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Target freq vs Actual freq.

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Target freq vs Actual freq.

Ruecan
Hi,

I am sending a stream on packets between two USRP N210, and at the Tx side i am setting a center freq. of 2.490 GHz for eg.
when outputting the Tune Result, I am always getting a difference of some khz. (Target Freq: 2.490000 GHz
                                                                                                                    vs
                                                                                                                      Actual freq: 2.489993GHz)

* I wonder if that is related to the low accuracy of the frequency synthesizers of the RF front end or is it a software setting issue
* Is the information of the actual freq being centered on returned from the FPGA, or from the GNU Radio.

* Then, is that frequency mismatch is it specific to the target freq and the RF daughter-board or what.

* If that is not the unique- freq. offset to account for (am using the actual freq. of the Tx to set the Center Rx freq.)
or I will need a PLL implemented in the Rx Side to have center frequency as exact as possible.

All help will be appreciated.
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Re: Target freq vs Actual freq.

Marcus D. Leech
On 05/02/2013 05:59 PM, NaceurElOuni wrote:

> Hi,
>
> I am sending a stream on packets between two USRP N210, and at the Tx side i
> am setting a center freq. of 2.490 GHz for eg.
> when outputting the Tune Result, I am always getting a difference of some
> khz. (Target Freq: 2.490000 GHz
>
> vs
>
> Actual freq: 2.489993GHz)
>
> * I wonder if that is related to the low accuracy of the frequency
> synthesizers of the RF front end or is it a software setting issue
> * Is the information of the actual freq being centered on returned from the
> FPGA, or from the GNU Radio.
>
The frequency-setting precision of the USRP+daughtercard combinations is
under 1Hz.  THe *accuracy* (remember the difference between the two
   from high-school math/physics) is dependant on the master frequency
reference used.  The on-board TCXO on the N210 is specced, as I recall,
   to +/- 2.5PPM, and if you use an external GPSDO reference, you can
achieve 50PPB accuracy.  The +/- 2.5PPM of the on-board TCXO puts
   it in the "pretty darned good for most applications" category.  The
average hand-held "walkie-talkie" typically is about an order of magnitude
   worse than this, whether for commercial or amateur-radio purposes.

What you're seeing in the printout is standard anomalies of the way IEEE
floating-point works, and the "error" you show there is roughly equivalent
   to an error of 7PPM, if I"ve done my math correctly.

But having said that, in REAL LIFE, all RX algorithms that are
demodulating "stuff" particularly "stuff" that is narrowband, will
require a mechanism
   to provide for frequency offset between the TX and RX.  It's a fact
of life, as sure as the Sun will rise in the morning.  No two crystal
oscillators
   are exactly the same, and if you want to build RF systems that are
broadly interoperable among devices that may have somewhat-different
   frequency offsets, stability, and accuracy, you're going to have to
deal with it in the receiver.




--
Marcus Leech
Principal Investigator
Shirleys Bay Radio Astronomy Consortium
http://www.sbrac.org



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