On 04/26/2019 11:36 AM, Fabian Schwartau via USRP-users wrote:
Hi,
I am am using LO sharing. So there should not be any phase offset and
no mirrored spectrum, no matter when the USRP comes around to change
the frequency. Even not using LO-sharing, the spectrum should NOT be
mirrored.
The apparent I/Q swap issue I agree should not happen, and I have a
query in to R&D about it.
Am 26.04.2019 um 17:12 schrieb Marcus D. Leech via USRP-users:
On 04/26/2019 05:13 AM, Fabian Schwartau via USRP-users wrote:
Hi,
a couple of days ago I filed a bug report which caused the USRPs to
switch but noone has responded yet. I did now encountered other
problems wich might be related to that issue. Can somone from Ettus
(or someone else) take a look at that?
Bug report is here: https://github.com/EttusResearch/uhd/issues/271
Best regards,
Fabian
So, the original design reason for timed-commands was to support
simultaneous tuning (and assertion of a resynch pulse) across
*multiple USRPs*,
in support of reducing/eliminating mutual phase offset among
identically-hardwared USRPs, for daughtercards that supported
phase-resynch.
When you use timed commands within a single USRP, hoping for similar
effects, you won't get them, because the commands in the queue
are *necessarily* executed sequentially. Even if there was a
"parallel execution" structure within the FPGA to handle the
commands, most of
the commands you care about ultimately end up on an SPI or I2C
bus, and those are inherently serial, and multiple devices
(synthesizers,
variable-gain amplifiers, clock-plls) typically share a single
such bus on a per-slot basis. In order to have "true parallelism"
*within* a single
USRP, you'd need dedicated control buses to each device that is
controlled by I2C/SPI/whatever, *in addition* to parallel execution
within the
FPGA.
When you're setting a bunch of PLL synthesizers sequentially, you can
expect that they won't agree on phase, even when being driven
by a common clock. The mechanism for achieving phase coherence
with TwinRx is to use LO sharing.
https://kb.ettus.com/TwinRX
This app-note talks about LO sharing with TwinRX
https://kb.ettus.com/Direction_Finding_with_the_USRP%E2%84%A2_X-Series_and_TwinRX%E2%84%A2
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