For efficient operation of OpenBTS, replace the on-board USRP clock with this 52Mhz board. 

Reduces CPU usage and provides better frequency stability to allow for better phone acquistion, particularly at the GSM1800 and GSM1900 bands.

Comes with an SMA connector, an SMA cable, and installation instructions.  Most of these instructions are available at http://gnuradio.org/trac/wiki/OpenBTS/ClockModifications.

You will have to modify the USRP motherboard to use this clock board. 

Small enough to fit inside USRP enclosure.