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

[Markus Heller M.A. (relix GmbH)]

I kindof have a similar problem. Guess I havent understood this yet. 

I want to generate a very basic signal at 10.000000 MHz. A little up or
down is not relevant, but I want to hear a signal at all. I have a very
sensitive HF receiver right next to my dummy load, so that should work
with a very faint rest. 

In GRC I have a signal generator and a USRP2 sink. 

Can you please tell me reference values for 
* sample rate
* interpolation on the usrp2

Is the product of these two required to be 100M? Or 50M? 

Thanks in advance for the clarification.

BR
Markus

Am Mittwoch, den 03.11.2010, 17:27 -0400 schrieb Dan Harasty:
> Marcus wrote:
> > 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.
> Can't this problem also be solved with a transmit-side correction?
> 
> If the "error" in this particular USPR2 LO is about 900MHz - 
> 899.99701MHz = 0.00299 MHz, then won't telling the USPR2 to modulate to 
> 900.00299 MHz pretty much get the tones to the right place?
> 
> I imagine the correction will change with device temperature and age, 
> and also by intended "center frequency".
> 
> But as a first cut, won't that work?
> 
> -- Dan Harasty
> 
> 
> 
> On 11/3/2010 4:59 PM, Marcus D. Leech wrote:
> > 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.
> >
> 
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