External Clock input on original LimeSDR


Im wanting to feed the limeSDR-usb (the original limesdr, not the mini) a external clock reference from a leo Bodnar gpso (Mini Precision GPS Reference Clock : Leo Bodnar Electronics)

After looking at various threads on here im confused as alot of them are focused on the limeSDR mini and im not sure how that differs vs the limeSDR-USB in this regard (if it does at all)

questions are:
what frequency should the external reference clock be on?
Does the original limeSDR need any HW modification to take an external clock reference?
Can I also use clk output port to output this reference on a separate configurable frequency? (to feed an LNB PLL on 24Mhz)
Optionally, If i used a gpso with dual reference outputs instead, could i feed the limesdr a reference on 24Mhz (and use the second gpso reference output to also feed the LNA PLL @ 24Mhz too)

many thanks

Hi @j4mbob,

you can look into this page https://wiki.myriadrf.org/LimeSDR-USB. Check hardware description and Schematic documents from this page.

Check Clock distribution section. By default reference clock input is connected to ADF4002 frequency synthesizer. It has range of 5MHz to 400Mhz. Take in mind if you want to bypass onboard clock circuitry and feed external reference clock into clock buffer that reference clock feeds PLL’s of LMS7002 IC and according to datasheet it should be in 10MHz-52MHz range.

EDIT: Check “Clock distribution” section in link above. By default REF CLK IN is connected to IC23 phase detector, if you want to discipline the onboard oscillator no changes needed. If you want to completely bypass onboard VCTCXO you have to solder some resistors to feed external reference clock into clock buffer.

By default REF CLK OUT port is not connected, to connect it you have to solder some resistor, check schematic according to your board version.

Just to be clear: isn’t that just if you want to completely bypass the onboard VCTCXO? I thought by default an external reference will go to the phase detector (IC23) input and discipline the onboard oscillator? Which is what most would presumably want to do when connecting an external reference.

@andrewback, correct, I have edited my original post.

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Thanks for clarifying!

@j4mbob for an example of the original LimeSDR USB being used with a Leo Bodnar reference and srsRAN to create a 4G network, see:


Using the LibreCellular fork of srsRAN it’s then possible to use an extra device argument to specify an external reference clock frequency:

Excellent, thanks for the responses both.

I was worried that i might need to make modifications to the board so relieved to find out that isnt the case.

I ended up ordering a dual output gpso from Leo Bodnar, which can output on two different freqs. so I wont need to use the clock out on the Lime now anyway.

I think i can feed the lime with 10mhz from the gpso and 24Mhz to the lnb PLL from the Leo which should solve my issue.

many thanks!


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