On 03/05/2020 11:10 AM, Rob Kossler via USRP-users wrote:
Hi Alejandro,
Phase sync is not possible with CBX
<https://kb.ettus.com/CBX#Phase_Synchronization>. Time sync and freq
sync should work, but keep in mind the following:
* The actual freq transmitted is a function of the quantized LO
setting and the more finely quantized DUC setting. The final
value is not necessarily "exactly" what you ask for. So, the delta
between your 2 signals may not be exactly 80 MHz (perhaps off by a
fraction of a hertz because of the quantizations).
* The two transmissions do not share the same LO. Each channel has
its own PLL that is locked to the 10 MHz ref, but there will be
some variability between the two PLL
Rob
Mostly what Rob said above.
Whether two same-type synthesizers will produce LO signals that "march
together" even in the presence of a common high-quality reference,
depends a lot on the implementation details of the synthesizer.
Additionally there are a couple of different things to consider when
making phase-coherent systems:
(A) Will the synthesized LOs "march together" -- I THINK that on
the CBX, the answer is YES, albeit with higher phase-noise than
strictly-shared
LO.
(B) Will there always be guaranteed zero phase-offset. With the
CBX, the answer is NO. Taking a quick look at the MAX2870 data-sheet
(the synthesizer chip used on the CBX), I don't see any
"phase reset" feature, which SOME synthesizer chips can use to drag
muiltiple
synthesizers together into mutual zero-phase-offset after tuning.
The mutual phase noise for a strictly-shared-LO scheme will ALWAYS be
better than with muiltiple-synthesizers all sharing the same reference
clock. Whether that higher mutual phase noise is acceptable is
very-much application specific.
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