First of all, thanks a lot for your detailed reply. I appreciate it. I
did
as you told in the last email, i.e., I transmitted a square wave
(switching
between 0.5 to -0.5). The sqaure wave’s period was 1 ms and the sampling
rate was 1 MHz. I have attached the real part of the outputs with the
email.
The output shows a phase shift after every 500 samples, i.e., half
period
of the square wave with 1 MHz sampling rate. The sinusoidal nature of
the
output probably comes from frequency offset of the two USRP’s. I
expected
this for an internal clock source.
However, I see a 6 kHz frequency offset (3 sine period per 0.5 ms) even
with the presence of an external clock. The external clock is driving
both
USRP’s. The E LED is on. I am using a sine wave with 10 MHz frequency &
7
dBm amplitude as the external clock. I also put the clock source options
in
grc as external. Do I need to make any other changes in the GRC blocks
to
inform USRP about the external source?
Any suggestions will be appreciated. Thanks for all of your help.
and
Hard to say from this info. A few things to note on, though. First,
What I would recommend is to create a transmitter that transmits a
long string of 1’s followed by a long string of 0’s (100 or 200 each).
When you plot the last 1000 samples, you should see something that
moves between two amplitudes. I wouldn’t trust what you see from one
run to another, so just do it at the same time.
Tom
–
Muhammad Nazmul I.
Graduate Student
Electrical & Computer Engineering
Wireless Information & Networking Laboratory
Rutgers, USA.
I got a partial answer to my previously posted question :). When I pass
the
complex baseband I & Q with a costas loop block, the output indeed
looks
like a square wave.
Does it mean that external reference clock does not correct the
phase/carrier offset error? Does it only solve the timing error issue?
I got a partial answer to my previously posted question :). When I pass the
complex baseband I & Q with a costas loop block, the output indeed looks
like a square wave.
Does it mean that external reference clock does not correct the
phase/carrier offset error? Does it only solve the timing error issue?
Thanks,
Nazmul
Glad that you are able to get far enough to recover it. As for the
remaining 6 kHz offset, what’s the RF frequency? What does 6 kHz
translate into for a parts per million? While I would expect them to
be the same with both locked to the same external clock, we are
talking about reality here, so things aren’t always that cooperative.
I can’t think what would cause this kind of an offset, though, as it
seems rather large.
Maybe someone with more hands-on hardware experience with precision
equipment can jump in here.
Thanks for your replies. The function generator is producing a sine wave
with 1 V (10 MHz). This is going to the ref clocks. The E light is ON
and I
have changed the clock source option to be external in
gnuradio-companion
environment. I tried with square wave, too. But the output did not
change.
The transmission is taking place at 450 MHz carrier frequency. I am
using
USRP2’s with RFX400 & FLEX400 daughterboard.
In fact, I see a 6 kHz frequency offset even without the reference
clock.
Does it suggest that the external reference clock is somehow not going
through? I don’t know if I should change more options in
gnuradio-companion
toolbox to make the reference clock work.
Does it mean that external reference clock does not correct the
talking about reality here, so things aren’t always that cooperative.
of the clock signal you’re feeding into the USRP2?
Hi Tom,
The output shows a phase shift after every 500 samples, i.e., half
USRP’s. The E LED is on. I am using a sine wave with 10 MHz frequency
(gr-uhd/apps) for reception. Thereafter, I use the
either
./uhd_rx_cfile -N 1000 -f 450M --samp-rate 5M file.dat (Since I am
run to another, so just do it at the same time.
Electrical & Computer Engineering
Electrical & Computer Engineering
–
Muhammad Nazmul I.
Graduate Student
Electrical & Computer Engineering
Wireless Information & Networking Laboratory
Rutgers, USA.
Thanks for your replies. The function generator is producing a sine wave
with 1 V (10 MHz). This is going to the ref clocks.
It’s possible that a function generator output, when loaded down by two
REF
inputs in parallel, might not produce the amplitude necessary to
reliably
work, nor might it have the necessary rise time.
You’ll need to measure these to ensure they meet the REF in
requirements.
The symptoms you are seeing are an indication they do not.
Johnathan
This forum is not affiliated to the Ruby language, Ruby on Rails framework, nor any Ruby applications discussed here.