spectral analysis

[Sharmi]

Hi,

I am working on a PXI project where I need to generate a single tone signal (which will be modulated by vital signs) and then demodulate the signal and view its
spectrum.
I am using PXI 5673 and 5663. Also I am using the following vi-
 RFSG Single Tone Generation vi for transmission and
 MT Demodulate AM for reception.
 MT Generate AM (for testing AM demodulation)

I have modified the second vi to view the magnitude spectrum. For testing purpose I am using the 2nd and 3rd vi.
However, I am not getting the correct magnitude spectrum no matter what modulating frequency I use.I am directly feeding the I channel signal into the Spectral
Measurements vi but the result is not what I expect(the FFT peak is not at the modulating frequency). 

I have also tried the FFT vi but still I am not getting the peaks at the modulating frequency.

I checked NI forums and found the information below-
https://decibel.ni.com/content/docs/DOC-6032

Do I need to use script to get the peak at the modulating frequency?

I have attached my vi.
Could someone please help me?

Thanks so much,
Sharmi

[Peter-E]

Hi Sharmi!

Did you make any changes in the RFSG Single Tone Generation VI before generating the signal? I would like to see both of the VIs you are using (both the generation and the acquisition) and see the type of modulation you are doing on the generation side. If you could, also post a screenshot of the magnitude spectrum that you are currently reading, and we can then compare those results with what you are generating.  

What results are you expecting? If you are modulating and demodulating correctly, the magnitude spectrum should simply be the single tone signal that you generated.

Another thing you can try is if you open up the RFSA Soft Front Panel after running the VI above (without any changes), you will be able to see the magnitude spectrum of the generated tone properly (apart from the code or modulation/demodulation).

Have a great day!