Frequency to time domain

[Shazlan]

Dear all,

I've acquired some frequency domain data from PXI-5660 using 'Read average power spectrum.vi' and 'SMT unit conversion'. Now, there is a requirement to convert this data into time domain. I've tried by simply passing this data through an inverse FFT VI but I didn't work. Any advices or suggestions on how I should proceed?

Thanks for the help!

Shazlan

[Andy_Hinde]

Hello Shazlan,
Unfortunately after a power spectrum is calculated, the phase information is no longer available. This is important because without the phase component of a complex FFT, you cannot perform an inverse FFT to get back the time domain data. To go back and forth you would have to calculate a complex FFT then a power spectrum from the FFT (keeping the mag and phase data).

Thanks,
Andy

[Shazlan]

Thanks Andy. Ok, that's what I was afraid of.

Anyhow, I've captured some pulse and CW signals from some radars and was told to get as much info from the signal as possible. This being the case, I can't do much on the pulse signal since (if I'm not mistaken) to get information, such as pulse width, PRI, etc, the signal has to be in time domain. Am I right? Please advice.

Also, any suggestions on what should I do with the CW signals? I mean, which analysis should I perform on the signal and what should I be looking for? My knowledge on CW-type radar and whatever characteristic are very limited. Again, please advice.


Shazlan

[Yardov]

Hi Shazlan,

You are correct, pulse with, pulse repetition interval, etc are time domain measurements and you will need the time domain representation.
Something you can do, is to acquire the time domain data using the 5660. This is called IQ data since it will be returned in the form of complex numbers I and Q (I+Q*j). There is a shipped example called "ni5660 Getting Started IQ.vi" that you can use.

For the type of measurements of a CW tone I would not be sure for the specific application but you can look at this document and find that many specifications are calculated using CW signals, so you can use these measurements for your own. Hope this helps as starting point.

[R Mishra]

Hi All,

I am working on DAC bench automation.I am sending a digital signal at some sampling frequency to DAC(Digital to analog) board.

The output from the DAC board is connected to Spectrum Analyzer(Agilent E4440A).

Using LabVIEW driver I am able to get the Frequency domain signal in my Graph.

I wanted to calculate the THD,THD+N,SFDR,Harmonics and SNR.Please anybody suggest how i can get these values.I am having below information:-

1. Sampling Frequency

2. The amplitude of the received signal is in dBm and base (X-Axis) is in frequency.

3. Fundamental Frequency.

4. Start Frequency and Stop Frequency.

5. There is the attached sampled data and snapshot of the frequency domain graph.

Thanks.

Ritesh Mishra