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
[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,
We are sorry that his writing was not very clear. But we need your
suggestions. Thank you very much for your help in advance.
Mamoru Y. / RISH, Kyoto University
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
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.
In this case, I input RF signals of 130 MHz + 0.05 MHz to RF1 and set as
Center frequency: 130 MHz
When I don’t set center frequency, two USRP2’s signal don’t have phase
So I suppose this is due to the phase difference between NCOs of two
What should I do?