External 40MHz tcxo by Phil Crump

I’ll point you at this thread: LimeSDR Mini external clock reference config permanent? and I’d recommend reading the two links on the last line of my post in this thread LIMESDR on SRSLTE : unable to find network on LTE device

And maybe check the 3rd and last page of the schematic for a LimeSDR mini.

P.S. That wire that in the photo that they are using to connect the clock, must be generating a nice amount of RFI at 40MHz (and all the harmonics from a 3.3v CMOS square wave). My guess would be that they removed the the U.fl (J8) from the Mini (and moved the 0 ohm resistor) . Or another possibility is that they left the 0 ohm resistor alone and removed the RTX5032A chip and are connecting to the “ground” (pin 4) and “RF out” (pin 6) pads from that part.

Thinking about it a bit more at 40MHz the wavelength would be ~7.5 meters, so probably not that bad at all. Even the 3rd harmonic at 120MHz (~2.5m) would not bad either. Nor the 5th harmonic at 200MHz (~1.5m), because it would not be a very efficient antenna. And by the time you get up to say the 37th harmonic at 1480MHz (~ 0.2m) the power at that harmonic would be so low that it is negligible. Just because the length of wire used is so short (~0.02m) it is probably not a problem at all.