Hi Marcus,
Thank you for your reply.
I compared the USRP configuration of your grc file with that of mine. I
found that the key to solve this issue is set_time_next_pps or
set_time_unknow_pps should be called.
I don't know why those two functions have an effect on phase error. I
haven't found any special description of these two functions in the N310
manual.
Best regards,
Damon
On 2019/3/21 上午1:07, Marcus D. Leech wrote:
On 03/20/2019 12:55 PM, Damon wrote:
Hi Marcus,
Thanks for your reply.
Yes, it happen with any other frequency changes, not only 460.01M and
469.03Mhz.
I'm building a 4-channel coherent receiving system with n310. It
needs to compensate the phase errors of multiple receiving channels.
So I need to measure the phase errors first and then compensate them.
But in the range of 100MHz bandwidth (from center_freq-50MHz to
center_freq+50MHz), the phase difference of two receiving channels of
different dboards varies too much with frequency, so it's very
difficult to compensate the phase errors. As a contrast, the phase
errors of the two receiving channels of X310 with ubx vary very samll
with frequency, so it is easy to compensate the phase difference.
Looking forward to your test results
Best regards,
Damon
I used the attached script.
Now, I'm only looking at +/- 2MHz, rather than your 100Mhz bandwidth.
I found that the phase noise and offset did not change noticably
tuning across the entire 4Mhz band. I don't have a machine fast
enough here to
sweep my TX across 100Mhz, but with the N310 RX at a fixed tuning,
and sweeping the TX (in this case, a Marconi transceiver test set), I
did not see
any significant phase shift or change in mutual phase noise as the
TX swept across the 4MHz band.
On 2019/3/20 上午10:40, Marcus D. Leech wrote:
On 03/19/2019 09:12 PM, Damon wrote:
Hi Marcus,
The phase responses of two channels of different dboards (ubx-160)
in a X310 are very consistent. When the frequency of transmitting
signal changes from 460.01MHz to 460.03Mhz, the phase difference
between two RX channels of different dboards in X310 remains
unchanged. But for the phase difference between two RX channels of
different dboards of N310, there are dozens or hundreds degree of
changes.
Can you try to reproduce the test and help me to figure out how to
solve it?
I'm not sure if it's a hardware bug, a driver bug or USRP setup
problem.
I cannot imagine a hardware or driver bug that could produce this
behavior. It would mean that the receiver was somehow changing LO
frequency when the TX frequency changed.
Does this happen with *other* frequency changes, or just
460.01<--->460.03. I wonder if you have an interfering signal that
is being
"uncovered" by TX frequency change, and you're simply measuring
that interfering signal?
The only other thing that occurs to me is filtering that is
extremely non-linear phase. But that would create such a mess that
most applications
would likely not work--clearly they do.
I can try to reproduce in my lab tomorrow, but, like the last tests
I did, I very much expect to not be able to reproduce.
Best regards,
Damon
On 2019/3/19 上午6:10, Marcus D. Leech wrote:
On 03/18/2019 01:26 PM, Damon wrote:
Hi Marcus,
Sorry, I can't reproduce the first observation in this discussion
thread. A new problem about phase response has arisen.
I am testing the phase coherence performance of four receiving
channels of N310. A B200 is transmitting single tone continuous
wave to a one to four splitter. The 4 outputs of the splitter are
connected to 4 RX channels of N310. Attached please find the GRC
file of this test.
The RX frequency of 4 channels of N310 is set to 460MHz, and
keep running in the test.
The TX frequency of B200 is set to 460.02MHz first, and then to
460.03MHz. I thought the phase difference between different
dboards would change very little when the signal frequency
difference is very small, similar to the performance of X310.
However, the fact is that the phase difference between the two
dboards of N310 varies considerably with the signal frequency
transmission. For example, in the attached picture, when the
signal frequency is 460.01MHz, the phase difference between
channel 2 and channel 0 is -118 degrees, the phase difference
between channel 1 and channel 0 is 0 degrees; when the
transmission frequency is adjusted to 460.03MHz, the phase
difference between channel 2 and channel 0 is 117 degrees, and
the phase difference between channel 1 and channel 0 is 0
degrees. It is very difficult to understand that the phase
difference of two receiving channels of two different dboards has
changed by 235 degrees with the signal frequency change of 20
KHz. The phase difference of two receiving channels of the same
dboard is basically unchanged.
Best regards,
Damon
Since the LO on the daughterboards has no idea that you've changed
input frequency, this is clearly a measurement thing, and it's up
to you
to understand what you're measuring, and why.
On 2019/3/16 上午7:47, Marcus D. Leech wrote:
On 03/14/2019 04:37 PM, Damon wrote:
Hi Marcus,
The UHD Version is v3.14.0.0-rc1.
Best regards,
Damon
I don't see this issue at all, using v3.14.0.0-rc3
How are you measuring phase, what does you flow-graph look like?
Have you increased the gain enough to assure that the inherent
system
noise is not dominating your phase measurements?
Hi Ali,
The daughterboards have their own clock generators, but they
are not
exactly 'independent'. At least they don't have to be, as
they share the
same reference clock. Look at the block diagram:
https://kb.ettus.com/images/9/9d/USRP_N310_N300_DB_Schematic.pdf
and "Ref Clock" block. I don't have N310 and I know that
reality can be
a bit far from expectations (i.e. look at my "What makes
sense and what
doesn't in the way carrier frequency is set for TwinRX
currently?" post).
But maybe the daughterboards can be configured to use that
reference clock.
Best Regards,
Piotr Krysik
The LMK clock generator uses the reference clock from the
mainboard, so
there should not be any mutual phase-jitter/drift issues. I
can test this
on my N310 in the coming day or two.
What version of UHD is in use?
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