If you don't have expensive laboratory equipment and you want to know accuracy of your USRP clock, you can use an application Kalibrator (Copyright (c) 2009 Joshua Lackey, Alexander Chemeris). Of cource you cannot achieve such precision measurement like with hi-tech lab equipment, but it's enough to find out modest clock calibration errors.
Latest version of Kalibrator today is 0.2 and is available here. Starting from the next update of OpenBTS Kalibrator will be distributed as a part of OpenBTS standard package.
Kalibrator takes advantage of the existing real networks and their accuracy. So you need to be in range within some operator's BTS. You pass the downlink frequency of the previously scanned ARFCN as a command line parameter for Kalibrator. Using some math Kalibrator can estimate the value of frequency difference between the frequency it tunes USRP to and the frequency of a BTS working in +-200kHz range near that frequency.
To build Kalibrator with gnuradio 3.2.2 (ver. 3.1.3 is not currently supported) run 'make' and you will get binary file 'kal' which is ready for use.
Now you need to know which ARFCN is better to use. With any GPRS modem supporting AT commands it is possible to find an ARFCN with the strongest signal. Without such modem, you have to scan whole band by manual iteration through downlink frequencies (it can be inconvenient especially on 1800/1900 band). Search this file for specific downlinks all_gsm_channels_arfcn.txt.Suitable channels must have at least -70 to -80dBm strenght, else you got to much overruns and the result could be misguiding. That's just good to have more channels to scan to eliminate possible wrong error. There is an example of network scan from GPRS modem (offset values are added manually).
Here is obvious how the values were obtained.
[openBTS@openBTS kal-0.2]# ./kal -f 946600000 -u
By default RX and side are set to ones used by OpenBTS, so usually you need only one mandatory parameter '-f'. You can use -u is optional parameter for continous display.
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注： Open BTS Clock Calibration（原文出处，翻译整理仅供参考!）