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

From: Milo Wong
Subject: Re: [Discuss-gnuradio] Problems of lost of BINS
Date: Wed, 16 Sep 2009 14:31:33 -0700
User-agent: Thunderbird (X11/20090817)

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.


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?


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?



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