application that was designed to post-process data from various sources. The tools included in DIAdem are:
1. NAVIGATOR: Allows you to import data from files (ASCII, binary, Excel, etc.) as well as DataBases (SQL/ODBC, Citadel, ASAM, etc.). The unique DataPlugin technology available within DIAdem allows you to import vitually and data file by creating a DataPlugin that describes the data format and gets linked to DIAdem. More information at: http://www.ni.com/diadem/dataplugins.htm
2. VIEW: Graphical and Numerical data inspection tool. Use cursors to zoom and scroll through your data, edit your data graphically and numerically and compare different tests.
3. ANALYSIS: DIAdem has a wide range of analysis functions, ranging from simple functions like integration and differentiation to FFTs, Order analysis and 3D data analysis.
4. REPORT: The DIAdem REPORT tool allows you to create reusable report layouts that can contain multiple axis systems (2D, 3D and polar plots) as well as tables (2D and 3D) and variables. The REPORT tool generates templates that can be usedwith multiple data sets due to the inclusion of automatic (or manula if required) scaling as well as extensive use of variables for annotations. Finished reports can be exported to printers, graphics files, the Windows Clipboard, HTML pages and PDF files (PDF writing is build into DIAdem, no extra software required). Using Scripts, it is also possible to create Word, Excel or Powerpoint reports from DIAdem automatically.
5. SCRIPT: The DIAdem scripting tools allows users to record macros or write scripts that automate complete sequences, for the import of data, via extensive analysis, to creating a publication ready report. Scripts can make reports "intelligent" by modifying the appearance of a report based on the data or calculation results that are going to be displayed in the report.
I recommend you check out the following link for more information: http://www.ni.com/diadem/
DIAdem is an extremly powerful tool for report generation. I am leaving to go on a business trip to Asia, but I would be more than happy to give you an Web presentation on DIAdem after I have returned. We could use your data and create a DIAdem report Script together online. My Email address is [email protected] and I will be back the week of Thanksgiving.
Otmar
Otmar D. Foehner
Business Development Manager
DIAdem and Test Data Management
National Instruments
Austin, TX - USA
"For an optimist the glass is half full, for a pessimist it's half empty, and for an engineer is twice bigger than necessary."

FFT Frequency Resolution vs. Graph Update Rate of a Vibration Signal

Hello,
First, what I know:
The frequency resolution (Hz) of an fft is 1/acquisitionTime (e.g. if 2 seconds of data are acquired, the frequency resolution is 1/2s = 0.5Hz).
What I'm Wondering:
Let's say I'm displaying an acquired signal on a graph and freq res = 0.5Hz. The graph update rate is therefore approx 2sec.  This may appear to be an application with a problem from a user's perspective.
Can I increase the update rate?
It seems that with vibration data (i.e. mostly non-periodic) I could create a buffer that is 2sec long.  The first data acquisition sequence will fil the buffer with 2sec of data. Then I could just acquire something shorter than 2sec of data (let's say 1/4sec). If I am acquiring data continuously and I push the 1/4sec of data into the front of my buffer then perform the fft again, I have effectively increased my graph update rate to 1/4sec and my users think I'm great.
Can anyone shed any light on whether or not (why/why not) this will work?
Thanks for your time,
Chris

The best tool NI provides for your application is the Zoom FFT. This allows you to specify the span (frequency range of the FFT) and the zoom (the delta f such as 0.5hz) in the FFT. The Zoom FFT has a built in buffer, called overlap and allows you to set the overlap so that it updates at the appropriate rate, event though you may need many seconds of data to get the delta-f in the FFT you are looking for. We package all this and manage units to display the correct units for power density, power, linear, magnitude, rms, 0 to peak, peak to peak, etc. We even provide a express VI for the zoom FFT and it is also part of the Sound and Vibration Assistant, requiring no programming at all.
http://digital.ni.com/public.nsf/allkb/F70DB3441DC5AB9986256E97005BA825
http://zone.ni.com/devzone/cda/tut/p/id/6107
http://zone.ni.com/devzone/cda/epd/p/id/5505
http://sine.ni.com/nips/cds/view/p/lang/en/nid/12152
Preston Johnson
Principal Sales Engineer
Condition Monitoring Systems
Vibration Analyst III - www.vibinst.org, www.mobiusinstitute.com
National Instruments
[email protected]
www.ni.com/mcm
www.ni.com/soundandvibration
www.ni.com/biganalogdata
512-683-5444

Freq Data from FFT

I am trying to extract the Frequency data from my FFT like I extract the Magnitude data, as a 1-D array, in order to easily write it to an excel file.
As shown, I can get the delta-freq value, and I could make a for loop that calculates the values with the dF value, but I would much rather just extract the whole array like I do with 'magnitude'. Is there an option for this, or should I just write the loop and calculate the values?
Solved!
Go to Solution.

Build yourself an array of frequencies, such as this:
(Just realised the indicator label is Time Array - as I usualy do this for waveforms!)
You can also pass the magnitude array into the for loop with an autoindexing input, whilst omitting the array size primitive wired to the iteration count.
Edit - to answer your question, no, there's no other easy method I'm aware of.
CLD

How is calculated the amplitude in the FFT function?

Hello,
I have a time signal, for example an 1KHz tone. I use the FFT complex function to calculate its Fourier Transform, and I want to Know, which is the relation beetween the amplitude of this FFT and my time signal.
I have an example of a sine of 1KHz with an amplitude of 1V. When I calculate the FFT the result is one delta at the correspondient frecuency, but with an amplitude of 12149. My signal has 65536 samples.
Could you help me please?

With the complex FFT, I do believe you have to normalize the by half the size of your sample set. This should give you back the original magnitude of your signal. Take a look at this screenshot.
Attachments:
fft_magnitude.jpg ‏65 KB

Importing multiple excel sheets into DIAdem

Hello,
I have an Excel file with multiple sheets. I am trying to import data from cells on each sheet into DIAdem. I am capable of importing anything I want so far using a VBS Script, but only from one sheet at a time. In order to import from another sheet, I have to select that sheet on Excel, save the file and close it. Then the next time the file is accessed by DIAdem, it will open on the sheet that was last saved on.
Is there a way I can create a VBS file that would check if there are any other sheets on the same file, move to a different sheet and import data from it? I am thinking some kind of IF statement that would realize when the last row from a sheet was imported, and then move on to the next sheet.
Any ideas/comments/suggestions will be greatly appreciated!
Thanks, Andy

Hi arkangel,
The GroupDefaultSet() command changes the default group, but that only affects ADDITIONS to the Data Portal after that point, such as new FormulaCalculation channels, excerpted section channels from VIEW, new ANALYSIS channels from an FFT calculation, etc.
If you want to reference channels of the same name from multiple groups, and you have DIAdem 9.x, you can use the [GroupName]/[ChannelName] adressing scheme to query off the "Time" channel from multiple groups. If you have DIAdem 9.1, then you can also use the [GroupIndex]/[ChannelName], [GroupName]/[ChannelIndex], or {GroupIndex]/[ChannelIndex] adressing options. For example:
For i = 1 TO GroupCount
ChNum = CNo("[" & i & "]/Time")
IF ChNum > 0 THEN Msg = Msg & "Group " & i & " --> " & ChnLength(ChNum) & vbCRLF
NEXT
MsgBox Msg
For DIAdem 8.x you will need to use the CNo() function's optional second parameter (ChannelNoStart) in order to ensure that you start at the next channel AFTER the one you just found. Channel numbers was all there was in DIAdem 8.x.
Brad Turpin
DIAdem Product Support Engineer
National Instruments

DIAdem FFT delta-f

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