Re: [Discuss-gnuradio] questions about USRP2 sink block and upconversion

[Steve Mcmahon]

Marcus:

Thank you for your reply. Let me ask you another question to make sure I'm 
understanding this correctly. Remember, I have a USRP2 board with a WBX 
daughterboard. If I construct a flow graph where a signal source block 
generates a 7 khz cosine at a sample rate of 200 khz (100e6/500), and if this 
cosine is fed into a USRP2 sink block with its "frequency" parameter set to 
"900M", then will I see a spike in the WBX output spectrum at 907 MHz, or at 
893 MHz, or both? (assuming perfect precision of the internal reference clock 
on the USRP2) What if it were a sine instead of a cosine? Thanks again for your 
help. I really appreciate your time.

Steve McMahon



--- On Wed, 11/3/10, Marcus D. Leech <address@hidden> wrote:

> From: Marcus D. Leech <address@hidden>
> Subject: Re: [Discuss-gnuradio] questions about USRP2 sink block and 
> upconversion
> To: address@hidden
> Date: Wednesday, November 3, 2010, 8:59 PM
> On 11/03/2010 04:00 PM, Steve Mcmahon
> wrote:
> > Hello:
> >
> > I am still somewhat new to GNU Radio. I am running GNU
> Radio 3.3.0 under openSUSE 11.2, and I have two USRP2
> boards, each with a WBX daughterboard. I need some help
> understanding some fundamental things about GNU Radio and
> the USRP2 and upconversion.
> >
> > I am trying generate a tone at 900.001 MHz
> (900,001,000 Hz). I am using GRC to construct a simple flow
> graph where I have a signal source block generating a 1 khz
> cosine at a sample rate of 195.312 khz (=100e6/512),
> connected to a USRP2 sink block with the "decimation"
> parameter set to 512, and with the "frequency" parameter set
> to 900M. I then look at the output on a spectrum analyzer.
> My understanding was that I should see a clear spike at
> 900.001 MHz, but I don't. Instead I see a peak at 899.99701
> MHz. What am I doing wrong? I'm using the internal USRP2
> clock. Is this happening because the internal clock is good
> to only 7ppm?
> >   
> There are two sources of error--one, as you've observed is
> the precision
> of the reference clock on
>   the USRP2.  And the other is whatever residual
> measurement error your
> Spectrum Analyser has.
>   Synthesized LOs are only as good as the reference
> clock, at least from
> a frequency-precision
>   perspective.  If you want to do better than
> that, then you can use an
> external 10MHz reference
>   clock, such as a GPS frequency standard.
> 
> This is entirely normal for synthesized RF gear. 
> Measure just about any
> commercial radio out there
>   with a precision measurement device, and there'll be
> some residual
> frequency error, unless you get
>   lucky.
> 
> It's usual in telecom systems for there to be some kind of
> AFC on the
> receive side to compensate for
>   transmit-side frequency error.
> > In general, how do I need to setup the frequency of a
> USRP2 source if I want to place tones in the spectrum? I
> thought it was simple upconversion. If I want to modulate a
> multitone signal (say with sine components 1 KHz, 3 khz, and
> 7 khz) to obtain an upconverted signal with tones at 901
> MHz, 903 MHz, and 907 MHz, then I simply set the "frequency"
> parameter of the USRP2 sink to 900 MHz, right? How exactly
> does the USRP2 do the upconversion? What exactly does the
> "frequency" parameter do?
> >
> >   
> The USRP2 takes your quadrature-sampled baseband signal,
> and
> interpolates it up to the required
>   Tx-side sampling rate.  It programs the Tx LO
> on the daughtercard to
> the desired frequency,
>   and sends it on to the Tx mixer.  Sometimes,
> due to LO frequency step
> size limitations on specific
>   daughtercards, the USRP2 FPGA will use a DUC
> (Digital Up-converter)
> stage to get to
>   exactly the desired frequency.
> 
> I'm not sure whether you meant 901,903 and 907Mhz, or
> 900.001MHz,
> 900.003Mhz, and
>   900.007Mhz.
> 
> For purposes of experiment, you can have 3 different signal
> generators,
> add their outputs,
>   and send the resulting multi-tone baseband stream on
> to the USRP2.
> 
> 
> -- 
> Principal Investigator
> Shirleys Bay Radio Astronomy Consortium
> http://www.sbrac.org
> 
> 
> 
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