Re: [Discuss-gnuradio] External clock source Info?

[Per Zetterberg]

Gregory Maxwell wrote:
> On Tue, Aug 17, 2010 at 3:10 PM, Mark J. Blair <address@hidden> wrote:
>   
> > On Aug 17, 2010, at 2:24 PM, William Pretty Security Inc wrote:
> >     
> > > It seems that 52MHz /64MHz precision clock references are like hen’s teeth, so 
> > > I’m working on a design.
> > > What I need to know is what sort of level is the USRP1 looking for ? Is it 3.3V 
> > > CMOS ?
> > >
> > > Once I get the design working, I’ll make them available at a reasonable price J
> > >       
> > I think you will get bonus points if your design can accept an external 10MHz 
> > reference provided by a GPS-disciplined oscillator. If there was some 
> > convenient way to adjust out the crystal aging for use when the external 
> > reference isn't available, that would be even better. I've been thinking of 
> > designing something along these lines, but I won't complain if you do the hard 
> > work and I can just buy one from you. ;)
> >     
>
>
> A while back I decided I wanted a better clock in my USRP1s and went
> hunting. I didn't have a lot of luck, and the few places I sent
> inquiries to didn't get back to me.  My attention was eventually
> pulled off onto other projects... but you can count me in as someone
> who would like to partake in the results of this work.
>
> I'd also like to echo the 10MHz comment.  GPSDOs Clocks with excellent
> long term stability show up at fairly low prices on ebay all the time
> (excess from cell site deployments, I assume).  I have a couple of
> them.   What they don't usually have is the very low phase noise that
> I'd want for a clock which is eventually going to multiplied up to GHz
> levels.   So a USRP1 clock board which was primarily a 10->64MHz
> up-converter with very low phase noise would be exactly what I would
> want.
>
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>   
I enjoy hearing the discussion on clocks as I have also wanted  very low 
phase-noise. For the USRP2 I hope that a good 10MHz reference will do 
the trick.  A while ago I did some tests which show good results when 
the transmitter and receiver are locked to the same 10MHz (see below). 
From this I assume that the same good results would occur with 
excellent 10MHz references. I have ordered T1220-T17-3.3-SM-10.0MHz 
from  http://www.greenrayindustries.com/tcxo.html. I haven't tested with 
that one yet.


== My experiments:

I have been experimenting with transmitting a tone between two USRP2s 
equipped with basic daughterboards. The daughterboards are tuned to 
20MHz, and the transmitted signal is generated in base-band as a cisoid 
with 5MHz frequency. The decimation and interpolation rate is 25MHz in 
both TX and RX.

In the figure
http://www.s3.kth.se/~perz/usrp/phase_noise1.jpg 
<http://www.s3.kth.se/%7Eperz/usrp/phase_noise1.jpg> non of the USRP2s 
are locked to external reference.
In the figure
http://www.s3.kth.se/~perz/usrp/phase_noise2.jpg 
<http://www.s3.kth.se/%7Eperz/usrp/phase_noise2.jpg> the two USRPs are 
locked to the 10MHz ref output of a marconi2024 generator.
In the figure
http://www.s3.kth.se/~perz/usrp/phase_noise3.jpg 
<http://www.s3.kth.se/%7Eperz/usrp/phase_noise3.jpg> the two USRP2 are 
locked to the 10MHz ref of two _different_ marconi2024.

What I gather from this is that the reference of my marconi2024 is 
better than that built in the usrp2 (first and third figure). Though, it 
can be much better (second figure). However, then I would need an 
extremely clean reference. My marconi2024 is specified to have 
phase-noise better than -124dB/Hz at 20kHz. The results seem to suggest 
that's not true. However, the generator is 15 years old. Is it likely 
that it has aged ?



BR/
Per