Re: [Discuss-gnuradio] 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