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 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.
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