[USRP-users] Tuning resolution of TwinRX + DDC?

Marcus D. Leech patchvonbraun at gmail.com
Fri Mar 22 10:03:43 EDT 2019

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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