[Discuss-gnuradio] sampling jitter

[Per Zetterberg]

Dear All,

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 non of the USRP2s are 
locked to external reference.
In the figure
http://www.s3.kth.se/~perz/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 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 ?

Any response will be appreciated!

BR/
Per