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Re: [Discuss-gnuradio] Question regarding frequency offset correction

From: Marcus Müller
Subject: Re: [Discuss-gnuradio] Question regarding frequency offset correction
Date: Mon, 14 Jul 2014 19:36:19 +0200
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:24.0) Gecko/20100101 Thunderbird/24.5.0

Hi Piotr,
there's various control loop based offset correction, look for the PLL
and FLL examples.
There's a feedback loop base class in GNU Radio for monolithic
correction. For the frequency correction estimate feedback, you could
easily use a message bus, that will occasionally inform the upstream
frequency correction block.


On 14.07.2014 18:14, Perper wrote:
> Hi all,
> In many applications very good synchronization of carrier and sampling
> frequencies is required.
> Available sources of signal not always have good clock reference. One of
> the examples is cheap RTL SDR receiver based on a DVB-T dongle.
> Without any additional effort to correct frequency offset it is
> impossible to decode GPS or GSM transmissions with such receivers.
> The frequency offset cannot be calibrated once as it changes with time
> and temperature. Good way to fight with it is to implement some
> correction algorithm that continuously computes frequency offset
> estimates and applies correction by:
> - performing frequency shifting and re-sampling in software,
> - or changing some hardware parameter that enables tuning of the
> frequency of an internal oscillator (like 'ppm' option in RTL SDR source).
> My question: is it possible to build working frequency correction with
> available GNU Radio blocks? Can you point some successful example? Or if
> not - can you share some ideas how it can be done? I'm especially
> interested in situations where frequency offset correction and
> estimation are in separate blocks i.e:
>                         ______freq. offset_________
>                         |                         |
>                         v                         |
> |sig.source|-->|freq.offset|-->(processing)-->|freq. offset|
>                |correction  |                  |estimation  | 
> --
> Best Regards,
> Piotr Krysik
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