# GPSDO synchronization with LimeSDR-USB

**URL:** <https://discourse.myriadrf.org/t/gpsdo-synchronization-with-limesdr-usb/7818>\
**Category:** LimeSDR\
**Created:** [3 February 2023 11:50 UTC](https://discourse.myriadrf.org/t/gpsdo-synchronization-with-limesdr-usb/7818 "2023-02-03T11:50:19Z")\
**Posts on this page:** 3\
**Page:** 1

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**Author:** ![JackRA](https://avatars.discourse-cdn.com/v4/letter/j/c37758/32.png) [@JackRA](https://discourse.myriadrf.org/u/JackRA)\
**Post date:** [3 February 2023 11:50 UTC](https://discourse.myriadrf.org/t/gpsdo-synchronization-with-limesdr-usb/7818/1 "2023-02-03T11:50:19Z")

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Given a 2.5 volt p-p GPSDO 10 MHz signal to the Osc. In port, what Python command can be used to ‘lock in’ that signal to the internal VCXO of Lime? Also, once that command is given what command can be used to confirm that it has occurred?

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**Author:** ![andrewback](https://avatars.discourse-cdn.com/v4/letter/a/22d042/32.png) [@andrewback](https://discourse.myriadrf.org/u/andrewback)\
**Post date:** [4 February 2023 12:35 UTC](https://discourse.myriadrf.org/t/gpsdo-synchronization-with-limesdr-usb/7818/2 "2023-02-04T12:35:24Z")

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No idea with Python and I suspect it will depend on what library you are using. But to give you an example, you can find the native LMS API driver for the LibreCellular srsRAN fork here (device “lime”):

> **[srsRAN/lib/src/phy/rf at lc/main · myriadrf/srsRAN](https://github.com/myriadrf/srsRAN/tree/lc/main/lib/src/phy/rf)**
>
> lc/main/lib/src/phy/rf

With this we can just specify `refclk=10e6` as a driver argument to set an external reference. See:

[https://librecellular.org/user/configuration.html#examples](https://librecellular.org/user/configuration.html#examples)

If successful and the SDR locks, you will see an LED illuminate.

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<div class="post-metadata">

**Author:** ![pgreenland](https://yyz2.discourse-cdn.com/flex034/user_avatar/discourse.myriadrf.org/pgreenland/32/4207_2.png) [@pgreenland](https://discourse.myriadrf.org/u/pgreenland)\
**Post date:** [18 February 2023 20:17 UTC](https://discourse.myriadrf.org/t/gpsdo-synchronization-with-limesdr-usb/7818/3 "2023-02-18T20:17:25Z")

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Before switching to the lime API version of srsRAN I was using LimeUtil to switch to the refclk input, which has an option for it:

`LimeUtil --refclk --fref 10000000`

After which SoapySDR seemed to just carry on as normal.

Not sure if that same command confirms the lock status though I’m afraid.