Re: [Discuss-gnuradio] Using external clock references in GRC environment

[Nazmul Islam]

Hello Jonathan,

Thanks a lot for your email. I am actually using 1 Mega chip per second in my experiments. I am using different chip sequence periods (63, 127, etc.) to see how they work. I I just wanted to emphasize on the "external ref clock source" option in the previous email and I forgot to correct the sampling rate value before taking the screenshot. I am really sorry for throwing you off. I have attached the screen shots of my transmitter and receiver with the email. 

In the receiver, I am just taking the baseband I&Q samples that is stored in "Input.dat". I am taking the correlation between the chip sequence & these samples. Thereafter, I am taking the magnitude of the real & imaginary parts.

I also think that timing error is leading to the wrong correlation values for higher chip sequence. I thought that the timing error would vanish if I use an external reference clock. I did that and put the UHD:USRP clock source options as "external". I don't know why the timing error still exists, i.e., I keep getting undetectable peaks with an external source. 

Thanks,

Nazmul

On Wed, Jun 6, 2012 at 12:41 PM, Johnathan Corgan <address@hidden> wrote:

On Wed, Jun 6, 2012 at 9:22 AM, Nazmul Islam <address@hidden> wrote:

 

I am sorry for not clarifying on the "strange output" last time. In my experiment, I am transmitting an LFSR PN sequence. Thereafter, I correlate the transmitted PN sequence with the received baseband complex I & Q. If there were no timing error between the two USRP's, then I would get high correlation after every chip sequence period.

It looked like on your last email that your GRC flowgraph has the USRP source sample rate set to 32Ksps.  This is not a sample rate producible by the USRP; in fact, in the bottom portion of the GRC window, you should see an error message telling you what it actually set the sample rate to.  If this is different from the transmitter, then you will get what you are seeing (or worse.)

You should look at what the allowable sample rates are for your transmitter and receiver USRPs, then pick a common one and set that as the variable 'samp_rate' in your GRC flowgraph on both sides.  For purposes of your experiment, I'd choose the lowest sample rate possible; later, you might try increasing this as CPU performance allows.

Johnathan

-- 
Muhammad Nazmul Islam

Graduate Student
Electrical & Computer Engineering
Wireless Information & Networking Laboratory
Rutgers, USA.

PN_Correlator_transmitter.png
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PN_Correlator_receiver.png
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