On 03/22/2019 09:39 AM, Paul Boven via USRP-users wrote:
Hi everyone,
We recently did VLBI observations centered at 1332.49 MHz, using a
X310 with TwinRX. The USRP was locked to external references for 10MHz
and PPS for this. To get the best phase noise, I used 'integer-N'
tuning. We noticed that overall, our receiver frequency was off by
0.118 Hz. This causes the phase of the recorded signal to rotate in
about 9 seconds, compared to the other stations participating.
As far as I understand, when using integer-N tuning, the DDC simply
takes care of the remaining offset. What would be the tuning
resolution of the TwinRX itself (using int-N), and what is the
resolution of the DDC which corrects for this?
Regards, Paul Boven.
The frequency step in INTEGER_N mode is either 6.25MHz (Rev A/B) or
12.5MHz (Rev C).
I don't know what the resolution of the DDC is, but traditionally they
have been "a fraction of a Hz". So the only way to get "exact" (with
respect to the common reference) tuning would be to use integer-N at a
multiple of the PFD frequency as above.
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