On 07/30/2019 12:43 PM, Sammy Welschen via USRP-users wrote:
Sorry for not specifying, but I was talking about two N310 or N320
devices. Do you know how it is in that case?
With the N320, with the sample-clocks in-phase when using an external
reference, I *THINK* you should see very little phase offset, even
with the up-converter in-circuit.
Am Fr., 19. Juli 2019 um 16:58 Uhr schrieb Marcus D. Leech via
USRP-users <usrp-users@lists.ettus.com
<mailto:usrp-users@lists.ettus.com>>:
On 07/19/2019 05:24 AM, Sammy Welschen via USRP-users wrote:
Thanks for your reply.
I am a bit confused now. Since the LO for this stage is derived
from the sample clock, wouldn't I be in the same situation as if
I only shared 10 MHz reference and PPS signals?
Quote from https://files.ettus.com/manual/page_usrp_n3xx.html:
My understanding was that we were talking about the N320, and only
a single unit?
I need to confirm, but I think the upconverter LO on the N320 is a
single (clock-derived) oscillator, shared among the channels in
the unit. In that case, the
phase relations will be static across powerups, assuming that
you share the other LOs.
In the case of multiple N320s, I don't yet have an answer, since
it depends critically on how that upconverter LO is produced, and
how the external-reference
PLL works.
----------------------
Reasons to use an external LO include:
Improving phase alignment: The N310 itself has no way of aligning
phase between channels, and phase will be random between runs. By
applying an external LO, the phase ambiguity is reduced to 180
degrees, produced by a by-2 divider in the AD9371 transceiver IC.
Improving phase noise: The quality of the onboard LO depends on
the external reference clock, among other things. By providing a
custom LO signal, it is possible to more accurately tune,
assuming the externally generated LO signal is coming from a
high-quality oscillator.
----------------------
I would still have a certain fixed phase relation between
channels, but with more fluctuations and a phase difference that
changes on every turn on of the devices (i.e. the same situation
as when sharing 10 Mhz and PPS signals). See for example the
plots on pages 24-25 of
https://forums.ni.com/ni/attachments/ni/grp-1008/110/1/Fundamentals%20of%20Phase-Coherent%20RF%20Measurements.pdf
for an illustration of what I mean.
Am Do., 18. Juli 2019 um 19:25 Uhr schrieb Marcus D Leech via
USRP-users <usrp-users@lists.ettus.com
<mailto:usrp-users@lists.ettus.com>>:
I have just been corrected by one of my colleagues at
Ettus.
While there is an up conversion stage for frequencies
below 450Mhz, the LO for that stage is fixed frequency,
and derived from the sample clock and coherent across
channels.
So there should be no random phase offset among channels
even below 450Mhz when LO sharing.
This is correct as far as I know. Although I don’t have
an N320 in my lab, it uses an up conversion scheme for
lower frequencies. That scheme does not participate in
the LO sharing.
Sent from my iPhone
On Jul 18, 2019, at 11:17 AM, Sammy Welschen via
USRP-users <usrp-users@lists.ettus.com
<mailto:usrp-users@lists.ettus.com>> wrote:
I have to acquire phase coherent data on multiple
channels using USRP N310/320 devices.
Am I correct in assuming that for frequencies below
450 MHz, there is no way to remove random phase
differences between channels due to the additional
frequency shift involved (shown for example in the
block diagram
http://www.ettus.com/wp-content/uploads/2019/03/N320BlockDiagram.png
<https://urldefense.com/v3/__http:/www.ettus.com/wp-content/uploads/2019/03/N320BlockDiagram.png__;%21fqWJcnlTkjM%219UAjRx1UqNGGgw_MyGxeNp4dcd08afk3IjpbsIlbrDYEk9H2AfliCLM52OcRdC4$>)?
As I understand it, by using the same LO signal for
all channels I could remove the differences for
frequencies above 450 MHz, but this is of no use
for frequencies below 450 MHz due to this the
additional stage.
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