Re: [Discuss-gnuradio] changing the vctcxo

[Roshan Baliga]

Matt Ettus wrote:
 > The rev 4 and newer USRPs have a clock generator chip that is 
spec'ed to
> take a signal as low as 150 mV p-p.  However in my testing I found I 
> needed a voltage as high as 8 or 9dBm.  8dBm is about .8V, so that 
> should still work.

Sorry if this isn't relevant to everybody on the list, but it is *very* 
important to those of us who are using external clocks with the Rev 4 USRPs.

I tried out varying clock inputs, and verified Matt's claim that 800 mV 
pp was required for consistent USRP operation. This didn't seem correct 
to me, because Analog (who makes the clock distribution chip) normally 
is very good about their specifications.

After further testing (and datasheet reading) we discovered two things:

(1) If you're using an external clock, in addition to the changes 
specified under "Rev 4 USRPs - For the Slave Boards" at:

http://www.comsec.com/wiki?USRPClockingNotes

You should probably also remove C924, the 0.01 uF cap going to X2 pin 3.

(2) Because the holes for J2001 come with solder in them, when 
soldering/pushing an SMA connector on, it is very easy to break or 
mangle the signal trace from J2001 to C927 (and therefore, to U702, the 
AD9513 clock distribution IC).

I believe that (2) is why Matt and I noticed a higher input signal 
requirement. On my board, with J2001 physically disconnected from C927, 
the leakage was enough to send a dirty/noisy clock into U702, which lead 
to flakey USRP performance.

However, once we connected the signal pin on J2001 correctly to C927, 
and removed C924, the USRP worked flawlessly, even down to 200 mV pp input.

-Roshan

NB For those EE types, you may wonder why I recommend removing C924. 
It's true that if pin 3 on X2 is DC, leaving C924 in shouldn't make a 
difference. However, removing C924 makes me feel better, because I know 
that U702 is completely disconnected from X2, and isn't affected if pin 
3 on X2 changes at all.