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Re: [Discuss-gnuradio] Frequency Offset

From: Eric Blossom
Subject: Re: [Discuss-gnuradio] Frequency Offset
Date: Tue, 1 Sep 2009 19:02:09 -0700
User-agent: Mutt/1.5.18 (2008-05-17)

On Wed, Sep 02, 2009 at 09:14:52AM +0800, fangming he wrote:
> Hi, all
> I set the frequency of USRP to be 910MHz, but I noticed that
> the central frequency of the signal is 914MHz. Why there is 4Mhz frequency 
> offset.
> I also found that there is also a signal comeing up in 896Mhz. The amplitude 
> of
> the signal in 914MHz is 20dB higher than the amplitude of the signal in 
> 896Mhz.

Did you call u.tune(..., 910e6, ...) to set the frequency?

If so, it adjusts the LO and DDC together such that the signal that
you want (910e6) ends up at DC in the baseband.  

If you are looking at the usrp_fft.py display, it displays the LO
frequency in the status area, but the actual frequency that
corresponds to baseband (post DDC) will be the frequency that you asked for.

> I also tried to change the LO Frequency Offset from -8MHz to
> +8MHz, but the frequency of the signal is still 914MHz.

Do not mess with the LO offset until you understand in detail what's
going on;  and even then, think twice about it ;-)


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