On 03/21/2019 07:08 AM, Damon wrote:
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
They'll have an effect on initial phase error, because each half of the
N310 has its own timekeeper. If those aren't brought into alignment, then
the sample streams will be unaligned. But that should have no effect
on ongoing phase drift--ONLY on initial phase error.
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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