On Thu, Jun 7, 2012 at 7:11 PM, Tom Rondeau <t...@trondeau.com> wrote:
> On Thu, Jun 7, 2012 at 1:30 PM, Nazmul Islam <mnis...@winlab.rutgers.edu> > wrote: > > 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. > > Tom > 6kHz is way too high. They should be cycle-locked. What is the amplitude of the clock signal you're feeding into the USRP2? --n > > > > On Thu, Jun 7, 2012 at 12:00 PM, Nazmul Islam < > mnis...@winlab.rutgers.edu> > > wrote: > >> > >> Hi Tom, > >> > >> 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. > >> > >> Nazmul > >> > >> On Fri, May 25, 2012 at 1:40 PM, Tom Rondeau <t...@trondeau.com> wrote: > >>> > >>> On Thu, May 24, 2012 at 7:07 PM, Nazmul Islam > >>> <mnis...@winlab.rutgers.edu> wrote: > >>> > Hello, > >>> > > >>> > I want to transmit a continuous stream of 1's or 0's (with bpsk > >>> > modulation) > >>> > and record the received I-Q stream. I am trying to use the > >>> > 'digital_bert_tx.py' code for transmission and the uhd_rx_cfile code > >>> > (gr-uhd/apps) for reception. Thereafter, I use the > read_complex_binary > >>> > code > >>> > to read the data in Matlab. > >>> > > >>> > Surprisingly, I am receiving similar type of I-Q stream (around 0.3 > + j > >>> > 0.3) > >>> > for both 1 and 0 transmission. I am using the following commands: > >>> > > >>> > self._bits = gr.vector_source_b([1,], True) (I > >>> > either > >>> > transmit infinite 1 or infinit 0's. When I transmit infinite 0's, I > >>> > replace > >>> > '1' by '0' in the command) > >>> > > >>> > ./digital_bert_naz_tx.py -r 5M -m bpsk -f 450M --gain 0.1 > >>> > --non-differential (I am using non-differential since I want to > see > >>> > the > >>> > different amplitude levels for '1's or 0's) > >>> > > >>> > ./uhd_rx_cfile -N 1000 -f 450M --samp-rate 5M file.dat (Since I am > >>> > using > >>> > bpsk, sample-rate should be equal to bit rate, I assume) > >>> > > >>> > Ideally, the I-Q stream of bpsk should show 180 degree phase shift > for > >>> > 1 and > >>> > 0 transmission. I am getting the same value for both transmission. > Can > >>> > anyone suggest where I am making mistakes? > >>> > > >>> > > >>> > Thanks, > >>> > > >>> > Nazmul > >>> > >>> > >>> Nazmul, > >>> Hard to say from this info. A few things to note on, though. First, > >>> 1000 samples isn't that much. There are startup transients in > >>> hardware, so you might just be seeing effects of those. I'd capture > >>> ten thousand or a million and just read out the last 1000. > >>> > >>> Also, the transmitter and receiver are running on two different > >>> clocks, so their frequency and phases aren't going to match, unless > >>> you've locked them to the same source. It'd be hard to say what you'll > >>> see, exactly, due to this. That's why we use recovery loops for all of > >>> these things. > >>> > >>> 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 Islam > >> > >> Graduate Student > >> Electrical & Computer Engineering > >> Wireless Information & Networking Laboratory > >> Rutgers, USA. > >> > > > > > > > > -- > > Muhammad Nazmul Islam > > > > Graduate Student > > Electrical & Computer Engineering > > Wireless Information & Networking Laboratory > > Rutgers, USA. > > > > _______________________________________________ > Discuss-gnuradio mailing list > Discuss-gnuradio@gnu.org > https://lists.gnu.org/mailman/listinfo/discuss-gnuradio >
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