A bias in the dither algorithm could explain that, which is why turning
off dither clears up the problem. The drifting-out-of-lock problem is
due to charge-pump current settings being too low in some cases.
On 2016-05-26 11:47, Juha Vierinen wrote:
> Would either of these issues in the rtlsdr driver consistently tune the two
> dongles on slightly different frequencies, even if you ask them to tune to
> exactly the same frequency? This is what the problem seems to be.
>
> juha
>
> On Wed, May 25, 2016 at 10:35 AM, <mle...@ripnet.com> wrote:
>
> 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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