Lessons learned with an external clock / USRP1 interface
In order to use even a high-priced SDR for GSM experimentation, an accurate clock source is indispensable. According to the GSM standard, a clock with “absolute accuracy better than 0.05 ppm for both RF frequency generation and clocking the timebase” is required. This means that with a standard USRP1 with frequency accuracy of 2.5 ppm, an external clock is definitely required. (An external clock port is provided on the front panel.) Using an accurate external clock source is essential when using OpenBTS.
The crew at hack4fun wanted better USRP clock accuracy for OpenBTS, so they decided to interface a FA-SY 1 external clock from Funkamateur.de with their USRP1. The results are chronicled in their project log. They eventually discovered that *coincidentally*, their RFX900 daughterboard on the USRP was fried, and they also experienced issues with the external clock’s calibration.
The experience is leading the hack4fun crowd in the direction of developing a new, easy to use external clock for USRP1 OpenBTS enthusiasts.
(For those interested, a succinct explanation of the proper way to install an external clock to the USRP1 can be found on page 13 of Nicolas Krassas’ GSM Fun article. It is also important to note that the FA-SY 1 is a CMOS output, and requires modification to decrease its output voltage to prevent USRP input burn out.)This entry was posted in project logs, RF, SDR and tagged FA-SY 1 clock, GSM, OpenBTS, USRP.