problem with FFT

[Al-rawachy]

Hi all,

I am using RFSA fetch IQ as mentioned in the attached message. This Vi gives me data in time doamin (y axis in voltage). When I use FFT to get power spectrum (in dBm) I always get wrong amplitude but the x-axis is right(frequency axis).  Has anybody face like this problem before? thanks in advance

regards

[mikeporter]

How is the amplitude "wrong"?

Mike...

[Al-rawachy]

I mean in term of values, they are wrong.  let's say i generate -30 dbm, but I got a round 50 dbm.. Also when I do power sweep. let's say 10 values between -20 and -30. I also get values between -50 and - 75 dbm. ?? any idea??

[Bob_Schor]

Please don't show us pictures of a tiny part of your VI -- we can't "see" the rest of your code, and more important, we can't try running it ourselves to understand its behavior.  It is also harder for us to try things to fix it.  Attach the VI itself -- if you need to attach 3 or 4 VIs (or VIs and data), put everything in a folder, compress the folder, and attach the .ZIP file, please.

Bob Schor

[JÞB]

I'm going to rant on the help file first.   #^@#^@#^ Darn it! the polymorphic selector label should have at lease some consistancy with the help file drop-down menus for the poly instances!  </Rant>  Put the words in the same order.  use the same words.  not too difficult in English, I don't know how it localizes.  Still, it is annoying as an ex-spouse that the poly labels don't match between the help file and the block diagram!

Now that that is off my chest.  did you read the help?

Y returns the complex-value time domain data array.

The real and imaginary parts of this complex data array correspond to the in-phase (I) and quadrature-phase (Q) data, respectively. To calculate the instantaneous power of a sampled I/Q point, use the equation (I2 + Q2)/2R, where R is the input impedance in ohms (Ω). For NI RF vector signal analyzers, R=50 Ω.

[Al-rawachy]

sorry about this, kindly have a look for the attached code.

Jeff,  Yeah sure I read the help and you can see that I did exactly as mentioned in the help. but this will give me the right amplitude but in time domain. thus, I tried to insert it first to FFT then use the things in the help.

Thanks in advance

Regards

[Bob_Schor]

Your "forum.zip" file is actually a .rar, which I was able to open after I changed its extension to .rar.  Not sure how you managed to do that ...

Bob Schor

[Al-rawachy]

this is another one.

[Al-rawachy]

Hi,

I was looking back to my old version of the code and I found that when I send one rf signal, the FFT gives me right value of Power. but when I use power sweep which is generating more than one signal, the FFT gives me wrong power measurements as per attached in the above example..  any advice?

Regards