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

From: Markus Heller M.A. (relix GmbH)
Subject: Re: [Discuss-gnuradio] questions about USRP2 sink block and upconversion
Date: Thu, 04 Nov 2010 04:11:26 +0100

Hi Marcus,

great, now I know what was wrong. I didn't expect my dummy load to be so
safe. It was a real signal lock :-)

For all the others, here's a short experiment report:

Here are the parameters:

      * Sample rate 200k
      * Signal source at 1 KHz, amplitude at a minimum of 0.001 - to
        make sure that a bad SWR does no harm. BTW, this is a good
        counter check that you can really influence the output power!
      * USRP2 sink: interpolation 500, frequency 14 MHz

Result: I hear a clear signal at 14.000 MHz, +/- 1 kHz

Thanks very much!

BR & 73s

Am Mittwoch, den 03.11.2010, 22:46 -0400 schrieb Marcus D. Leech:
> On 11/03/2010 06:57 PM, Markus Heller M.A. (relix GmbH) wrote:
> > 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
> >   
> The maximum interpolation rate on the USRP2 is 512, which means that the
> narrowest baseband
>   signal you can squirt over the Ethernet to it is 100Msps/512, or
> 195312.5Hz.
> So, the sample rate for your (complex) signal should be at a minimum,
> 195312.5Hz, and then
>   request an interpolation rate of 512 when setting up the USRP2 sink.
> Although, it may be "tidier" to use an interpolation of 500, and thus a
> sample rate of 200KHz.
> Create a 1Khz sinusoidal signal, sampled at 200KHz, send that to your
> USRP2, tuned to
>   10.0000MHz.  You should see that on your HF receiver in CW mode,
> somewhere around
>   10.001MHz or so (+/- handwaving).
> The interpolation tells the FPGA in the USRP2 how to "expand" the
> bandwidth of your signal to
>   match the fixed sample rate of the DAC that feeds the analog Tx chain.
> The sample rate of your signal source had better match what you told the
> USRP2 for interpolation,
>  or your signal won't be interpreted correctly.
> Different types of signals have different inherent bandwidths, and you
> chose sample rates and
>   interpolation rates to "fit" the signal at hand.  A typical amateur
> radio SSB signal, for example,
>   is only a couple of Khz wide, but remember that the *minimum*
> bandwidth you can squirt over
>   the Ethernet, and get interpolated up to the DAC rate is 100Msps/512. 
> Which means that
>   narrow-band signals tend to get oversampled by quite a bit inside the
> flow-graph before
>   they end up being presented to the USRP2.

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