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Re: [Discuss-gnuradio] questions about USRP2 sink block and upconversion
From: |
Marcus D. Leech |
Subject: |
Re: [Discuss-gnuradio] questions about USRP2 sink block and upconversion |
Date: |
Wed, 03 Nov 2010 16:59:33 -0400 |
User-agent: |
Mozilla/5.0 (X11; U; Linux i686; en-US; rv:1.9.1.12) Gecko/20100907 Fedora/3.0.7-1.fc12 Thunderbird/3.0.7 |
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
- [Discuss-gnuradio] questions about USRP2 sink block and upconversion, Steve Mcmahon, 2010/11/03
- Re: [Discuss-gnuradio] questions about USRP2 sink block and upconversion,
Marcus D. Leech <=
- Re: [Discuss-gnuradio] questions about USRP2 sink block and upconversion, Dan Harasty, 2010/11/03
- Re: [Discuss-gnuradio] questions about USRP2 sink block and upconversion, Markus Heller M.A. (relix GmbH), 2010/11/03
- Re: [Discuss-gnuradio] questions about USRP2 sink block and upconversion, Marcus D. Leech, 2010/11/03
- Re: [Discuss-gnuradio] questions about USRP2 sink block and upconversion, Markus Heller M.A. (relix GmbH), 2010/11/03
- Re: [Discuss-gnuradio] questions about USRP2 sink block and upconversion, Marcus D. Leech, 2010/11/03
- Re: [Discuss-gnuradio] questions about USRP2 sink block and upconversion, Markus Heller M.A. (relix GmbH), 2010/11/04
- Re: [Discuss-gnuradio] questions about USRP2 sink block and upconversion, Marcus D. Leech, 2010/11/03
Re: [Discuss-gnuradio] questions about USRP2 sink block and upconversion, alexander levedahl, 2010/11/03
Re: [Discuss-gnuradio] questions about USRP2 sink block and upconversion, Steve Mcmahon, 2010/11/04