Hello,
I have captured I-phase and Q-phase (quadrature) data of a radio signal with QPSK modulation. The two data are in two separate text files with an array of data in a list.
How do I do FFT using Mathcad please?
Thank you
I would be very interested in your previously unknown data.
I would also like to see a demodulated version that shows the time history phase shifted data from I and Q as my simple sample showed.
Hi,
http://iitg.vlab.co.in/?sub=59&brch=163&sim=1065&cnt=1
You must demodulate the I and Q data back to the original signal data before frequency examination.
Cheers
Hi,
Regardless of the signal, in general to calculate its spectrum you should follow the procedure of the example here below, do you agree?
Greetings
FM
This is a very challenging analysis to follow. I think it's good! But I don't have either the time or inclination to verify it completely.
This looks like a set of "tools" that has been developed, used, and trusted for a considerable time.
Yes, of course, it is one of my work done some years ago, the set up required a lot of time. For the case in particular, the time was irrelevant, I just had to put together the various topics.
Hello,
I have downloaded the IQ spectra.zip file.
I am struggling with copying the 45001 data points into Mathcad. I am using copy and paste command and Mathcad crashes.
Initially, I stated IQ:= and then try to insert a table (Insert>Data Control>Table). But the table option was greyed out
How did you imported the data into Mathcad please?
Hello MF,
Thank you for all the help and for taking your time.
I see you have done two analysis for IQ, please correct me if I am wrong: FFT and discrete FFT.
However, the results in terms of shape and bandwidth are different between the two analysis.
The DFT replica is somewhat resembles the shape of the Matlab and measured results but BW is not correct. It should be 20MHz.
The FFT result is also not showing the correct BW. I used your IQ spectra file and entered sampling time as 2.222E-08 i.e. Sampling frequency is about 45MHz. That gave me a BW of 10MHz instead of the expected 20MHz. I made an attempt to use the log for y-axis, but I had no success. Please see plot below.
For the FFT plot shown above, I left the signal frequency as it was in your file i.e. 1/T2sp. But my understanding is that 1/T2sp should yield the sampling frequency and not signal frequency. Do you think that is contributing the error in the FFT plot?
NB: I measured the LTE 20MHz wide signal in the signal analyser (in IQ domain). The IF BW was set to 25MHz to capture the complete signal.
Hi,
Hello,
Thank you for the help, mf.
I tried to recreate the waveform based upon the information that you supplied to me earlier today.
But Mathcad is complaining of some undefined variables e.g. fcv, Xre, mgv etc.
Maybe I used too many italics.
I have e-mailed the Mathcad file.
Hi,
File attached.
I hope everything goes well finally!
Greetings
FM
Hi MF,
Thank you for the revised version.
I seem to be struggling with the cursors. They are not active for some reason even by sliding the rule.
The Cu parameter is undefined.
Also, the import Excel is not working anymore by following your guide lines. The data doesn't get imported at all. I did the following: 1) create table, 2) Right click in table, import Data wizard and selected the file. It says the file could not be imported for reading.
The way to overcome is copy and paste from my old Mathcad file.
Hello,
Thank you. I got the markers operation working after activation. I am still struggling of importing Excel data into the table. I resolve this issue by copying and pasting from the previous file.
Regarding my last question, here is my explanation.
IQ Data
In the last part, inverse FFT is used to recover the original signal. The net results are shown below. As you can see there are several observations:
I see that re-center command is used to Shift zero-frequency component to center of spectrum. How does the command know to which samples to re-centre?
Hi,
The last part is only indicative of what else can be done, since I used cfft, to reconstruct the signal I have to use icfft and not ICFFT. Unfortunately in the reconstruction of the signal with "makeri" the phases are altered, so you get a different signal from the original as you can see in the relative mathcad help. mg10, mg11, mg12, are the Fourier transforms of the autocorrelation functions of the signal I, of the signal Q and of the complex signal IQ, considered as random processes. Furthermore, to represent the IQ signal on the constellation, you first need to quantize each sample, read with a click, and convert from analog to digital ( four bits for example). Then you have to represent it on the corresponding constellation and so on.
We have no more time to satisfy your umpteenth question ....