On 03/03/2020 12:31 PM, Rob Kossler via USRP-users wrote:
Hi Len,
I have run some testing of this capability. A few remarks:
* In my test setup, I had a signal generator running into a 1:4
splitter with outputs connected to the four N310 Rx2 inputs. I
used another signal generator as the LO source running into a 1:2
splitter with outputs connected to the N310 Rx LO inputs with
amplitude near 3 dBm <https://kb.ettus.com/N300/N310#Front_Panel>
at N310 LO inputs.
* When using external LO, the RF range is limited to a max of 4 GHz
(rather than 6 GHz which is the limit with internal LO)
* I had problems with UHD 3.14 because there was some bug related to
resetting the phase of the DDC. I had success with 3.15. But even
with success, there is still a 180 degree ambiguity between
channels 1/2 and channels 3/4. I am not certain regarding the
circumstances that can cause the ambiguity to flip, but I think it
only occurs at application startup such that once your application
is running, you can expect consistent results with no ambiguity flips.
This is probably due to the 2XLO phase-splitter inside the AD9371
chips--it's based on a flip-flop, whose initial state is unpredictable. Any
2XLO phase-splitter has this property--including on the WBX
daughtercard used on N210/X310 etc. The mixer has a 2XLO phase-splitter,
so also shows this 180deg phase ambiguity.
* When you launch your application and create a usrp device
instance, the UHD software will automatically try to run some IQ
calibrations (RX_QEC_INIT) expecting that the RF is at 2.5 GHz,
which implies that your external LO must be at 5 GHz at startup.
After startup is complete (and this calibration is complete) you
can change the freq of your LO and retune the RF of the various
channels keeping in mind that the external LO must always be twice
the desired RF. If you want to avoid the startup calibration
(with LO at 5 GHz), you can specify explicitly which calibrations
<https://files.ettus.com/manual/page_usrp_n3xx.html#n3xx_mg_calibrations>
you want using the device args and choose not to run RX_QEC_INIT.
But, I'm not sure if skipping this calibration could have negative
effects on signal quality.
Let me know if you have questions.
Rob
On Thu, Feb 27, 2020 at 12:22 AM Len via USRP-users
<[email protected] <mailto:[email protected]>> wrote:
Folks,
I'm looking for information from any subscriber(s) to this forum
that have successfully used the USRP N310 to provide 4 Rx channels
which are both coherent and synchronized. I've heard this could
be challenging and I know people have been able to do this
successfully so I was wondering where I might find some detailed
information/instructions on how best to accomplish this. I was
told this forum would be a good place to start. So any block
diagrams illustrating the Master LO and Master Clock hookups
needed along with any available calibration procedure(s)
describing how to actually check/verify coherence and
synchronization between the 4 channels would be great.
Thanks,
Len
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