There are a couple of issues with the rtlsdr driver used by gr-osmocom
in this regard: 

  (A) The charge-pump loop current is too constrained for the higher
frequencies 

  (B) The "dither" option appears to have a bias that causes a (small)
frequency offset. 

The driver that AirSpy uses fixes both of these, although without
"dither", the tuning granularity is worse.  Not sure this matters. 

On 2016-05-25 09:28, Marcus Müller wrote:

> That, or simply, the output clock VCO changes its reaction to the
> control voltage under certain circumstances (temperature, frequency) so
> much that the control loop loses the ability to reach stationary
> exactness (e.g. due to natural limits on the magnitude of the VCO
> voltage). These devices definitely were made with cost in mind - not
> with maximum reliability, and hence I can believe that for example with
> the Elonics E4000 tuner, the charge pump used to generate the VCO
> voltage simply might deteriorate with temperature.
> 
> Cheers,
> Marcus
> 
> On 25.05.2016 14:25, Sylvain Munaut wrote: Hi,
> 
> of the drift remains a mistery (probably due to the implementation of the PLL,
> but I cannot understand why Phase Locked Loops would drift in Phase !). If 
> the phase comparator is digital ( i.e. a XOR ) and the input clock
> is somewhat analog, the gate thresholds might vary depending on
> temperature, thus shifting the cycle a bit.
> 
> Just a thought.
> 
> Cheers,
> 
> Sylvain
> 
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