# LimeSDR Mini external clock voltage level and calibration

**URL:** <https://discourse.myriadrf.org/t/limesdr-mini-external-clock-voltage-level-and-calibration/6075>\
**Category:** LimeSDR\
**Created:** [9 May 2020 14:07 UTC](https://discourse.myriadrf.org/t/limesdr-mini-external-clock-voltage-level-and-calibration/6075 "2020-05-09T14:07:46Z")\
**Posts on this page:** 3\
**Page:** 1

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**Author:** ![sigrokBlack](https://avatars.discourse-cdn.com/v4/letter/s/b9bd4f/32.png) [@sigrokBlack](https://discourse.myriadrf.org/u/sigrokBlack)\
**Post date:** [9 May 2020 14:07 UTC](https://discourse.myriadrf.org/t/limesdr-mini-external-clock-voltage-level-and-calibration/6075/1 "2020-05-09T14:07:46Z")

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Hi, I want to use an external clock on my limesdr mini. After research I have two questions left:  
First:  
The LMK00105 states a maximal CLK-IN voltage of 2 Vpp. My ocxo has an 10MHz square wave output which swings between 0V and 3.3V. That said I should be able to connect my ocxo directly to the limesdr without attenuation. Is this correct?

Second:  
After changing to an external reference clock the LimeSDR Mini doesn’t pass a certain calibration. Is the calibration in question the one of the VCTCXO ADC? So this doesn’t affect the tx or rx calibration in any way ? (I am asking because I learned that the limesdr transmits a constant carrier if the tx calibration fails which I want to avoid)

Regards

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**Author:** ![Zack](https://yyz2.discourse-cdn.com/flex034/user_avatar/discourse.myriadrf.org/zack/32/89_2.png) [@Zack](https://discourse.myriadrf.org/u/Zack)\
**Post date:** [18 May 2020 07:23 UTC](https://discourse.myriadrf.org/t/limesdr-mini-external-clock-voltage-level-and-calibration/6075/2 "2020-05-18T07:23:33Z")

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Hi @sigrokBlack,

Not sure, how did you connect external reference clock. Hence assuming you remove ed R59 and soldered R62, check schematic:

> **[LimeSDR\_Mini\_1v1\_schematic\_r1.PDF](https://github.com/myriadrf/LimeSDR-Mini/blob/master/hardware/1v1/Project%20Outputs%20for%20LimeSDR_Mini_1v1/LimeSDR_Mini_1v1_schematic_r1.PDF)**

1. CLKin voltage of LMK00105 is up to Vdd hence 3.3V is fine.

2. If my assumption above is correct then VCTCXO DAC have no influence. You have to change reference clock frequency in LimeSuiteGUI in SXR/SXT/CGEN tabs to tell what your reference frequency is.

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**Author:** ![sigrokBlack](https://avatars.discourse-cdn.com/v4/letter/s/b9bd4f/32.png) [@sigrokBlack](https://discourse.myriadrf.org/u/sigrokBlack)\
**Post date:** [18 May 2020 12:29 UTC](https://discourse.myriadrf.org/t/limesdr-mini-external-clock-voltage-level-and-calibration/6075/3 "2020-05-18T12:29:53Z")

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Hi, thanks for your help, that helped but it started a third question:

> You have to change reference clock frequency in LimeSuiteGUI in SXR/SXT/CGEN tabs to tell what your reference frequency is.

So using `LMS_SetClockFreq()` in a C program would have the same effect as setting the reference manually using LimeSuiteGUI?

Regards!