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.

On Wed, May 11, 2011 at 12:48 PM, mehmet kabasakal

[email protected]wrote:

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

On 05/11/2011 07:57 AM, Tom R. wrote:

calculation as the error value in a control loop to correct your

frequency offset

Tom

With the range of observed values given by Mehmet, it looks like a total

frequency error of about 12.5PPM, which is pretty darned good.

–

Marcus L.

Principal Investigator

Shirleys Bay Radio Astronomy Consortium

Hi Marcus,

May i ask you how you calculate the freq error i.e. 12.5 ppm ?

Is there a formula?

Mehmet.

2011/5/11, Marcus D. Leech [email protected]:

On 06/02/2011 03:01 AM, mehmet kabasakal wrote:

Hi Marcus,

May i ask you how you calculate the freq error i.e. 12.5 ppm ?

Is there a formula?

Mehmet.

Just take the error in Hz, divide by the “expected” frequency, and

multiply by 1.0e6

–

Principal Investigator

Shirleys Bay Radio Astronomy Consortium