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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