Trouble Locking to External Reference with LimeSDR USB

Hello, I have been trying to link my LimeSDR-USB to an external reference. But currently I am unable to get all of the PLLs to lock. I used several different sources, a square wave source at 10 MHz and a sinusoidal source at 50 MHz with Vpp of approximately 3.0V. Just wondering if anyone has any suggestions for a path forward or if anyone has had any particular success with a specific configuration.

Hi, make sure your 10 MHz reference is not too strong. I’ve had to add 6 dB of attenuation to mine because it comes in at 5 Vpp, when the maximum is 3.3 Vpp if I recall correctly.

Thanks for response, I did check that it was within the 0.8 to 3.3Vpp range. Any other ideas of what it might be?

@Zack, any suggestions?

Could you please describe all the steps you do.

Took 10 MHz reference clock source from signal generator and checked using oscilloscope that the output was square and within the 0.8 to 3.3 VPP range.
Connected Lime to Computer using usb 3.0 connection
Opened LimeSuite GUI
Connected LimeSDR to LimeSuite GUI
Entered ADF4002 Menu
Connected Reference Clock signal to J19 U.FL connector
Clicked Calculate R,N and Upload Button. LED flashed but PLL never locked.

I’m using a leo bednar gpsdo, 10 Mhz signal, fed to the ref clock in port, and enabling external clock via the C API, and it works great for me.

Did this set up take a lot of time to get working?


I am using gnuradio in my final implementation for this project, does anyone have suggestions on the best way to enable the external clock input?