# LimeSDR ext ref clock freq and type?

**URL:** <https://discourse.myriadrf.org/t/limesdr-ext-ref-clock-freq-and-type/647>\
**Category:** LimeSDR\
**Created:** [25 January 2017 22:46 UTC](https://discourse.myriadrf.org/t/limesdr-ext-ref-clock-freq-and-type/647 "2017-01-25T22:46:05Z")\
**Posts on this page:** 1\
**Showing post:** 4

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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 April 2017 14:44 UTC](https://discourse.myriadrf.org/t/limesdr-ext-ref-clock-freq-and-type/647/4 "2017-04-04T14:44:51Z")

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There are two U.FL connectors near the USB 3.0 connector that are marked REF CLK IN and OUT, that are used for synchronisation. You connect OUT from one board to IN on another board. Then if you start LimeSuiteGUI and select Modules -\> Si5351C you will see there are radio buttons to change PLL source to CLK IN.

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_[View the full topic](https://discourse.myriadrf.org/t/limesdr-ext-ref-clock-freq-and-type/647)._