Re: [Discuss-gnuradio] Problems of lost of BINS

[Milo Wong]

Eric Blossom wrote:
> On Wed, Sep 16, 2009 at 11:34:38AM -0700, Milo Wong wrote:
>   
> > Hi folks,
> >
> > I have implemented a multi sub-carriers AM transmitter and here I still  
> > have a strange problem: I plotted the FFT of the modulated signal at  
> > baseband (as attached) and the number of the bins (spikes) I have on the  
> > plot was exactly the same as the number of the sub-carriers as I set in  
> > the program. This is correct so far as I know. But after the signal was  
> > tuned to 2.45GHz by USRP, the number of the bins(spikes) on my spectrum  
> > analyzer was not the same, which is only 4 or 6. While I have 20  
> > sub-carriers at different frequencies and it seems that I lost most of  
> > them after the signal was tuned to RF. Is there anybody who knows the  
> > reason behind that? I have been stuck at here for a long time. Any helps  
> > or suggestions will be greatly appreciated.
> >
> > Thanks,
> > Milo
> >     
>
> The USRP won't be transmitting at exactly 2.45GHz because of the
> tolerance in the oscillator.  Is the span on your SA set wide enough
> to see it, even if it's not exactly on frequency?
>
> Eric
>
>   
Thank you. Eric.

Yes. I think the span is wide enough(I searched from 2.3G to 2.7G). The 
center frequency is just a little bit below 2.45G(aroud 2.446G). Is that 
possible that amplitudes of the "lost" bins are too low to see and just 
like random noise on SA? or something else problems?

Thanks,

Milo