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

[Steve Mcmahon]

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?

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?

I would really appreciate any help I could get,
and I appreciate your time.

Steve McMahon