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Re: [Discuss-gnuradio] sampling jitter

From: Tom Rondeau
Subject: Re: [Discuss-gnuradio] sampling jitter
Date: Thu, 18 Mar 2010 19:55:10 -0400

On Wed, Mar 17, 2010 at 8:23 AM, Per Zetterberg
<address@hidden> wrote:
> 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

This all seems reasonable to me. If you have not recalibrated the
clock in 15 years, then, yes, it is definitely possible that it has


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