On 2021-12-06 13:42, Temir Karakurum wrote:
Hi,
I have some experience working with E310 and B210 and one of the
bottlenecks for my application is the frequency tuning/hopping time of
AD9361. I understand that N320 has the ability to use an external
reference oscillator but I am not sure what this actually entails.
Does this mean that the LO used in the homodyne IQ receiver
architecture is no longer generated by the PLL but is directly
supplied by the external reference? If so, does this mean that I can
sweep the frequencies as fast as I can generate LO frequencies in a
similar fashion to a swept spectrum analyzer? Is external LO mode
useful for spectrum sensing/frequency hopping applications or is it
mostly used for applications that requires
phase coherence?
Also are there any UHD examples or tutorials depicting how to build
tune_request objects under 'external LO' mode?
Best,
Temir
External LO is largely about providing phase-coherence. The AD9361 in
the E310 and B210 has NO support for external LOs, but the AD9371 in the
N310 does have support
for a 2XLO.
When you create the multi_usrp device object, you'll pass in an
"rx_lo_source=external" argument, and make sure the external LO, at
that moment, is operating at 5GHz.
Once device initialization has completed, you can
set_lo_source("external"). See:
https://www.gnuradio.org/doc/doxygen/classgr_1_1uhd_1_1usrp__source.html#ae24f390fef2a0692dc230a5284ad1002
Now, how that interacts with future set_rx_freq() or set_tx_freq() I'm
not clear on. The AD9371 in the N310 must necessarily complete some I/Q
calibration and compensation
functions every time it tunes to a new frequency. It's not clear
(and I cannot find examples) how you can trigger that calibration
process after you change your external LO.
In any case, the external LO needs to be at *TWICE* the desired tuned
frequency (because of the 2XLO mixers), and that input is limited to
8GHz, which means you can only
use external LO usefully for center frequencies between 300Mhz and 4GHz.
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