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Re: [Discuss-gnuradio] Multi-rtl - making multi-channel receiver out of


From: mleech
Subject: Re: [Discuss-gnuradio] Multi-rtl - making multi-channel receiver out of multiple RTL-SDR dongles
Date: Thu, 26 May 2016 12:28:14 -0400
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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, <address@hidden> 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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