Re: [Discuss-gnuradio] Recording continuous I-Q stream and frequency offset with an external reference clock

[Nazmul Islam]

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 <address@hidden> 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 <address@hidden> wrote:

On Thu, May 24, 2012 at 7:07 PM, Nazmul Islam
<address@hidden> 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.