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

[Marcus D. Leech]

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