[Hackrf-dev] PPM Adjustment

[Michael Ossmann]

There are several different ways to improve tuning frequency including;

- external frequency reference:  If the tuned frequency is wrong with
  respect to the external reference, then it is a bug.
- adjusting the load capacitance for the primary crystal:  This can be
  done in firmware.  It is possible that we haven't selected the best
  default, but we would need to test a bunch of different manufactured
  units to determine that.  The capacitance adjustment affects the
  crystal frequency in relatively large steps, so even the best
  selection is going to be sub-optimal for many units even if it is
  within 20 ppm (as it should be).
- adjusting the PLL parameters in the clock generator:  This would be
  done to calibrate vs. a particular unit's crystal frequency.  There is
  some potential to add phase noise by switching the clock generator
  from integer mode to fractional mode, but I'm seem to recall it not
  being anywhere near as bad as I originally thought it might be.
- adjusting the PLL parameters of the frequency synthesizer in the
  MAX2837 and/or RFFC5072:  This could be done in firmware, or it could
  be done by having the host software compute an offset and just telling
  the firmware to tune to a different frequency.
- correcting the frequency in software:  The frequency could be shifted
  in software on the host computer.


On Sun, Aug 17, 2014 at 11:56:31AM +0100, Simon Brown wrote:
>
> Hi,
> 
>  
> 
> Is there any way to calibrate the hackRF to get the frequency spot-on?
> Register changes maybe? Users are complaining.
> 
>  
> 
> Simon Brown G4ELI
> http://v2.sdr-radio.com
> 
>  
> 

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