Maximum peak frequency

Hello..
I'm a user of NI product (LABVIEW 8.5).
My problem is, i'm using DAQ VI that read the signal from the hardware and then i transfrom it into frequency domain by using FFT.
In the frequency domain, i get a graph with a set of different frequency. Here is the set of data
1. At freq= 0Hz, magnitude= 23 - this is the offset
2. At freq= 20Hz, magnitude= 5
3. At freq= 35Hz, magnitude= 18
4. At freq= 55Hz, magnitude= 10..
So, which VI should I use so that it will display the frequency of 35Hz which contain the maximum peak frequency of my signal?
TQ

1. It appears that your frequency resolution is 1 Hz. For FFT based systems df = fs/N. To get finer resolution you need more data points (increase N). You can either take more samples, although the insects might not cooperate, or you can pad the data. The FFT.vi will do that internally just by changing the FFT size input to a value larger than the number of samples in the dataset.
Your saved data has 3000 points and you get two outputs (at 8 and 9 Hz) to represent that peak. Changing the size to 15000 results in 7 points in a peak at about 8.6 Hz.
2. Your data looks very strange. It appears that about 40% of the input data is saturated at amplitude = 0.21. This has to distort your results far beyond the DC offset. You cannot do anything about that after the data has been recorded because all the information is lost in the saturated intervals. Please check the gain of your preamplifier.
Lynn

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Hi guys, I need some help here. I´m using a NI 9215 to do a frequency sweep from 30 to 90 Hz.
So far so good. Now I need a resulting resonance frequency at highest voltage.
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ok, here is my sweep.vi for better understanding
Message Edited by andinew on 08-11-2009 04:14 AM
Attachments:
sweep.vi ‏216 KB
create_log_frequencies.vi ‏15 KB

Hi Andi,
one thought first off: looking at your "create_log_frequencies.vi", I realized that it generates linear frequency hops, not logarithmic ones...
But on to your question: to find the frequency with the highest amplitude (this will be your resonace, I guess), you will have to look at the entire signal - your "sweep.vi" code iterates the measurement and interpretation five times per second, independantly of the speed of the sine sweep.
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Message Edited by SFK on 08-12-2009 04:12 PM
Attachments:
Multi-Function-Synch AI-AO mit Signalgeneration.vi ‏91 KB

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Attachments:
3.1onesidedwithnooffft.vi ‏19 KB

Dear 'Dad',
Thanks for the correction. I have made adjustment as you have mentioned. It seems to work but I noticed some new problems.
1)      If the following parameters are used ( sine wave 100Hz, Fs 400Hz, FFTsize 512, Sample Size 512) then everything is fine.
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3)      Things get even weird when Fs is increased to 500Hz. Then the 2nd peak moves from 300 to 400 Hz. !!!
Attachments:
3.1onesidedwithnooffft2.vi ‏17 KB

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Attachments:
graph.JPG ‏20 KB

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Attachments:
Moving_Average-1.vi ‏15 KB

Maximum peak frequency

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