Phase changes between two USRP-2. Why and how?

Dear Friends,

I am Mamoru Y., a professor in RISH, Kyoto Univ., who is
using USRP-1 for dual-band satellite beacon receiver. We use
our receivers are used for real research, and are very productive.
Thank you. Today I want to ask you a different thing.

We are running another project with USRP-2. This is to
develop a radar receiver. We plan to use several (about 7)
USRP-2-based receivers for the radar imaging. Thus the
receivers must be working with following conditions.

(1) Synchronized in frequency and timing with a transmitter.
(2) Phase between receivers kept constant.

The condition (1) is successful with synchronizing the USRP-2
with outside REF 10MHz or 100MHz, and applying a transmitter pulse
to PPS port. Now we face a problem for (2). We can check this
by applying the same signal to two USRP-2 receivers. Our
symptom is,

[a] Phase between output data from two USRP-2 is stable in a single
run for hours. (GOOD)

[b] But at different runs of the USRP-2, the phase difference
are different (= look random). (BAD)

Our URSP-2 radar receiver is at 130MHz. Then we use Basic-RX
board. One more finding is,

[c] If center frequency of the USRP-2 is zero (=0Hz),
then the phase difference is stable at different runs. (WHY?)

Result of [b] and [c] may show that NCO in the FPGA are oscillating
at the same frequency but at different (random) initial phases. At the
“zero-frequency” setup, NCO output may be fixed at a certain value
like 1+0j, then [c] is realized. I would like to know,

– Is this story true?
– How can we set NCO initial phase equalized at different USRP-2?

Wakisaka-kun, one of our students asked a question to this mailing
list on October 1st, but not yet get an answer. I attach his E-mail
here. The figure is found from,
http://www.rish.kyoto-u.ac.jp/~yamamoto/Observation_data.png

We are sorry that his writing was not very clear. But we need your
suggestions. Thank you very much for your help in advance.

Regards,


Mamoru Y. / RISH, Kyoto University
[email protected]
Phone +81-774-38-3814, Cell +81-90-5653-7555

=== Original question (E-mail from Mr. Wakisaka) ===

Subject: About synchronize two USRP2s using external reference (SMA)

I tried to synchronize two USRP2s using external reference (SMA).
I input 10-MHz signal to REFCLOCK pin and a trigger in PPS IN.

I modified rx_streaming_samples.cc and wrote the following code
u2->config_mimo(usrp2::MC_WE_LOCK_TO_SMA);
I use txrx_raw_eth_20100608.bin and u2_rev3-20100603.bin.
but I failed to synchronize two USRP2s.

I synchronized start timing using PPS but two USRP2’s signal
have phase difference. This phase difference change every observation.
Some figures are attached.

– Figure –
http://www.rish.kyoto-u.ac.jp/~yamamoto/Observation_data.png

In this case, I input RF signals of 130 MHz + 0.05 MHz to RF1 and set as
follows:
Center frequency: 130 MHz
Decimation: 4

When I don’t set center frequency, two USRP2’s signal don’t have phase
difference.
So I suppose this is due to the phase difference between NCOs of two
USRP2.

What should I do?

Many Thanks.

Youhei Wakisaka.

=========================

Hi Mamoru,

We have written a multi-channel sampler for USRP2. It is fairly phase
coherent and also starts out with the same phase across all receivers
(we have tested only up to four, but I don’t see why more receivers
wouldn’t work). We use equal length cables to feed a PPS and a 10 MHz
reference to all of our usrp2 devices.

What fpga image and firmware are you using with your usrp2? We started
out with the gnuradio ones, but they started out with a random initial
phase, even though the relative phase between the devices was locked.
This was because the firmware didn’t reset the NCO phase when it
received a PPS. This problem is corrected with the UHD version of the
firmware.

I’d be happy to send you our multi-channel sampler code.

juha

2010/10/8 Mamoru Y. [email protected]:

Juha, and Friends,

Thanks for the prompt reply. Then my guess on the “phase difference
at different runs” problem is correct, and is random initial phase
of NCO.

FPGA image and firmware that we use are,

I use txrx_raw_eth_20100608.bin and u2_rev3-20100603.bin.

I want to try the UHD version. Would you please send your code
to me? Can you tell me how much they (GNU Radio original and UHD)
are different? We use C++ to develop our radar receiver program.
Thanks for your help.

Friends, if there is a corrected FPGA image with the “GNU Radio
original” setup, please let me know. This is another way
for us to follow.

Regards,

Mamoru Y.
RISH, Kyoto Univ.

phase, even though the relative phase between the devices was locked.

(1) Synchronized in frequency and timing with a transmitter.

at the same frequency but at different (random) initial phases. At the

but I failed to synchronize two USRP2s.
Center frequency: 130 MHz
Youhei Wakisaka.

=========================


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Mamoru Y. / RISH, Kyoto University
[email protected]
Phone +81-774-38-3814, Cell +81-90-5653-7555

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