To do accurate frequency measurements of Andrés EB4FJV‘s 2.3GHz beacon, I have needed to setup my LimeSDR to use my DF9NP 10MHz GPSDO as an external reference. This is a quick HOWTO with the steps to make the LimeSDR use an external reference.
The LimeSDR uses an ADF4002 phase detector to discipline its 30.72MHz VCTCXO to an external reference. The external reference should be connected to the LimeSDR J19 U.FL connector. It needs to have a level between 0.8Vpp and 3.3Vpp into 50 Ohm. DC bias is acceptable, since the external reference input is AC coupled. The ADF4002 specifies that an input frequency can be between 20MHz and 350MHz, but a frequency below 20MHz can also be used provided that the slew rate is greater than 50V/us. Thus, if you feed in a 10MHz reference, you should use a square wave.
To enable the external reference, we open
LimeSuiteGUI and got to Options > Connection settings, then we select our LimeSDR device and click Connect. Then we go to Modules > ADF4002, we enter the reference frequency in the “Fref, MHz” field and click Calculate R, N & Upload.
The FPGA LED2 in the LimeSDR board should light up indicating the status of the ADF4002. It is red for no lock, green for lock, and off when the ADF4002 is disabled. If everything is working correctly, this LED should be green. Then we can close
LimeSuiteGUI and open an SDR software such as gqrx. Note that the ADF4002 should be configured using LimeSuiteGUI every time the LimeSDR board is powered up.
There is more information about the LimeSDR clock configuration here.
A final note: the DF9NP 10MHz GPSDO outputs a sine wave, so I have had to bypass the lowpass filter to get something resembling a square wave, as otherwise the ADF4002 wouldn’t lock.