N200 and N210 Device Notes
The N210 has a variety of clock configurations that may work or not work with OpenBTS to varying degrees. All N210's include an on-board 2.5ppm TCXO that serves as a stable reference, though not GSM specification compliant. The internal TXCO is used if the '--with-extref' option is not specified and no GPSDO kit is present.
The front panel input can be used with any external reference and is enabled with the '--with-extref' configure option.
From the UHD manual, the reference specifications are:
Using an external 10 MHz reference clock, square wave will offer the best phase noise performance, but sinusoid is acceptable.
The reference clock requires the following power level:
USRP2: 5 - 15 dBm
N2XX: 0 - 15 dBm
Of various clock options, the worst performing case arises when the external reference is enabled with no reference present causing the internal oscillator to run free. The resulting center frequency can be off by a large amount - visible as many kHz on a spectrum analyzer. Handset compatibility in this specific configuration varies, but performance is very likely to be poor do to the frequency error.
Ettus Research™ GPSDO Kit
For use of the optional, but recommended, GPSDO Kit, see the Ettus Research installation instructions.
Please note the J510 setting which switches the reference connection from front panel connector to the GPSDO clock; pin 1 of the jumper pins is noted by the large white triangle. Improper setting of the jumper will disable use of the GPSDO and cause poor clocking performance.
Also note that the GPSDO Kit configures automatically through the UHD driver. The '--with-extref' configure setting is not necessary and has no effect.