Re: [Discuss-gnuradio] USRP2 External Clock

[Matt Ettus]

Kyle Pearson wrote:
> I'm aware that the internal clock can be locked to an external 10MHz
> reference. I take it that there is no way to use an external 100MHz
> clock instead of the internal clock then.
>   

The internal clock (a very clean VCXO) is locked to the external 
reference.  You have a lot of flexibility as to the reference frequency, 
as I said in the previous message, but you cannot pull the VCXO further 
than a few kHz, and you cannot use the reference as the main system 
clock.  So there is no way, for example, to put in a 120 MHz clock and 
have everything run off of that.  You are basically stuck with 100 MHz 
unless you are willing to solder.

If you are willing to do some soldering, there are pin-compatible VCXOs 
in other frequencies which are available.  The part is the CVHD950 from 
Crystek, and you can get them from Mouser.  They come in the following 
frequencies:

   54 MHz
   74.1758
   74.25
   80
   100
   120 MHz
   122.88
   125

and possibly others.  Note than anything over 100 MHz may make it hard 
to get the FPGA to meet timing requirements.  Also, the ADC is only 
specified to 105 MHz, but there is a pin-compatible version spec'ed to 
125 MHz.

Matt


> -Kyle
>
> On Wed, Oct 22, 2008 at 4:17 PM, Eric Blossom <address@hidden> wrote:
>   
> > On Wed, Oct 22, 2008 at 04:12:07PM -0400, Kyle Pearson wrote:
> >     
> > > I'm wondering if it is possible to connect the USRP2 to an external
> > > 100MHz clock like the USRP was able to be modified so it could connect
> > > to an external 64MHz clock. I don't see any place to solder on a SMA
> > > connector but there are a set of pins labeled Test CLK. Could that be
> > > used for inputting an external clock or is it just for testing the
> > > clock output?
> > >