Forum: GNU Radio external clock is not working

Hi, I am doing experiments on two usrp n210. And i am using an external
10MHz function generator to sync the frequency of the two usrp. I
connect the TTL output of the function generator to the ref input on the
usrp. But i still can see the frequency offset on the received signal. I
doubt the function generator is not working. Does anyone have idea about
this? Any reply will be appreciated.