On Wed, May 11, 2011 at 12:48 PM, mehmet kabasakal
<address@hidden> wrote:
Hi List,
I am trying to find the spectrum symmetry of a modulated signal.
That's why i use the fft block on GRC. (The flow i used is attached).I
use AM modulation as an example. And i write a block that finds the
left and right side of the spectrum and make the calculation
P = (PL - PU)/(PL+PU).
But every time i run the flow the point of the carrier is changing. I
use 2048 point fft, hence i expect to find a carrier at the 1024th
point and every 2048 point after (3072, 5120, etc) until i stop the
flow.
But this value changes any time i run the code( some values i take as
an ex: 1010, 1012, 1018, 999, 1045, 1044, etc). I also attached the
code for the calculation. I couldn't figure out what i did wrong.
Thanks for any help.
Mehmet.
Mehmet,
You are seeing the frequency drift of the oscillator in both the transmitter and receiver. They will never be perfectly accurate. You should probably test your block using a simulation setup. That is, tie the transmitted signal straight to the receiver in GRC and bypass the USRPs altogether to make sure your block is correct.
If you are looking for a symmetrical signal, you could then use your calculation as the error value in a control loop to correct your frequency offset :)
Tom