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From: | Marcus Müller |
Subject: | Re: [Discuss-gnuradio] Syncronization issues, using a GPSDO |
Date: | Tue, 24 May 2016 17:34:12 +0200 |
User-agent: | Mozilla/5.0 (X11; Linux x86_64; rv:38.0) Gecko/20100101 Thunderbird/38.6.0 |
Hi Meelis, what USRP are you using? This is indeed an interesting plot. So let's look at the clocks and control mechanisms involved (this is assuming you're using something like the N2xx or X3xx with the matching GPSDO):
One more thing: Tuning to a given frequency consists of two
things, first of all of course the physical setting of the
synthesizers mentioned under 3., and secondly on correcting the
residual frequency offset (as the name suggests, these
synthesizers have a finite set of possible frequencies and can't
hit every frequency exactly) by effectively multiplying the Master
Clock Rate (X3xx: typically 200 MS/s, alternatively 184.32 or 120
MS/s, N2xx: 100MS/s) sample stream with a complex oscillation (so,
in RX, the samples coming from the ADC are first shifted in
frequency and then filtered and decimated to your 10MHz sampling
rate). So when watching out for phase synchronous receiption, you do not
only have to take care that sampling starts at the same instant,
but also that the daughterboard synthesizers were tuned exactly at
the same time leading to the same phase (if your daughterboard has
that capability at all). Also, you must make sure that the phase
accumulator in the FPGA, used for the "digital" part of tuning
mentioned above, has the same value on all devices. Best regards, On 24.05.2016 16:37, Meelis Nõmm wrote:
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