Power Spectrum peak calculator

Hi,
I am using the "fft power spectrum.vi" to calculate an FFT on a wave and the output of that is some bundled wire called power spectrum. I was wondering is there an easy way to take this output and find and display all the various peaks in the power spectrum? Thanks.
Intern NSWCCD Carderock.

Yes, I did try that, but the problem is I don't understand the inputs required for it as it requires
"X" a set of points, but when you run the power spectrum you get a bundled wire with various parts and I'm not
sure which of these to wire into the peak detector as everything I have tried has given me wierd values. Thanks for
any help.
Intern NSWCCD Carderock.

Similar Messages

Keeping Power Spectrum Peaks on a Graph

Hello,
I am trying to read in a signal which varies in frequency as time progresses. I am taking this function and running a real time power spectrum (FFT Power Spectrum.vi) on it. As time goes the peaks move around according to teh frequency which is coming in. What I want to do is to keep the old peaks on the graph as the new peaks come in. So for example if I had a signal which was at 50 htz then 75 htz, at the end of the run I want to see both peaks on my grpah. Is this possible? Thanks.
Intern NSWCCD Carderock.

If you use the Spectral Measurements Express VI, you can configure it to do peak hold. In the configuration screen, turn averaging on, click on the Peak Hold radio button, and set the averaging number to 1. Then all your peaks will be displayed as they come and go. See attached vi. If you want to see how the express vi does it, you can convert the express vi to a normal vi and then look into the block diagram. I believe one of the functions has a peak hold input.
- tbob
Inventor of the WORM Global
Attachments:
SpectralMeasure.vi ‏98 KB

Power Spectrum no longer working correctly after upgrading to 8.0.1

Recently, I installed Labview 8.0.1 to correct a problem I was experiencing in 8.0. The installation resolved my problem, but now the Power Spectrum no longer gives the correct result. My program has not changed other than the update. I checked my circuit and everything is functioning properly. Before changing to 8.0.1 the power spectrum peaked at 40Hz and 80Hz, which was correct, and now it peaks at 0Hz and 10Hz. On top of everything, the peaks are much higher than they should be. I expect peaks with a max of 2.5millivolts and now they are 1volt. I don't understand what is going on. Nothing changed, but the software version. Can someone please help me?

In addtion to Altenbach's comments -
I'd recommend trying another VI function to get the power spectrum for
your signal to see if the behavior is the same. You can find some
examples of VIs using the power spectrum functions using the NI Example
Finder.
You can go to LabVIEW's Example Finder by going to Help >> Find Examples.
Then search for FFT. There should be some results that include FFT and
Power Spectrum Units.vi, Power Spectrum Measurement.vi, Basic Spectral Measurements.vi, and Spectrum Measurements.vi. Try opening and
running these examples to see if the VIs are returning the proper
results.
Mark Walters
Application Engineer
National Instruments

Power Spectrum VI Performance

I am looking to accelerate the performance of the Power Spectrum and
Peak Finder VI's in LabVIEW. Right now to process 10000 points
the power spectrum takes 1.3 Miliseconds. While this number is
not huge, when doing 3000 of these analyses in a shot, it gets large,
quickly. I would like to know if there's anything to be done to
speed it up. For example, the nubmers i am sending it are fairly
small Single Precision Floats, would it work better with larger
numbers?...
Thanks for all your thoughts in advance.
Eugene

Hi
I read in some place, and time ago, that those vis (power spectrum,fft, psd, etc) are optimized for power of 2 number points. I am not sure if this keeps being true, but you can try to use, say, 8192 points, or zero pading to 16384 to see if speed increases.
Hope it helps.
Alipio
"Qod natura non dat, Salmantica non praestat"

Optimizing Power Spectrum Performance

I'm continuously acquiring data from two channels on a PXIe-5122 at a rate of 50 MS/s (100 MS/s total). I would like to compute the power spectral density of each channel while acquiring the data. I'm currently using the FFT Power Spectrum and PSD VI. I benchmarked the performance of the FFT Power Spectrum and PSD VI on its own and was processing at a rate of about 3 MS/s per channel.
Is there a more efficient VI should I should be using to calculate the PSD?
Is calculating the PSD at these rates even feasible? If not, any suggestions for alternatives? (Would an LV FPGA be an option?)
Thanks!
geoff
Geoffrey Schmit
Fermi National Accelerator Laborary

Hello Geoff,
This is Andrew Brown, an Applications Engineer from National Instruments. The FFT Power Spectrum and PSD VI you mentioned is the recommended method of determining the PSD of data channels. Calculating the PSD given your sampling rates (100 MS/s) would definitely be much faster using the LabVIEW process. However, I was unable to find a specific benchmark.
Some additional resources on using LabVIEW FPGA are located at NI LabVIEW FPGA and in the LabVIEW FPGA Help for FFT. Please let me know if you have related questions on this issue. Thanks, and have a great day!
Regards,
Andrew Brown
Applications Engineer
National Instruments

WAV power spectrum

I am conducting a sound measurement test using a IEPE microphone and the NI USB 9233. I used the SVXMPL_WAV Recorder (DAQmx).vi to record a Wav file of the sound and then the SVXMPL.WAV Power Spectrum.vi to produce a power spectrum of the wave file. I had a concern that scaling was occurring because the spectrum seemed louder than expected. I read in another thread that no scaling occurs in the vi. Next I used SVMPL_FFT Subset (DAQmx).vi to produce a power spectrum over the same frequency range (20 to 20,000 Hz). In general, the two method agree well except that the peaks in the spectrum are at lower frequencies for the WAV Power spectrum vi than that obtained for the SVMPL_FFT vi. I was wondering if any one could please explain why this is occurring?
Regards,
Bill

Hello Bill!
Thanks for your posts back and the information. So what I have done is I ran approx 1 KHz signal through a series of different programs to see if I could reproduce anything you were seeing. I also used the program you posted as well. However when I run the WAV Recorder (DAQmx) vi and then put that wave file into the WAV Power SPectrum I see the signal at about 1KHz like I expect. Then I aslo use your program to see if I can get the same results with the same function generated signal. I always se my signal in the same matter and the frequency components line up accordingly. See the pictures attached below. Thanks for posting your code and it seems like I am unable to reproduce what you are seeing. Did you make any changes to the VI's in your code? If you could perhaps post the wav files that you are producing to see what I can make of them? Take a look at the screen shots that I was able to produce.
Cheers!
Corby_B
http://www.ni.com/support
Attachments:
Power Spectral Density.jpg ‏225 KB
WAV Power Spec.jpg ‏202 KB
Power Spectrum.jpg ‏142 KB

Windowing and Power Spectrum

Hi,
I have a simple .vi that is attempting to calculate the total PSD over a 500Hz bandwidth. My question pertains to the Windowing function and
what affect it has on the power calculation. I have much on windowing etc. in the NI's help but cannot seem to arrive at the value I am simulating to. I have including my .VI and the associated input file. (this file represents Pulse Densisty Modulated data output from an ADC)
thanks for any help
Attachments:
PSD of PDM.zip ‏127 KB

You may want to try "FFT Power Spectrum and PSD.vi". it takes care of the compensation for the window effect automatically.
Be careful that "Amplitude" compensation is used for Power Spectrum, and "Energy" compensation for PSD. The difference is a scaling factor of 1.5 for Hanning window.
Attachments:
psd.vi ‏20 KB

Cross power spectrum

hi
I'm trying to obtain cross power spectrum of two time domain data (signal conditioned accelerometer o/p's) using 'cross power spectrum.vi'. The vi o/p one being the 'cross power xy spectrum mag' & the other being the 'cross power xy spectrum phase'. I need to plot the frequency with 'cross power xy spectrum phase' or in other words, I need the corresponding frequency value for the calculated 'cross power xy spectrum phase'. This is required for the calculation of phase delay (phase diff / corresponding frequency) and inturn calculate phase velocity
Regards
Ajit

Hi EISmitho
thanks for the suggestion, accordingly I've incorporated the vi you had suggested. The result obtained from the changes made above contained the frequency values also. But I was bit secpticle about the result as the tabulated frequency was just like a serial no. (1,2,3,...)Is it just a coincidence or have I gone wrong anywhere??Would u pls go through it once and respond to me.
thaks for the help
regards
Ajit
Attachments:
crs.zip ‏97 KB

What are the units for the power spectrum output? (units in)^2/Hz?

I can't seem to find the answer in the manual that came with the software or anywhere online. Maybe I am just looking in the wrong places.
Thanks
David

Hi David,
I assume you are referring to the step, "Power Spectrum", in Analysis»Frequency Domain Measurements
Per the help that I see, "This step can return the spectra in root-mean-square, peak, and peak-to-peak
units."
Also, in the configuration tab of the step, for magnitude you can select either dB or linear.
Hope that helps!
-Sam F, DAQ Marketing Manager
Learn about measuring temperature
Learn how to take voltage measurements
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What is the difference between power spectrum and PSD?

Hi all,
I input a time domain signal to the FFT power spectrum VI and FFT PSD VI seperately, and got two outputs in frequency domain with similar shapes but different magnitudes.
I refer to some signal and system textbook, and find PSD(w)=1/pi*Power Specrum or equivalently PSD(f)=2* Power Specrum.But as I observed from the LabVIEW outputs, this relation is not correct. I input a 1KHz sin wave with amplitude=1, Fs=512KHz, #s=2048 , the PS peak is 0.5 and the PSD peak is around 0.013.
So I wondered what's the real formula LabVIEW is using to calculate PSD?
Sunny

PSD is related to spectrum resolution, thus related to sample rate.
df= Sample rate/ #samples
I think they follow this relationship:
Amplitude FFT power spectrum = Amplitude PSD * df * 1.5

Raw time series with power spectrum

I want to generate another VI with raw time series and power spectrum. Use soundgen.vi, with a sampling frequency of 1 kHz and datasize of 2000, so that it covers 2 sec.
How do I display the raw time series data and a power spectrum of it?
Find the rest of the peaks and identify them. Why the peaks are where they are?
Attachments:
soundgen.vi ‏15 KB

To start do a search in the examples that came with LV for FFT. This will show you a good cross-section of what LV has to offer in terms of analysis capabilities. The thing to remember is that you can perform this analysis on data directly from a device or post-process data that you read from a datafile. After going through the examples if you have specific questions, we'll be able to give more specific answers...
Mike...
Certified Professional Instructor
Certified LabVIEW Architect
LabVIEW Champion
"... after all, He's not a tame lion..."
Be thinking ahead and mark your dance card for NI Week 2015 now: TS 6139 - Object Oriented First Steps

Signal-to-Noise ratio from Power Spectrum?

Hi,
I have a feeling this is a basic issue but for some reason I'm not getting it. I hope this forum can help.
I have a Power Spectrum (freq on the x-axis and power on the y-axis) and I am trying to determine the signal to noist ratio of the rf signal live.
I am using LV 8.5 and have the spectrum toolkit and am using an RFSA in a PXI chassis.
Any suggestions?
Thanks
- rf curious

Hi Rf Curious,
This questions does not have a simple
answer but let me share my thoughts and you can decide what it makes
most sense for your application. For signal to noise ratio (SNR) you
should have ideally the power of the signal and divide it by the power
of the noise. The problem here is that you are looking at both at the
same time (rf live signal). Depending of what type of signal is it, you
can recreate it and substract it from the spectrum (you will need to
recreate the phase and magnitude and delete it from the phase and
magnitude of the spectrum). This is normally done with simple signals
such as a tone.
If you don't have access to the magnitude and
phase of the data, and you only see the spectrum, then you will have to
do things different. You can probably measure the power of the signal
by measuring the power of the band where the signal is located. Then,
you can terminate your input and measure the power of the noise of the
instrument (with same attenuation and RBW settings) and have a rough
estimate. This method should have better results when S >> N. You
can also take the adjacent channel (or further) noise reference.
Finally,
if your signal has a pretty flat spectrum, then you can only take the
difference between the noise and the signal since they both have the
same RBW.
If by now, you have found other methods, feel free to post your ideas here for any other person that come across this problem.
Regards,
Gerardo O.
RF Systems Engineering
National Instruments

Basic accelerometer-power spectrum readings

Hi all,
I'm new to Labview and am trying to create a simple power spectrum logger. I've had a look at multiple tutorials but am stuck getting to places (most of the tutorials require sound and vibration toolkit, are really old and incompatible with Labview 2011 etc). The program I'm trying to do is:
1) Collect signals from accelerometer (4kS/s)
2) Generate power spectrum
3) Save to disk (on board the cRIO)
My setup is:
- cRIO 9014
- NI 9234
- Labview 2011
So far, I've managed to get the signal readings (XYZ axis) and merge them, put them into a 'Collector' block and then output them to the power spectrum block. I've very crudely used a 'Time Delay' block to set the sample rate (which I know is probably not accurate, but I cannot figure out how else to do it). I'm having a problem figuring out how to input the sample rate into the collector, which is why my power spectrum is giving me 0 to 0.5Hz (it's likely assuming the sample rate is 1Hz). I've attached my .vi.
Would someone be able to point me to a tutorial/example/alternative solution? Thank you very much in advance.
Attachments:
accelero.vi ‏93 KB

I think what you want is the CompactRIO vibration data logger. It does not require the S&V measurement suite, though you will find it very useful.
Check this out!
http://zone.ni.com/devzone/cda/epd/p/id/6388
It has a waveform acquisition template for the FPGA. It has a data logging state design.
You can see a Video of it working here
http://www.ni.com/mcm/wind/
Let us know if this helps.
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

Save Power spectrum

Hi
I would like to save a power spectrum ( about 80000 lines of data) as a file that i can later plot. When i write the cluster (coming out of FFT Powerspewctrum.vi) to an excel (excel works only upt o 56000 or something like that, so i can´t use it anyways) or .txt file using the a combination of open -write -close file i get weird number and letters and not the data i´d like to have. Which data format ( iwould like to make graphs in origin) and vi´s should i use to export the spectrum or is it the best to extract the y-value of the spectrum (magnitude) using the unbundle by name .vi and export it as txt?
Thanks for any help

I believe this is also something i can use for my application, however, when trying to recreate the code in LabVIEW 2011 SP1, the connections between the For Loop and the Close File return an error that the data types are different.The Path wire returns that the source is 1-D array of byte stream file reference with the sink being byte stream file reference. The Error wire returns that the source is 1-D array of cluster of 3 elements with the sink being a cluster of 3 elements. How would i go about fixing this?
I plan on using this to write to the file from an accelerometer connected to a NI-9234 c-series module through a cDAQ-9178 usb chasssis. So my intention is to use this as a subVI to connect the accelerometer signal from the DAQ Assist of another VI. I am trying to record the spectral data. Attached is the attempt at recreating your code.
Attachments:
Save Power Spectrum.vi ‏55 KB

Memory leak with Power Spectrum function

Hi
I have a memory leak on my application. By observing with the "Desktop Execution Trace Toolkit", the Power Spectrum function (from NI_AALPro.lvlib) seems to have two "Reference leak".
The first one refers to the function "Open VI reference"; I got around this problem by replacing the "Open VI reference" by an "Initialize" input boolean, but I don't understand the second memory leak.
Any idea ?
Thanks...
(See attachments : Desktop Execution Trace + code)
LV8.6.1 + Desktop Execution Trace Toolkit 2009
Same problem with LV2012 + Desktop Execution Trace Toolkit 2012
Attachments:
DesktopExecutionTraceToolkit.png ‏82 KB
MemoryLeakPowerSpectrum.zip ‏7 KB

Hi Mathilde,
Thank you for using NI Discussion Forums!
I reproduce this problem with LV2012 + Desktop Execution Trace Toolkit 2012. I will look further into this.
Are there many calls of this function in your code? Could it be a problem for you?
Thank you.
Regards,
Audrey_P
National Instruments France
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Power Spectrum peak calculator

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