Re: [Discuss-gnuradio] Multiple USRPs

[Marcus Leech]

Robert McGwier wrote:

> This is not quite right.  The DIFFERENT oscillators on the USRP boards 
> have different frequencies and thus constantly changing phase angle 
> with respect to each other and with probability 1, the frequencies of 
> both oscillators will change with changing temp, air flow, etc..  The 
> only way to avoid this is the buffer the clock from one board and use 
> it to drive the other board.  The system will NOT be coherent unless 
> it IS coherent.  ;-).
>
> Bob

This raises a question I've always had about phase-coherence of PLL 
oscillators that use a common
 clock source.  Will they be coherent "enough" for things like 
astronomical interferometry?

The individual PLLs will still have "close in" phase noise components 
that are unrelated to
 on another.  I suppose that given that you typically integrate over 
several seconds, such
 artifacts get cancelled out.  But at very fine timescales, I imagine 
that PLLs locked to
 a common clock are *not* a good way to get decent coherency?